GHSA-XPV3-W29H-X7CV

Vulnerability from github – Published: 2026-09-29 17:56 – Updated: 2026-10-13 15:47
VLAI
Summary
Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)
Details

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

Severity
6.8 (Medium)
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "oauthlib"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0"
            },
            {
              "fixed": "4.0.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-49265"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-208"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-29T17:56:31Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nA timing side-channel vulnerability exists in the PKCE (RFC 7636) implementation \nof the Authorization Code Grant flow. The `code_challenge_method_plain` function \nuses Python\u0027s standard `==` operator for string comparison instead of a \nconstant-time comparison function, potentially allowing timing-based attacks.\n\n## Affected Component\n\n- File: `oauthlib/oauth2/rfc6749/grant_types/authorization_code.py`\n- Functions: `code_challenge_method_plain`, `code_challenge_method_s256`\n- Vulnerability Type: CWE-208 (Observable Timing Discrepancy)\n\n## Technical Details\n\nPython\u0027s `==` operator uses short-circuit evaluation when comparing strings:\n1. Returns `False` immediately if lengths differ\n2. Compares characters left-to-right, stopping at first mismatch\n\nThis means comparison time varies linearly with the length of the common prefix \nbetween the attacker-supplied verifier and the stored challenge, creating a \nmeasurable timing oracle.\n\n## Proof of Concept\n\nTested locally against oauthlib source (network jitter eliminated to isolate \npure Python execution time):\n\n| Input | Result | Time (10M iterations) |\n|---|---|---|\n| Wrong first char (`B` + `A`*49) | Fast reject | 0.34106s |\n| 49 chars correct (`A`*49 + `B`) | Deep compare | 0.37847s |\n| **Difference** | | **0.03741s** |\n\nThe ~37ms delta over 10M iterations corresponds to nanosecond-level differences \nper call, which are statistically exploitable under controlled conditions.\n\n## Attack Scenario\n\n1. Attacker intercepts `authorization_code` via Custom URI Scheme Hijacking\n2. PKCE blocks token request \u2014 attacker lacks `code_verifier`\n3. Attacker sends repeated requests to `/token` endpoint measuring response times\n4. Using timing oracle, attacker recovers `code_verifier` character by character\n5. Attacker obtains Access Token \u2192 Account Takeover\n\n\u003e **Note:** Practical exploitability is limited due to the single-use nature of \n\u003e authorization codes and real-world network noise. However, the vulnerable \n\u003e pattern should be corrected as a defense-in-depth measure.\n\n## Recommended Fix\n\nReplace `==` with `hmac.compare_digest()` for constant-time comparison:\n\ncr: Elvin Latifli",
  "id": "GHSA-xpv3-w29h-x7cv",
  "modified": "2026-10-13T15:47:41Z",
  "published": "2026-09-29T17:56:31Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-49265"
    },
    {
      "type": "WEB",
      "url": "https://github.com/oauthlib/oauthlib/pull/963"
    },
    {
      "type": "WEB",
      "url": "https://github.com/oauthlib/oauthlib/commit/40b0ab56da3682c2484a4b78bbff309f8025d950"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/oauthlib/oauthlib"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-4114.yaml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)"
}



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