Software Security Practice Exam

Question 1 Software Security

What is a JWT (JSON Web Token)?

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default. API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser.

Question 2 Software Security

What is a CSP nonce?

A CSP nonce is a unique random token generated for each page load that allows specific inline scripts to execute. The server adds Content-Security-Policy: script-src 'nonce-abc123' and tags allowed scripts with the matching nonce attribute. This provides fine-grained control over inline scripts without using 'unsafe-inline', strengthening XSS protection. Argon2 is the winner of the 2015 Password Hashing Competition. Three variants:

Argon2d: data-dependent, resistant to GPU attacks
Argon2i: data-independent, resistant to side-channel attacks
Argon2id: hybrid (recommended)

It's configurable on time, memory, and parallelism, making it resistant to both GPU and ASIC attacks. CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request. OpenID Connect is an identity layer built on top of OAuth 2.0. While OAuth 2.0 handles authorization, OIDC adds authentication:

Returns an ID token (JWT) with user identity claims
Provides a /userinfo endpoint
Standardized scopes: openid, profile, email

It answers "who is this user?" while OAuth answers "what can they access?"

Question 3 Software Security

What is Cross-Site Scripting (XSS)?

bcrypt is a password hashing function based on the Blowfish cipher. Key features:

Built-in salt — prevents rainbow table attacks
Configurable work factor (cost) — can be increased as hardware improves
Deliberately slow — resistant to brute force

Usage: bcrypt.hash(password, saltRounds). Output format: $2b$12$salt...hash... HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. Secrets management is the practice of securely storing and accessing sensitive data like API keys, passwords, and certificates. Best practices:

Use a vault (HashiCorp Vault, AWS Secrets Manager)
Never hardcode secrets in source code
Use environment variables or secret injection
Rotate secrets regularly
Use .gitignore for config files
Audit access to secrets

Question 4 Software Security

What is the difference between encoding, encryption, and hashing?

API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

Defense in depth is a security strategy using multiple layers of protection so that if one layer fails, others still provide security. Layers include:

Network: firewalls, VPNs, IDS/IPS
Application: input validation, authentication
Data: encryption, access controls
Physical: server room access
Monitoring: logging, alerting

No single control is considered sufficient.

Encoding: transforms data for compatibility (Base64, URL encoding) — not for security, easily reversible
Encryption: transforms data to protect confidentiality — reversible with key (AES, RSA)
Hashing: one-way transformation to a fixed-size digest — not reversible (SHA-256, bcrypt)

Each serves a different purpose; using the wrong one creates vulnerabilities. Broken Access Control (OWASP #1) occurs when users can act outside their intended permissions. Examples:

IDOR: accessing another user's resources by changing IDs
Modifying URL parameters to access admin pages
Missing function-level authorization checks
CORS misconfiguration

Prevention: deny by default, enforce access control server-side, test authorization on every request.

Question 5 Software Security

What is OpenID Connect (OIDC)?

A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default. Broken Access Control (OWASP #1) occurs when users can act outside their intended permissions. Examples:

IDOR: accessing another user's resources by changing IDs
Modifying URL parameters to access admin pages
Missing function-level authorization checks
CORS misconfiguration

Prevention: deny by default, enforce access control server-side, test authorization on every request. A rainbow table is a precomputed table of hash-to-password mappings used to reverse hash functions. Instead of brute-forcing, attackers look up the hash directly.
Prevention:

Use a unique salt per password — makes precomputed tables useless
Use slow hashing algorithms (bcrypt, argon2)

Modern password hashing functions include automatic salting. OpenID Connect is an identity layer built on top of OAuth 2.0. While OAuth 2.0 handles authorization, OIDC adds authentication:

Returns an ID token (JWT) with user identity claims
Provides a /userinfo endpoint
Standardized scopes: openid, profile, email

It answers "who is this user?" while OAuth answers "what can they access?"

Question 6 Software Security

What is Broken Access Control?

Broken Access Control (OWASP #1) occurs when users can act outside their intended permissions. Examples:

IDOR: accessing another user's resources by changing IDs
Modifying URL parameters to access admin pages
Missing function-level authorization checks
CORS misconfiguration

Prevention: deny by default, enforce access control server-side, test authorization on every request. HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption

API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

Argon2 is the winner of the 2015 Password Hashing Competition. Three variants:

Argon2d: data-dependent, resistant to GPU attacks
Argon2i: data-independent, resistant to side-channel attacks
Argon2id: hybrid (recommended)

It's configurable on time, memory, and parallelism, making it resistant to both GPU and ASIC attacks.

Question 7 Software Security

What is session management security?

Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy. The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser. Rate limiting restricts the number of requests a client can make within a time window. Purposes:

Prevent brute force attacks
Protect against DDoS
Ensure fair usage

Common algorithms:

Token bucket
Sliding window
Fixed window
Leaky bucket

Typically returns 429 Too Many Requests with Retry-After header.

Question 8 Software Security

What is CORS (Cross-Origin Resource Sharing)?

Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs. DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval(). CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request. The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser.

Question 9 Software Security

What is a rainbow table attack?

MFA requires two or more independent authentication factors:

Knowledge: something you know (password, PIN)
Possession: something you have (phone, security key)
Inherence: something you are (fingerprint, face)

Even if one factor is compromised, the attacker cannot authenticate. TOTP and WebAuthn/FIDO2 are common implementations. A rainbow table is a precomputed table of hash-to-password mappings used to reverse hash functions. Instead of brute-forcing, attackers look up the hash directly.
Prevention:

Use a unique salt per password — makes precomputed tables useless
Use slow hashing algorithms (bcrypt, argon2)

Modern password hashing functions include automatic salting. CSP is an HTTP header that tells the browser which sources of content are allowed to load. It mitigates XSS by blocking inline scripts and unauthorized sources.
Example: Content-Security-Policy: default-src 'self'; script-src 'self' cdn.example.com; style-src 'self' 'unsafe-inline'
Key directives: default-src, script-src, style-src, img-src, connect-src. CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request.

Question 10 Software Security

What is secrets management?

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. Secrets management is the practice of securely storing and accessing sensitive data like API keys, passwords, and certificates. Best practices:

Use a vault (HashiCorp Vault, AWS Secrets Manager)
Never hardcode secrets in source code
Use environment variables or secret injection
Rotate secrets regularly
Use .gitignore for config files
Audit access to secrets

IDOR occurs when an application exposes internal object references (like database IDs) and doesn't verify the user has authorization to access them. Example: changing /invoice/1001 to /invoice/1002 to view another user's invoice.
Prevention:

Check authorization on every request
Use indirect references (UUIDs)
Implement proper access control

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session.

Question 11 Software Security

What is a digital signature?

A digital signature proves authenticity and integrity of data using asymmetric cryptography:

Sender hashes the message
Sender encrypts the hash with their private key
Recipient decrypts with sender's public key
Recipient compares hashes to verify

Used in: code signing, TLS certificates, JWT signing, blockchain transactions. SQL injection occurs when untrusted input is included in SQL queries without proper sanitization, allowing attackers to execute arbitrary SQL. Example: SELECT * FROM users WHERE name = '' OR '1'='1'. Prevention:

Parameterized queries: WHERE name = ?
Use an ORM
Input validation and escaping
Principle of least privilege for DB accounts

Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

Defense in depth is a security strategy using multiple layers of protection so that if one layer fails, others still provide security. Layers include:

Network: firewalls, VPNs, IDS/IPS
Application: input validation, authentication
Data: encryption, access controls
Physical: server room access
Monitoring: logging, alerting

No single control is considered sufficient.

Question 12 Software Security

What is the principle of least privilege?

A rainbow table is a precomputed table of hash-to-password mappings used to reverse hash functions. Instead of brute-forcing, attackers look up the hash directly.
Prevention:

Use a unique salt per password — makes precomputed tables useless
Use slow hashing algorithms (bcrypt, argon2)

Modern password hashing functions include automatic salting. Clickjacking (UI redressing) tricks users into clicking on hidden elements by overlaying a transparent iframe over a legitimate page. The user thinks they're clicking a button but actually interact with the hidden frame.
Prevention:

X-Frame-Options: DENY header
Content-Security-Policy: frame-ancestors 'none'
JavaScript frame-busting (less reliable)

The principle of least privilege states that every user, program, and process should have only the minimum permissions necessary to perform its function. Examples:

Database accounts with only required queries
API keys scoped to specific operations
Container processes running as non-root
IAM roles with minimal AWS permissions

Limits the blast radius of a security breach. CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request.

Question 13 Software Security

What is a salt in password hashing?

A salt is a random value added to a password before hashing. Each user gets a unique salt, stored alongside the hash. Benefits:

Two users with the same password get different hashes
Defeats rainbow table attacks
Defeats precomputation attacks

Example: hash(salt + password). bcrypt and argon2 handle salting automatically. The principle of least privilege states that every user, program, and process should have only the minimum permissions necessary to perform its function. Examples:

Database accounts with only required queries
API keys scoped to specific operations
Container processes running as non-root
IAM roles with minimal AWS permissions

Limits the blast radius of a security breach. A MITM attack occurs when an attacker secretly intercepts and relays communication between two parties who believe they're communicating directly. The attacker can eavesdrop, modify, or inject messages.
Prevention:

Use HTTPS/TLS for all communications
Implement HSTS
Certificate pinning
Verify certificate chains
Use secure Wi-Fi (WPA3)

The SameSite cookie attribute controls when cookies are sent with cross-site requests:

Strict: cookie only sent for same-site requests — strongest CSRF protection
Lax: sent for top-level navigations but not for forms/AJAX — good balance
None: always sent (requires Secure flag)

Lax is the browser default since Chrome 80.

Question 14 Software Security

What is the OAuth 2.0 Authorization Code flow?

Argon2 is the winner of the 2015 Password Hashing Competition. Three variants:

Argon2d: data-dependent, resistant to GPU attacks
Argon2i: data-independent, resistant to side-channel attacks
Argon2id: hybrid (recommended)

It's configurable on time, memory, and parallelism, making it resistant to both GPU and ASIC attacks.

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session. The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser. A salt is a random value added to a password before hashing. Each user gets a unique salt, stored alongside the hash. Benefits:

Two users with the same password get different hashes
Defeats rainbow table attacks
Defeats precomputation attacks

Example: hash(salt + password). bcrypt and argon2 handle salting automatically.

Question 15 Software Security

What is input validation?

Input validation is verifying that user-supplied data meets expected format, type, length, and range requirements before processing. Two approaches:

Allowlist (preferred): accept only known-good patterns
Denylist: reject known-bad patterns (less secure)

Validate on both client-side (UX) and server-side (security). Never trust client-side validation alone. The SameSite cookie attribute controls when cookies are sent with cross-site requests:

Strict: cookie only sent for same-site requests — strongest CSRF protection
Lax: sent for top-level navigations but not for forms/AJAX — good balance
None: always sent (requires Secure flag)

Lax is the browser default since Chrome 80. DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval(). Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs.

Question 16 Software Security

What are parameterized queries?

OpenID Connect is an identity layer built on top of OAuth 2.0. While OAuth 2.0 handles authorization, OIDC adds authentication:

Returns an ID token (JWT) with user identity claims
Provides a /userinfo endpoint
Standardized scopes: openid, profile, email

It answers "who is this user?" while OAuth answers "what can they access?" Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs. Rate limiting restricts the number of requests a client can make within a time window. Purposes:

Prevent brute force attacks
Protect against DDoS
Ensure fair usage

Common algorithms:

Token bucket
Sliding window
Fixed window
Leaky bucket

Typically returns 429 Too Many Requests with Retry-After header.

Authentication (AuthN): verifies who you are — login with username/password, biometrics, MFA
Authorization (AuthZ): determines what you can do — permissions, roles, access control

Authentication always comes first. Example: logging in proves your identity (authentication); your role determines which pages you can access (authorization).

Question 17 Software Security

What are important HTTP security headers?

Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request. Key security headers:

Strict-Transport-Security (HSTS): force HTTPS
Content-Security-Policy: control resource loading
X-Content-Type-Options: nosniff: prevent MIME sniffing
X-Frame-Options: prevent clickjacking
X-XSS-Protection: legacy XSS filter
Referrer-Policy: control referrer information
Permissions-Policy: control browser features

Question 18 Software Security

What is defense in depth?

Defense in depth is a security strategy using multiple layers of protection so that if one layer fails, others still provide security. Layers include:

Network: firewalls, VPNs, IDS/IPS
Application: input validation, authentication
Data: encryption, access controls
Physical: server room access
Monitoring: logging, alerting

No single control is considered sufficient. CSRF tricks a user's browser into making an unwanted request to a site where they're authenticated. Example: a malicious page submitting a hidden form to a banking site. Prevention:

CSRF tokens — unique tokens per session/request
SameSite cookie attribute
Check Origin/Referer headers
Require re-authentication for sensitive actions

Input validation: verify data meets expected format before processing — reject or sanitize bad input
Output encoding: transform data for safe display in a specific context — HTML encoding, URL encoding, JS encoding

Both are needed: validation prevents bad data from entering; encoding prevents stored bad data from causing harm when displayed. AES (Advanced Encryption Standard) is the most widely used symmetric encryption algorithm. Key features:

Block cipher with 128-bit blocks
Key sizes: 128, 192, or 256 bits
Modes: CBC, GCM (authenticated), CTR

AES-256-GCM is recommended — it provides both confidentiality and integrity (authenticated encryption).

Question 19 Software Security

What is the TLS certificate chain?

Encoding: transforms data for compatibility (Base64, URL encoding) — not for security, easily reversible
Encryption: transforms data to protect confidentiality — reversible with key (AES, RSA)
Hashing: one-way transformation to a fixed-size digest — not reversible (SHA-256, bcrypt)

Each serves a different purpose; using the wrong one creates vulnerabilities. A digital signature proves authenticity and integrity of data using asymmetric cryptography:

Sender hashes the message
Sender encrypts the hash with their private key
Recipient decrypts with sender's public key
Recipient compares hashes to verify

Used in: code signing, TLS certificates, JWT signing, blockchain transactions. A TLS certificate chain is a hierarchy of trust:

Root CA: self-signed, pre-installed in browsers/OS
Intermediate CA: signed by root CA
Server certificate: signed by intermediate CA

The browser walks up the chain to verify each certificate is signed by a trusted authority. Certificate pinning restricts which CAs are trusted for a domain. CSRF tricks a user's browser into making an unwanted request to a site where they're authenticated. Example: a malicious page submitting a hidden form to a banking site. Prevention:

CSRF tokens — unique tokens per session/request
SameSite cookie attribute
Check Origin/Referer headers
Require re-authentication for sensitive actions

Question 20 Software Security

What is HSTS (HTTP Strict Transport Security)?

HSTS tells browsers to only use HTTPS for a domain, even if the user types http://. Header: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Benefits:

Prevents SSL stripping attacks
Automatic HTTP to HTTPS redirect at browser level
Preload list: browsers ship with a list of HSTS-enabled domains

Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

Argon2 is the winner of the 2015 Password Hashing Competition. Three variants:

Argon2d: data-dependent, resistant to GPU attacks
Argon2i: data-independent, resistant to side-channel attacks
Argon2id: hybrid (recommended)

It's configurable on time, memory, and parallelism, making it resistant to both GPU and ASIC attacks. DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval().

Question 21 Software Security

What is command injection?

Reflected XSS occurs when user input from a request is immediately included in the response without proper encoding. Example: a search page that displays the query parameter without sanitization. Prevention:

HTML-encode all output
Use Content-Security-Policy headers
Validate and sanitize input

Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

SSRF tricks the server into making HTTP requests to unintended locations, often internal services. Example: an image URL parameter targeting cloud metadata endpoints.
Prevention:

Allowlist permitted URLs/domains
Block private IP ranges
Use network segmentation
Validate and sanitize URLs
Disable unnecessary URL schemes

A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default.

Question 22 Software Security

What is stored XSS?

Password hashing is a one-way transformation — you can verify a password but cannot recover the original. Unlike encryption (which is reversible), hashing ensures that even if the database is compromised, passwords cannot be recovered.
Use slow hash functions designed for passwords: bcrypt, argon2, scrypt. Never use MD5 or SHA-256 alone — they're too fast. Stored XSS (persistent XSS) occurs when malicious script is saved on the server (e.g., in a database, forum post, or comment) and served to users who view that content. It's more dangerous than reflected XSS because it affects all users who view the page, not just those who click a crafted link. Prevention: sanitize on input, encode on output. DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval(). The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy.

Question 23 Software Security

What is SQL injection?

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. The token bucket algorithm: a bucket holds tokens up to a maximum capacity. Tokens are added at a fixed rate. Each request consumes one token. If the bucket is empty, the request is rejected. Advantages:

Allows bursts up to bucket size
Smooths out request rate over time
Simple to implement

Used by AWS API Gateway, Stripe, and many rate limiters. SQL injection occurs when untrusted input is included in SQL queries without proper sanitization, allowing attackers to execute arbitrary SQL. Example: SELECT * FROM users WHERE name = '' OR '1'='1'. Prevention:

Parameterized queries: WHERE name = ?
Use an ORM
Input validation and escaping
Principle of least privilege for DB accounts

Key security headers:

Strict-Transport-Security (HSTS): force HTTPS
Content-Security-Policy: control resource loading
X-Content-Type-Options: nosniff: prevent MIME sniffing
X-Frame-Options: prevent clickjacking
X-XSS-Protection: legacy XSS filter
Referrer-Policy: control referrer information
Permissions-Policy: control browser features

Question 24 Software Security

What is the Same-Origin Policy?

HSTS tells browsers to only use HTTPS for a domain, even if the user types http://. Header: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Benefits:

Prevents SSL stripping attacks
Automatic HTTP to HTTPS redirect at browser level
Preload list: browsers ship with a list of HSTS-enabled domains

The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy. Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

Question 25 Software Security

What is the difference between symmetric and asymmetric encryption?

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session. The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser. Secure session management best practices:

Generate cryptographically random session IDs
Set cookies as HttpOnly, Secure, and SameSite
Implement session timeout (idle and absolute)
Regenerate session ID after login (prevents session fixation)
Invalidate sessions on logout
Store sessions server-side, not in cookies

IDOR occurs when an application exposes internal object references (like database IDs) and doesn't verify the user has authorization to access them. Example: changing /invoice/1001 to /invoice/1002 to view another user's invoice.
Prevention:

Check authorization on every request
Use indirect references (UUIDs)
Implement proper access control

Question 26 Software Security

What is Security Misconfiguration?

IDOR occurs when an application exposes internal object references (like database IDs) and doesn't verify the user has authorization to access them. Example: changing /invoice/1001 to /invoice/1002 to view another user's invoice.
Prevention:

Check authorization on every request
Use indirect references (UUIDs)
Implement proper access control

A MITM attack occurs when an attacker secretly intercepts and relays communication between two parties who believe they're communicating directly. The attacker can eavesdrop, modify, or inject messages.
Prevention:

Use HTTPS/TLS for all communications
Implement HSTS
Certificate pinning
Verify certificate chains
Use secure Wi-Fi (WPA3)

A CSP nonce is a unique random token generated for each page load that allows specific inline scripts to execute. The server adds Content-Security-Policy: script-src 'nonce-abc123' and tags allowed scripts with the matching nonce attribute. This provides fine-grained control over inline scripts without using 'unsafe-inline', strengthening XSS protection. Security Misconfiguration (OWASP #5) includes:

Default credentials left unchanged
Unnecessary features enabled (debug mode, directory listing)
Missing security headers
Overly permissive CORS
Verbose error messages exposing stack traces
Unpatched software

Prevention: automated configuration scanning, minimal installs, regular audits, infrastructure as code.

Question 27 Software Security

What is the difference between input validation and output encoding?

The OWASP Top 10 is a regularly updated list of the most critical web application security risks. The 2021 edition includes:

A01: Broken Access Control
A02: Cryptographic Failures
A03: Injection
A04: Insecure Design
A05: Security Misconfiguration
A06: Vulnerable Components
A07: Authentication Failures
A08: Data Integrity Failures
A09: Logging Failures
A10: SSRF

SSRF tricks the server into making HTTP requests to unintended locations, often internal services. Example: an image URL parameter targeting cloud metadata endpoints.
Prevention:

Allowlist permitted URLs/domains
Block private IP ranges
Use network segmentation
Validate and sanitize URLs
Disable unnecessary URL schemes

Input validation: verify data meets expected format before processing — reject or sanitize bad input
Output encoding: transform data for safe display in a specific context — HTML encoding, URL encoding, JS encoding

Both are needed: validation prevents bad data from entering; encoding prevents stored bad data from causing harm when displayed. A rainbow table is a precomputed table of hash-to-password mappings used to reverse hash functions. Instead of brute-forcing, attackers look up the hash directly.
Prevention:

Use a unique salt per password — makes precomputed tables useless
Use slow hashing algorithms (bcrypt, argon2)

Modern password hashing functions include automatic salting.

Question 28 Software Security

What is Argon2?

MFA requires two or more independent authentication factors:

Knowledge: something you know (password, PIN)
Possession: something you have (phone, security key)
Inherence: something you are (fingerprint, face)

Even if one factor is compromised, the attacker cannot authenticate. TOTP and WebAuthn/FIDO2 are common implementations. AES (Advanced Encryption Standard) is the most widely used symmetric encryption algorithm. Key features:

Block cipher with 128-bit blocks
Key sizes: 128, 192, or 256 bits
Modes: CBC, GCM (authenticated), CTR

AES-256-GCM is recommended — it provides both confidentiality and integrity (authenticated encryption). OAuth 2.0 is an authorization framework that allows third-party applications to access a user's resources without exposing their credentials. Key grant types:

Authorization Code: for server-side apps (most secure)
PKCE: for SPAs and mobile apps
Client Credentials: for machine-to-machine
Refresh Token: for obtaining new access tokens

Argon2 is the winner of the 2015 Password Hashing Competition. Three variants:

Argon2d: data-dependent, resistant to GPU attacks
Argon2i: data-independent, resistant to side-channel attacks
Argon2id: hybrid (recommended)

It's configurable on time, memory, and parallelism, making it resistant to both GPU and ASIC attacks.

Question 29 Software Security

What is SSRF (Server-Side Request Forgery)?

Common JWT security risks:

Using alg: "none" — always validate the algorithm
Storing JWTs in localStorage (vulnerable to XSS) — use httpOnly cookies
Not validating the signature
No expiration (exp claim)
Putting sensitive data in the payload (it's only encoded, not encrypted)
Not implementing token revocation

A digital signature proves authenticity and integrity of data using asymmetric cryptography:

Sender hashes the message
Sender encrypts the hash with their private key
Recipient decrypts with sender's public key
Recipient compares hashes to verify

Used in: code signing, TLS certificates, JWT signing, blockchain transactions. SSRF tricks the server into making HTTP requests to unintended locations, often internal services. Example: an image URL parameter targeting cloud metadata endpoints.
Prevention:

Allowlist permitted URLs/domains
Block private IP ranges
Use network segmentation
Validate and sanitize URLs
Disable unnecessary URL schemes

Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs.

Question 30 Software Security

What is API key security best practice?

Authentication (AuthN): verifies who you are — login with username/password, biometrics, MFA
Authorization (AuthZ): determines what you can do — permissions, roles, access control

Authentication always comes first. Example: logging in proves your identity (authentication); your role determines which pages you can access (authorization). API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

bcrypt is a password hashing function based on the Blowfish cipher. Key features:

Built-in salt — prevents rainbow table attacks
Configurable work factor (cost) — can be increased as hardware improves
Deliberately slow — resistant to brute force

Usage: bcrypt.hash(password, saltRounds). Output format: $2b$12$salt...hash... The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy.

Question 31 Software Security

What is AES encryption?

MFA requires two or more independent authentication factors:

Knowledge: something you know (password, PIN)
Possession: something you have (phone, security key)
Inherence: something you are (fingerprint, face)

Even if one factor is compromised, the attacker cannot authenticate. TOTP and WebAuthn/FIDO2 are common implementations.

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session. SSRF tricks the server into making HTTP requests to unintended locations, often internal services. Example: an image URL parameter targeting cloud metadata endpoints.
Prevention:

Allowlist permitted URLs/domains
Block private IP ranges
Use network segmentation
Validate and sanitize URLs
Disable unnecessary URL schemes

AES (Advanced Encryption Standard) is the most widely used symmetric encryption algorithm. Key features:

Block cipher with 128-bit blocks
Key sizes: 128, 192, or 256 bits
Modes: CBC, GCM (authenticated), CTR

AES-256-GCM is recommended — it provides both confidentiality and integrity (authenticated encryption).

Question 32 Software Security

What is rate limiting?

A salt is a random value added to a password before hashing. Each user gets a unique salt, stored alongside the hash. Benefits:

Two users with the same password get different hashes
Defeats rainbow table attacks
Defeats precomputation attacks

Example: hash(salt + password). bcrypt and argon2 handle salting automatically. Broken Access Control (OWASP #1) occurs when users can act outside their intended permissions. Examples:

IDOR: accessing another user's resources by changing IDs
Modifying URL parameters to access admin pages
Missing function-level authorization checks
CORS misconfiguration

Prevention: deny by default, enforce access control server-side, test authorization on every request. Rate limiting restricts the number of requests a client can make within a time window. Purposes:

Prevent brute force attacks
Protect against DDoS
Ensure fair usage

Common algorithms:

Token bucket
Sliding window
Fixed window
Leaky bucket

Typically returns 429 Too Many Requests with Retry-After header. Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

Question 33 Software Security

What is Content Security Policy (CSP)?

HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption

HSTS tells browsers to only use HTTPS for a domain, even if the user types http://. Header: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Benefits:

Prevents SSL stripping attacks
Automatic HTTP to HTTPS redirect at browser level
Preload list: browsers ship with a list of HSTS-enabled domains

CSP is an HTTP header that tells the browser which sources of content are allowed to load. It mitigates XSS by blocking inline scripts and unauthorized sources.
Example: Content-Security-Policy: default-src 'self'; script-src 'self' cdn.example.com; style-src 'self' 'unsafe-inline'
Key directives: default-src, script-src, style-src, img-src, connect-src. Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs.

Question 34 Software Security

What is the OWASP Top 10?

CSRF tricks a user's browser into making an unwanted request to a site where they're authenticated. Example: a malicious page submitting a hidden form to a banking site. Prevention:

CSRF tokens — unique tokens per session/request
SameSite cookie attribute
Check Origin/Referer headers
Require re-authentication for sensitive actions

The OWASP Top 10 is a regularly updated list of the most critical web application security risks. The 2021 edition includes:

A01: Broken Access Control
A02: Cryptographic Failures
A03: Injection
A04: Insecure Design
A05: Security Misconfiguration
A06: Vulnerable Components
A07: Authentication Failures
A08: Data Integrity Failures
A09: Logging Failures
A10: SSRF

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session. Clickjacking (UI redressing) tricks users into clicking on hidden elements by overlaying a transparent iframe over a legitimate page. The user thinks they're clicking a button but actually interact with the hidden frame.
Prevention:

X-Frame-Options: DENY header
Content-Security-Policy: frame-ancestors 'none'
JavaScript frame-busting (less reliable)

Question 35 Software Security

What is password hashing and why not use encryption?

Password hashing is a one-way transformation — you can verify a password but cannot recover the original. Unlike encryption (which is reversible), hashing ensures that even if the database is compromised, passwords cannot be recovered.
Use slow hash functions designed for passwords: bcrypt, argon2, scrypt. Never use MD5 or SHA-256 alone — they're too fast. The token bucket algorithm: a bucket holds tokens up to a maximum capacity. Tokens are added at a fixed rate. Each request consumes one token. If the bucket is empty, the request is rejected. Advantages:

Allows bursts up to bucket size
Smooths out request rate over time
Simple to implement

Used by AWS API Gateway, Stripe, and many rate limiters. The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy. Stored XSS (persistent XSS) occurs when malicious script is saved on the server (e.g., in a database, forum post, or comment) and served to users who view that content. It's more dangerous than reflected XSS because it affects all users who view the page, not just those who click a crafted link. Prevention: sanitize on input, encode on output.

Question 36 Software Security

What is DOM-based XSS?

Common JWT security risks:

Using alg: "none" — always validate the algorithm
Storing JWTs in localStorage (vulnerable to XSS) — use httpOnly cookies
Not validating the signature
No expiration (exp claim)
Putting sensitive data in the payload (it's only encoded, not encrypted)
Not implementing token revocation

DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval(). A salt is a random value added to a password before hashing. Each user gets a unique salt, stored alongside the hash. Benefits:

Two users with the same password get different hashes
Defeats rainbow table attacks
Defeats precomputation attacks

Example: hash(salt + password). bcrypt and argon2 handle salting automatically. Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

Question 37 Software Security

What is multi-factor authentication (MFA)?

MFA requires two or more independent authentication factors:

Knowledge: something you know (password, PIN)
Possession: something you have (phone, security key)
Inherence: something you are (fingerprint, face)

Even if one factor is compromised, the attacker cannot authenticate. TOTP and WebAuthn/FIDO2 are common implementations. CORS is a browser mechanism that allows servers to specify which origins can access their resources. Key headers:

Access-Control-Allow-Origin: allowed origins
Access-Control-Allow-Methods: allowed HTTP methods
Access-Control-Allow-Headers: allowed headers
Access-Control-Allow-Credentials: allow cookies

Preflight requests (OPTIONS) check permissions before the actual request.

Symmetric: same key for encryption and decryption — AES, ChaCha20. Fast, used for bulk data. Challenge: securely sharing the key.
Asymmetric: public key encrypts, private key decrypts — RSA, ECC. Slower, used for key exchange and digital signatures.

TLS uses asymmetric encryption to exchange a symmetric key, then symmetric encryption for the session. A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default.

Question 38 Software Security

What is the SameSite cookie attribute?

IDOR occurs when an application exposes internal object references (like database IDs) and doesn't verify the user has authorization to access them. Example: changing /invoice/1001 to /invoice/1002 to view another user's invoice.
Prevention:

Check authorization on every request
Use indirect references (UUIDs)
Implement proper access control

AES (Advanced Encryption Standard) is the most widely used symmetric encryption algorithm. Key features:

Block cipher with 128-bit blocks
Key sizes: 128, 192, or 256 bits
Modes: CBC, GCM (authenticated), CTR

AES-256-GCM is recommended — it provides both confidentiality and integrity (authenticated encryption). Parameterized queries (prepared statements) separate SQL code from data by using placeholders for user input. The database engine treats parameters as data, never as executable code.
Example: db.query("SELECT * FROM users WHERE id = ?", [userId])
This is the primary defense against SQL injection, supported by all major databases and ORMs. The SameSite cookie attribute controls when cookies are sent with cross-site requests:

Strict: cookie only sent for same-site requests — strongest CSRF protection
Lax: sent for top-level navigations but not for forms/AJAX — good balance
None: always sent (requires Secure flag)

Lax is the browser default since Chrome 80.

Question 39 Software Security

What is reflected XSS?

The SameSite cookie attribute controls when cookies are sent with cross-site requests:

Strict: cookie only sent for same-site requests — strongest CSRF protection
Lax: sent for top-level navigations but not for forms/AJAX — good balance
None: always sent (requires Secure flag)

Lax is the browser default since Chrome 80. The token bucket algorithm: a bucket holds tokens up to a maximum capacity. Tokens are added at a fixed rate. Each request consumes one token. If the bucket is empty, the request is rejected. Advantages:

Allows bursts up to bucket size
Smooths out request rate over time
Simple to implement

Used by AWS API Gateway, Stripe, and many rate limiters. Reflected XSS occurs when user input from a request is immediately included in the response without proper encoding. Example: a search page that displays the query parameter without sanitization. Prevention:

HTML-encode all output
Use Content-Security-Policy headers
Validate and sanitize input

A digital signature proves authenticity and integrity of data using asymmetric cryptography:

Sender hashes the message
Sender encrypts the hash with their private key
Recipient decrypts with sender's public key
Recipient compares hashes to verify

Used in: code signing, TLS certificates, JWT signing, blockchain transactions.

Question 40 Software Security

What is OAuth 2.0?

API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

OAuth 2.0 is an authorization framework that allows third-party applications to access a user's resources without exposing their credentials. Key grant types:

Authorization Code: for server-side apps (most secure)
PKCE: for SPAs and mobile apps
Client Credentials: for machine-to-machine
Refresh Token: for obtaining new access tokens

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. A TLS certificate chain is a hierarchy of trust:

Root CA: self-signed, pre-installed in browsers/OS
Intermediate CA: signed by root CA
Server certificate: signed by intermediate CA

The browser walks up the chain to verify each certificate is signed by a trusted authority. Certificate pinning restricts which CAs are trusted for a domain.

Question 41 Software Security

What are Insecure Direct Object References (IDOR)?

IDOR occurs when an application exposes internal object references (like database IDs) and doesn't verify the user has authorization to access them. Example: changing /invoice/1001 to /invoice/1002 to view another user's invoice.
Prevention:

Check authorization on every request
Use indirect references (UUIDs)
Implement proper access control

The token bucket algorithm: a bucket holds tokens up to a maximum capacity. Tokens are added at a fixed rate. Each request consumes one token. If the bucket is empty, the request is rejected. Advantages:

Allows bursts up to bucket size
Smooths out request rate over time
Simple to implement

Used by AWS API Gateway, Stripe, and many rate limiters. A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default. HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption

Question 42 Software Security

What is clickjacking?

MFA requires two or more independent authentication factors:

Knowledge: something you know (password, PIN)
Possession: something you have (phone, security key)
Inherence: something you are (fingerprint, face)

Even if one factor is compromised, the attacker cannot authenticate. TOTP and WebAuthn/FIDO2 are common implementations. Clickjacking (UI redressing) tricks users into clicking on hidden elements by overlaying a transparent iframe over a legitimate page. The user thinks they're clicking a button but actually interact with the hidden frame.
Prevention:

X-Frame-Options: DENY header
Content-Security-Policy: frame-ancestors 'none'
JavaScript frame-busting (less reliable)

A salt is a random value added to a password before hashing. Each user gets a unique salt, stored alongside the hash. Benefits:

Two users with the same password get different hashes
Defeats rainbow table attacks
Defeats precomputation attacks

Example: hash(salt + password). bcrypt and argon2 handle salting automatically.

Authentication (AuthN): verifies who you are — login with username/password, biometrics, MFA
Authorization (AuthZ): determines what you can do — permissions, roles, access control

Authentication always comes first. Example: logging in proves your identity (authentication); your role determines which pages you can access (authorization).

Question 43 Software Security

How does HTTPS/TLS work?

Input validation is verifying that user-supplied data meets expected format, type, length, and range requirements before processing. Two approaches:

Allowlist (preferred): accept only known-good patterns
Denylist: reject known-bad patterns (less secure)

Validate on both client-side (UX) and server-side (security). Never trust client-side validation alone. HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption

A CSP nonce is a unique random token generated for each page load that allows specific inline scripts to execute. The server adds Content-Security-Policy: script-src 'nonce-abc123' and tags allowed scripts with the matching nonce attribute. This provides fine-grained control over inline scripts without using 'unsafe-inline', strengthening XSS protection. A rainbow table is a precomputed table of hash-to-password mappings used to reverse hash functions. Instead of brute-forcing, attackers look up the hash directly.
Prevention:

Use a unique salt per password — makes precomputed tables useless
Use slow hashing algorithms (bcrypt, argon2)

Modern password hashing functions include automatic salting.

Question 44 Software Security

What are common JWT security pitfalls?

API key security best practices:

Never embed keys in client-side code or public repos
Use environment variables or secret managers
Scope keys with minimum permissions
Set expiration dates and rotate regularly
Use different keys for development and production
Monitor usage for anomalies
Revoke compromised keys immediately

DOM-based XSS occurs entirely on the client side — the vulnerability exists in JavaScript code that processes data from an untrusted source (like location.hash) and writes it to the DOM using unsafe methods like innerHTML or document.write(). The malicious payload never reaches the server. Prevention: use textContent instead of innerHTML, avoid eval(). The Authorization Code flow:

App redirects user to authorization server
User authenticates and grants consent
Server redirects back with an authorization code
App exchanges the code for an access token (server-to-server)
App uses the access token to call APIs

Most secure flow because tokens never pass through the browser. Common JWT security risks:

Using alg: "none" — always validate the algorithm
Storing JWTs in localStorage (vulnerable to XSS) — use httpOnly cookies
Not validating the signature
No expiration (exp claim)
Putting sensitive data in the payload (it's only encoded, not encrypted)
Not implementing token revocation

Question 45 Software Security

What is certificate pinning?

XSS is a vulnerability where an attacker injects malicious scripts into web pages viewed by other users. Three types:

Reflected: script in URL is reflected back in the response
Stored: script is permanently stored on the server (e.g., in a database)
DOM-based: script manipulates the page's DOM client-side

Prevention: output encoding, CSP, input validation. The principle of least privilege states that every user, program, and process should have only the minimum permissions necessary to perform its function. Examples:

Database accounts with only required queries
API keys scoped to specific operations
Container processes running as non-root
IAM roles with minimal AWS permissions

Limits the blast radius of a security breach. Certificate pinning associates a host with its expected public key or certificate, rather than trusting any certificate signed by a CA. The app only accepts connections with the pinned certificate.
Benefits: prevents MITM attacks even if a CA is compromised.
Drawbacks: difficult to rotate certificates, can cause outages if pins expire. HPKP was deprecated in favor of Certificate Transparency. OAuth 2.0 is an authorization framework that allows third-party applications to access a user's resources without exposing their credentials. Key grant types:

Authorization Code: for server-side apps (most secure)
PKCE: for SPAs and mobile apps
Client Credentials: for machine-to-machine
Refresh Token: for obtaining new access tokens

Question 46 Software Security

What is bcrypt?

HSTS tells browsers to only use HTTPS for a domain, even if the user types http://. Header: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Benefits:

Prevents SSL stripping attacks
Automatic HTTP to HTTPS redirect at browser level
Preload list: browsers ship with a list of HSTS-enabled domains

bcrypt is a password hashing function based on the Blowfish cipher. Key features:

Built-in salt — prevents rainbow table attacks
Configurable work factor (cost) — can be increased as hardware improves
Deliberately slow — resistant to brute force

Usage: bcrypt.hash(password, saltRounds). Output format: $2b$12$salt...hash... A JWT is a compact, URL-safe token format for securely transmitting claims between parties. Structure: header.payload.signature

Header: algorithm and token type
Payload: claims (user data, expiry)
Signature: verifies integrity using HMAC or RSA

JWTs are stateless — the server doesn't need to store session data. Base64Url encoded, not encrypted by default. Password hashing is a one-way transformation — you can verify a password but cannot recover the original. Unlike encryption (which is reversible), hashing ensures that even if the database is compromised, passwords cannot be recovered.
Use slow hash functions designed for passwords: bcrypt, argon2, scrypt. Never use MD5 or SHA-256 alone — they're too fast.

Question 47 Software Security

What is the difference between authentication and authorization?

The Same-Origin Policy is a browser security mechanism that restricts scripts from one origin from accessing resources of a different origin. Two URLs have the same origin if they share the same protocol, host, and port.
Example: https://example.com:443 differs from http://example.com:80.
CORS provides controlled exceptions to this policy. The SameSite cookie attribute controls when cookies are sent with cross-site requests:

Strict: cookie only sent for same-site requests — strongest CSRF protection
Lax: sent for top-level navigations but not for forms/AJAX — good balance
None: always sent (requires Secure flag)

Lax is the browser default since Chrome 80. Security Misconfiguration (OWASP #5) includes:

Default credentials left unchanged
Unnecessary features enabled (debug mode, directory listing)
Missing security headers
Overly permissive CORS
Verbose error messages exposing stack traces
Unpatched software

Prevention: automated configuration scanning, minimal installs, regular audits, infrastructure as code.

Authentication (AuthN): verifies who you are — login with username/password, biometrics, MFA
Authorization (AuthZ): determines what you can do — permissions, roles, access control

Authentication always comes first. Example: logging in proves your identity (authentication); your role determines which pages you can access (authorization).

Question 48 Software Security

What is CSRF (Cross-Site Request Forgery)?

Command injection occurs when user input is passed to a system shell without sanitization, allowing execution of arbitrary OS commands. Example: passing unsanitized input to os.system().
Prevention:

Avoid calling OS commands from application code
Use language APIs instead of shell commands
If unavoidable, use parameterized APIs like subprocess.run(["ping", host])

SSRF tricks the server into making HTTP requests to unintended locations, often internal services. Example: an image URL parameter targeting cloud metadata endpoints.
Prevention:

Allowlist permitted URLs/domains
Block private IP ranges
Use network segmentation
Validate and sanitize URLs
Disable unnecessary URL schemes

Stored XSS (persistent XSS) occurs when malicious script is saved on the server (e.g., in a database, forum post, or comment) and served to users who view that content. It's more dangerous than reflected XSS because it affects all users who view the page, not just those who click a crafted link. Prevention: sanitize on input, encode on output. CSRF tricks a user's browser into making an unwanted request to a site where they're authenticated. Example: a malicious page submitting a hidden form to a banking site. Prevention:

CSRF tokens — unique tokens per session/request
SameSite cookie attribute
Check Origin/Referer headers
Require re-authentication for sensitive actions

Question 49 Software Security

What is a Man-in-the-Middle (MITM) attack?

Input validation: verify data meets expected format before processing — reject or sanitize bad input
Output encoding: transform data for safe display in a specific context — HTML encoding, URL encoding, JS encoding

Both are needed: validation prevents bad data from entering; encoding prevents stored bad data from causing harm when displayed. Input validation is verifying that user-supplied data meets expected format, type, length, and range requirements before processing. Two approaches:

Allowlist (preferred): accept only known-good patterns
Denylist: reject known-bad patterns (less secure)

Validate on both client-side (UX) and server-side (security). Never trust client-side validation alone. The principle of least privilege states that every user, program, and process should have only the minimum permissions necessary to perform its function. Examples:

Database accounts with only required queries
API keys scoped to specific operations
Container processes running as non-root
IAM roles with minimal AWS permissions

Limits the blast radius of a security breach. A MITM attack occurs when an attacker secretly intercepts and relays communication between two parties who believe they're communicating directly. The attacker can eavesdrop, modify, or inject messages.
Prevention:

Use HTTPS/TLS for all communications
Implement HSTS
Certificate pinning
Verify certificate chains
Use secure Wi-Fi (WPA3)

Question 50 Software Security

What is the token bucket algorithm?

The token bucket algorithm: a bucket holds tokens up to a maximum capacity. Tokens are added at a fixed rate. Each request consumes one token. If the bucket is empty, the request is rejected. Advantages:

Allows bursts up to bucket size
Smooths out request rate over time
Simple to implement

Used by AWS API Gateway, Stripe, and many rate limiters. The principle of least privilege states that every user, program, and process should have only the minimum permissions necessary to perform its function. Examples:

Database accounts with only required queries
API keys scoped to specific operations
Container processes running as non-root
IAM roles with minimal AWS permissions

Limits the blast radius of a security breach. bcrypt is a password hashing function based on the Blowfish cipher. Key features:

Built-in salt — prevents rainbow table attacks
Configurable work factor (cost) — can be increased as hardware improves
Deliberately slow — resistant to brute force

Usage: bcrypt.hash(password, saltRounds). Output format: $2b$12$salt...hash... HTTPS uses TLS (Transport Layer Security) to encrypt communication. The TLS handshake:

Client sends supported cipher suites
Server responds with its certificate and chosen cipher
Client verifies the certificate against trusted CAs
Both parties establish a shared secret using asymmetric encryption
All subsequent data is encrypted with symmetric encryption