# OAuth2 authorization code injection via missing PKCE

- **ID:** `security/oauth2-authorization-code-injection-via-missing-pkce`
- **Domain:** security
- **Category:** auth_error
- **Error Code:** `OAUTH2_MISSING_PKCE`
- **Verification:** ai_generated
- **Fix Rate:** 85%

## Root Cause

OAuth2 authorization code flow without PKCE (Proof Key for Code Exchange) allows an attacker to intercept the authorization code and exchange it for tokens, bypassing the intended client.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| OAuth2.0 | active | — | — |
| Spring Security 5.7+ | active | — | — |
| Okta 2023.04.0 | active | — | — |
| Auth0 2023.01 | active | — | — |

## Workarounds

1. **Implement PKCE in the authorization request by generating a code_verifier (random string) and its SHA-256 hash as code_challenge, then send code_challenge_method=S256. Example in JavaScript: const codeVerifier = crypto.randomBytes(32).toString('base64url'); const codeChallenge = crypto.createHash('sha256').update(codeVerifier).digest('base64url'); Then include code_challenge and code_challenge_method in the authorization request, and send code_verifier in the token request.** (90% success)
   ```
   Implement PKCE in the authorization request by generating a code_verifier (random string) and its SHA-256 hash as code_challenge, then send code_challenge_method=S256. Example in JavaScript: const codeVerifier = crypto.randomBytes(32).toString('base64url'); const codeChallenge = crypto.createHash('sha256').update(codeVerifier).digest('base64url'); Then include code_challenge and code_challenge_method in the authorization request, and send code_verifier in the token request.
   ```
2. **Enable PKCE enforcement on the authorization server (e.g., in Auth0 tenant settings under 'Advanced OAuth' > 'OIDC Conformant').** (85% success)
   ```
   Enable PKCE enforcement on the authorization server (e.g., in Auth0 tenant settings under 'Advanced OAuth' > 'OIDC Conformant').
   ```
3. **Upgrade to OAuth2 libraries that enforce PKCE by default (e.g., Spring Security 5.7+ with oauth2Login).** (80% success)
   ```
   Upgrade to OAuth2 libraries that enforce PKCE by default (e.g., Spring Security 5.7+ with oauth2Login).
   ```

## Dead Ends

- **** — Enabling HTTPS only protects the transport layer; the authorization code can still be intercepted via malicious redirects or browser extensions, as PKCE is a separate mechanism. (60% fail)
- **** — Using state parameter alone is insufficient because if the attacker can predict or steal the state, they can still inject the code; PKCE uses a cryptographically random code verifier that cannot be guessed. (70% fail)
- **** — Shortening the authorization code lifetime reduces the window for attack but does not prevent injection; PKCE is the only standard mitigation. (50% fail)