Web security is the practice of protecting web applications, the users who interact with them, and the data they handle from malicious access, misuse, and disruption. Security failures do more than cause outages — they erode user trust, expose sensitive data, and can create legal liability, financial loss, and long-term operational harm. Treating security as a core feature, not an afterthought, is what separates resilient systems from fragile ones.
Two concepts sit at the center of most access decisions. Authentication answers the question "who is making this request?" by verifying identity, usually at login. Authorization comes after, deciding what an already-authenticated user is allowed to see or do. Conflating these is a common source of bugs. Closely related is the principle of least privilege: every user, service, and process should receive only the minimum access needed for its task. Combined with a secure-by-default and deny-by-default stance, this keeps blast radius small when something does go wrong.
Good web security is also proactive. Threat modeling identifies likely attackers, valuable assets, and entry points before code is written, while security reviews done early in design are far cheaper than retrofitting fixes after assumptions have shipped. A related idea is defense in depth: layering independent controls so that a single failure — a missed input check, a leaked token, a misconfigured header — does not fully compromise the system. Fail-closed behavior, where a security control denies access when it cannot decide, is generally safer than failing open. Practically, this means treating all untrusted input as hostile, keeping dependencies current, designing for least privilege by default, and assuming attackers will probe every exposed boundary.