GHSA-W2CX-738M-MC7W

Vulnerability from github – Published: 2026-09-29 23:16 – Updated: 2026-09-29 23:16
VLAI
Summary
PyJWT accepts public JWK containers as HMAC secrets
Details

Summary

PyJWT 2.13.0 contains an incomplete defense against algorithm confusion when an application mixes symmetric and asymmetric algorithms in one verification path. A public RSA, EC, or OKP JWK can be accepted as an HMAC secret when it is wrapped in a JWKS object, nested in an array, or represented in another container form that does not expose a top-level kty member.

Impact

An attacker who knows the public key material can forge HS256/HS384/HS512 tokens if the application simultaneously:

  • allows both HS* and asymmetric algorithms;
  • passes raw public JWK/JWKS JSON as key=; and
  • uses that same value as the HMAC secret.

This can allow forged JWT claims in affected application configurations. The issue does not affect applications that keep symmetric and asymmetric verification paths separate and follow PyJWT's algorithm-selection guidance.

Fix status

The fix is on master in commit 801cd12 (fix: reject public JWK container HMAC keys). HMACAlgorithm.prepare_key now rejects public JWK members found in objects, arrays, nested containers, BOM/UTF variants, and recursion-limit inputs. It also recognizes escaped JSON member names without treating ordinary string values as JWKs. Ordinary JSON secrets remain accepted byte-for-byte.

The change was tested with focused regression tests and the full local tox matrix. Available Python 3.9, 3.12, and 3.13 crypto/no-crypto suites, mypy, package metadata, and coverage passed; unavailable interpreters were skipped by the project configuration. A fresh independent Astra/max security review accepted the final diff with no blocking findings.

The affected range is = 2.13.0. The fix is on the unreleased development branch; the patched version will be recorded when a released 2.x version containing the fix is available. This advisory is being moved to draft pending that release.

Reporter credit

Credit: Charles Vosburgh / Trilobyte.

Original report

The original report and reproduction package are retained in the private advisory record.

Maintainer update — 2026-09-11

The verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.

Severity
7.4 (High)
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "PyJWT"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.13.0"
            },
            {
              "fixed": "2.14.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.13.0"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-102273"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-347"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-29T23:16:55Z",
    "nvd_published_at": "2026-09-28T21:17:15Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nPyJWT 2.13.0 contains an incomplete defense against algorithm confusion when\nan application mixes symmetric and asymmetric algorithms in one verification\npath. A public RSA, EC, or OKP JWK can be accepted as an HMAC secret when it\nis wrapped in a JWKS object, nested in an array, or represented in another\ncontainer form that does not expose a top-level `kty` member.\n\n### Impact\n\nAn attacker who knows the public key material can forge HS256/HS384/HS512\ntokens if the application simultaneously:\n\n* allows both HS* and asymmetric algorithms;\n* passes raw public JWK/JWKS JSON as `key=`; and\n* uses that same value as the HMAC secret.\n\nThis can allow forged JWT claims in affected application configurations. The\nissue does not affect applications that keep symmetric and asymmetric\nverification paths separate and follow PyJWT\u0027s algorithm-selection guidance.\n\n### Fix status\n\nThe fix is on `master` in commit `801cd12` (`fix: reject public JWK container\nHMAC keys`). `HMACAlgorithm.prepare_key` now rejects public JWK members found in\nobjects, arrays, nested containers, BOM/UTF variants, and recursion-limit\ninputs. It also recognizes escaped JSON member names without treating ordinary\nstring values as JWKs. Ordinary JSON secrets remain accepted byte-for-byte.\n\nThe change was tested with focused regression tests and the full local tox\nmatrix. Available Python 3.9, 3.12, and 3.13 crypto/no-crypto suites, mypy,\npackage metadata, and coverage passed; unavailable interpreters were skipped\nby the project configuration. A fresh independent Astra/max security review\naccepted the final diff with no blocking findings.\n\nThe affected range is `= 2.13.0`. The fix is on the unreleased development\nbranch; the patched version will be recorded when a released 2.x version\ncontaining the fix is available. This advisory is being moved to draft pending\nthat release.\n\n### Reporter credit\n\nCredit: Charles Vosburgh / Trilobyte.\n\n### Original report\n\nThe original report and reproduction package are retained in the private\nadvisory record.\n\n## Maintainer update \u2014 2026-09-11\n\nThe verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.",
  "id": "GHSA-w2cx-738m-mc7w",
  "modified": "2026-09-29T23:16:55Z",
  "published": "2026-09-29T23:16:55Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-w2cx-738m-mc7w"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-102273"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/commit/801cd128528c62d9b23fcd161d1a2e1c17982f95"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jpadilla/pyjwt"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/releases/tag/2.14.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "PyJWT accepts public JWK containers as HMAC secrets"
}



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