OAuth Security Checker: Common Login Flow Weaknesses and How to Test Them

OAuth vulnerabilities usually look fine in happy-path testing. They show up in edge cases — and attackers only need one edge case.

“Sign in with Google” feels secure by default. But OAuth security depends on your implementation details: state validation, redirect URI controls, token handling, and PKCE configuration.

The 4 OAuth Checks You Should Never Skip

1) Redirect URI allowlist

Never accept wildcard or loosely matched callbacks. Exact-match allowlists only.

2) State parameter validation

State must be unique per login attempt and validated server-side to prevent CSRF and account linking abuse.

3) PKCE enforcement

For public clients, require PKCE and reject flows without valid code_verifier.

4) Token storage hygiene

Keep tokens out of logs, URLs, and browser storage where possible. Prefer short lifetimes and rotation.

High-risk pattern: accepting any redirect URI under a domain prefix. Subdomain takeovers turn this into instant token theft.

Quick Test Sequence

Trigger a normal login flow and capture requests
Replay with modified redirect_uri and confirm rejection
Replay callback with missing/changed state
Test refresh token behavior and revocation path

Starter run command:

python3 edgeiq-oauth-checker.py --target "https://example.com/oauth/login" --deep

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