OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison

{
  "id": "4595100c99f294f75cea12d65669173aae9b8c1edcaacf74f44f7698ac9de211",
  "url": "https://osv.dev/vulnerability/CVE-2026-49265",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "OAuthLib: Timing Attack Vulnerability in PKCE code_verifier Comparison",
    "summary": "OAuthLib is an implementation of the OAuth request-signing logic for Python 3.6+. From 3.0.0 until 4.0.0, OAuthLib's PKCE checks in oauthlib/oauth2/rfc6749/grant_types/authorization_code.py compare attacker-supplied code_verifier values with stored challenges using ordinary string equality in code_challenge_method_plain and code_challenge_method_s256. Because the comparison can return after a length difference or the first mismatching character, repeated token endpoint measurements can reveal common-prefix timing information that could potentially allow recovery of the code_verifier, unauthorized access-token acquisition, and account takeover. Practical exploitation requires an intercepted, single-use authorization code and enough measurements to overcome real-world network noise; the local timing benchmark does not establish practical remote account takeover, so the weakness remains a defense-in-depth concern. This issue is fixed in version 4.0.0.",
    "evidence": "osv:CVE-2026-49265",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-49265",
      "aliases": [
        "CVE-2026-49265",
        "GHSA-xpv3-w29h-x7cv",
        "PYSEC-2026-4114"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/oauthlib/oauthlib",
              "type": "GIT",
              "events": [
                {
                  "introduced": "9dfa40c3bdab3aa788e711283afdf36c2fbdd1de"
                },
                {
                  "fixed": "145a9a4690cb4d9de30d15fcc2984e34c49df741"
                },
                {
                  "fixed": "74664d3fe037a354e180e305135c6bab1747a6b0"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "3.0.0"
                      },
                      {
                        "fixed": "4.0.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "74664d3fe037a354e180e305135c6bab1747a6b0"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "145a9a4690cb4d9de30d15fcc2984e34c49df741"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v4.0.0"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v3.3.0",
            "v3.1.0",
            "v3.0.1",
            "v3.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-49265.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:27.851305240Z",
      "provider": "osv",
      "references": [
        "https://github.com/oauthlib/oauthlib/releases/tag/v4.0.0",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49265.json",
        "https://github.com/oauthlib/oauthlib/security/advisories/GHSA-xpv3-w29h-x7cv",
        "https://github.com/pypa/advisory-database/tree/main/vulns/oauthlib/PYSEC-2026-4114.yaml",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-49265",
        "https://github.com/oauthlib/oauthlib/commit/74664d3fe037a354e180e305135c6bab1747a6b0",
        "https://github.com/oauthlib/oauthlib/pull/963"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-208"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49265.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-49265",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 494493,
  "published_at": "2026-10-13T15:47:11.854+00:00"
}