oauth-implementation

OAuth/OIDC Implementation Security (OWASP A07:2025)

What this checks

Prevents authentication bypasses from weak JWT validation, open redirects from loose redirect_uri matching, and CSRF from missing state parameters. These flaws allow account takeover and session hijacking.

Vulnerable patterns

• JWT decode call that accepts the none algorithm, or accepts an attacker-supplied algorithm header — enabling algorithm-confusion bypass.
• redirect_uri validated by prefix match, substring match, or regex — letting an attacker host a lookalike domain whose URL passes the check.
• JWT decode that omits audience (aud) validation — a token minted for a sibling service passes signature verification but should not be honored here.
• Tokens stored in browser web storage (e.g. localStorage) where any same-origin script can read them.
• OAuth authorization request sent with no state parameter, or with a state that is not bound to and verified against the initiating session.

Fix immediately

Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties. Translate each property into the audited file's language and OAuth/JWT library — use the library's documented secure-by-default APIs rather than mirroring an example from another stack.

1. JWT verification pins algorithms to an explicit allowlist that never includes none and never permits switching between symmetric and asymmetric families (the classic HS256/RS256 public-key-as-HMAC attack). For broader JWT handling guidance, see the authentication-failures skill.
2. Redirect URIs match exactly against an allowlist set. Prefix matching, substring matching, and regex matching all have known bypass classes — percent-encoded tricks, userinfo smuggling, lookalike subdomains. The only safe comparison is exact string equality against a set of registered URIs.
3. Every JWT decode validates aud (audience) and required claims. A token minted for a different service of the same issuer will pass signature verification; only the audience check catches the confused-deputy case. Require exp, iat, sub at minimum.
4. State parameter ties the callback to the initiating session. Generate a CSPRNG state, store it in the session before redirect, compare it on the callback. Without this the OAuth flow is CSRF-forgeable.
5. Tokens live in a Secure, HttpOnly, SameSite cookie or in memory — not web storage. Browser storage is readable by any same-origin script, so one XSS compromise exfiltrates every token.