GHSA-xpv3-w29h-x7cv

## Summary

A timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation of the Authorization Code Grant flow. The `code_challenge_method_plain` function uses Python's standard `==` operator for string comparison instead of a constant-time comparison function, potentially allowing timing-based attacks.

## Affected Component

- File: `oauthlib/oauth2/rfc6749/grant_types/authorization_code.py` - Functions: `code_challenge_method_plain`, `code_challenge_method_s256` - Vulnerability Type: CWE-208 (Observable Timing Discrepancy)

## Technical Details

Python's `==` operator uses short-circuit evaluation when comparing strings: 1. Returns `False` immediately if lengths differ 2. Compares characters left-to-right, stopping at first mismatch

This means comparison time varies linearly with the length of the common prefix between the attacker-supplied verifier and the stored challenge, creating a measurable timing oracle.

## Proof of Concept

Tested locally against oauthlib source (network jitter eliminated to isolate pure Python execution time):

| Input | Result | Time (10M iterations) | |---|---|---| | Wrong first char (`B` + `A`*49) | Fast reject | 0.34106s | | 49 chars correct (`A`*49 + `B`) | Deep compare | 0.37847s | | **Difference** | | **0.03741s** |

The ~37ms delta over 10M iterations corresponds to nanosecond-level differences per call, which are statistically exploitable under controlled conditions.

## Attack Scenario

1. Attacker intercepts `authorization_code` via Custom URI Scheme Hijacking 2. PKCE blocks token request — attacker lacks `code_verifier` 3. Attacker sends repeated requests to `/token` endpoint measuring response times 4. Using timing oracle, attacker recovers `code_verifier` character by character 5. Attacker obtains Access Token → Account Takeover

> **Note:** Practical exploitability is limited due to the single-use nature of > authorization codes and real-world network noise. However, the vulnerable > pattern should be corrected as a defense-in-depth measure.

## Recommended Fix

Replace `==` with `hmac.compare_digest()` for constant-time comparison:

cr: Elvin Latifli