CVE-2026-103001: PyJWT PyJWT.decode() Reintroduces Options-dict Mutation, Enabling Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gvp8-978c-rx2q",
  "modified" : "2026-09-30T23:53:17Z",
  "published" : "2026-09-30T23:53:17Z",
  "aliases" : [ "CVE-2026-103001" ],
  "summary" : "PyJWT.decode() reintroduces options-dict mutation, enabling silent claim-verification bypass on dict reuse",
  "details" : "### Summary\n\n`PyJWT.decode()`/`decode_complete()` mutates a caller-supplied `options` dict in place whenever `verify_signature` is falsy, adding `verify_exp`/`verify_nbf`/`verify_iat`/`verify_aud`/`verify_iss`/`verify_sub`/`verify_jti` keys directly onto that object. If application code reuses the same `options` dict across calls (a config object, a module-level constant, a wrapper's `self.options`), a later call that explicitly sets `verify_signature=True` for a full, normal verification silently inherits the leftover `verify_exp=False` etc. from an earlier `verify_signature=False` call on the same object -- and accepts an expired, wrong-audience, wrong-issuer, or wrong-subject token as valid with no exception. Signature verification itself is unaffected; this is specifically a claims-check bypass.\n\nThis is a regression of an already-fixed, closed issue: originally reported in #679 (2021), fixed in #743 (2022, via a shallow copy of `options`), and silently reintroduced by #1045 (merged 2025-03-05, a typing/logic refactor that dropped the 2022 shallow-copy line). It has been present since #1045 merged, so it likely shipped in several releases already in the wild.\n\n### Details\n\nFile: `jwt/api_jwt.py`, `PyJWT._merge_options()` (called from `decode_complete()`, which backs both `jwt.decode()` and `jwt.decode_complete()`):\n\n```python\ndef _merge_options(self, options=None):\n    if options is None:\n        return self.options\n    if not options.get(\"verify_signature\", True):\n        options[\"verify_exp\"] = options.get(\"verify_exp\", False)\n        options[\"verify_nbf\"] = options.get(\"verify_nbf\", False)\n        # ...5 more, same pattern -- all written directly onto the caller's dict\n    return {**self.options, **options}\n```\n\nNo `dict(options)` copy occurs before these writes, unlike the pre-2025-03-05 code (see #743's fix, `options = dict(options or {})`). Confirmed via `gh api repos/jpadilla/pyjwt/commits/29fbfc36` that #1045's diff removes that line. Confirmed still present on current `main` via a live fetch of `jwt/api_jwt.py`.\n\n### PoC\n\nTwo scripts, each run twice (separate processes), identical output both times (except Python object `id()`), saved locally and available on request:\n\n```python\nimport jwt, time\n\nINSECURE_OPTIONS = {\"verify_signature\": False}\n\nsecret = \"s3cr3t\"\nexpired_token = jwt.encode(\n    {\"exp\": int(time.time()) - 3600, \"data\": \"x\"}, secret, algorithm=\"HS256\"\n)\n\njwt.decode(expired_token, options=INSECURE_OPTIONS)\nprint(INSECURE_OPTIONS)\n# The dict has been silently mutated with 7 new keys, including verify_exp: False.\n\nINSECURE_OPTIONS[\"verify_signature\"] = True\nresult = jwt.decode(expired_token, secret, algorithms=[\"HS256\"], options=INSECURE_OPTIONS)\nprint(result)\n# No exception raised. Expected: jwt.ExpiredSignatureError, since verify_signature=True\n# was explicitly requested on this call.\n```\n\nReproduced deterministically across 2 independent process runs.\n\n### Impact\n\nPreconditions: the calling application must reuse a mutable `options` dict object across two or more `decode()`/`decode_complete()` calls with different intent (e.g. one \"insecure peek\" call followed by a \"fully verified\" call on the same dict, config object, or instance attribute). This is a real, plausible pattern but not universal; it does not affect code that constructs a fresh `options` dict per call.\n\nWhen triggered: a signed-but-expired (or wrong-aud/iss/sub/jti) token is silently accepted as fully verified, with no warning or exception. This is an authentication/claims-verification bypass, not a signature-verification bypass.\n\nScope: regression present since PR #1045 (merged 2025-03-05), affecting every PyJWT release since then up to and including the current 2.13.0.\n\nSuggested fix: restore a shallow copy inside `_merge_options()` itself (rather than relying on a caller-side copy), so every caller of `_merge_options` is covered:\n\n```python\ndef _merge_options(self, options=None):\n    if options is None:\n        return self.options\n    options = dict(options)  # never mutate the caller's dict\n    if not options.get(\"verify_signature\", True):\n        options.setdefault(\"verify_exp\", False)\n        options.setdefault(\"verify_nbf\", False)\n        options.setdefault(\"verify_iat\", False)\n        options.setdefault(\"verify_aud\", False)\n        options.setdefault(\"verify_iss\", False)\n        options.setdefault(\"verify_sub\", False)\n        options.setdefault(\"verify_jti\", False)\n    return {**self.options, **options}\n```\n\nFound via a property-based audit (copy/clone independence across widely-used Python libraries); both repro scripts were written and independently re-run before this report.\n\n## Maintainer update — 2026-09-08\n\nWe reproduced the reported behavior on PyJWT 2.13.0 at source SHA `7144e4534c34810f4525dc4578a32addd8212cff`. A caller-supplied mutable `options` mapping was modified when `verify_signature` was false; reusing that mapping after re-enabling signature verification could leave registered-claim checks disabled. This is a conditional claim-verification bypass, not a signature-forgery issue.\n\nA narrow fix has been prepared: PyJWT now copies the options mapping before applying defaults. Regression coverage includes `decode()`, `decode_complete()`, constructor input preservation, mutable-mapping compatibility, and reuse of an options mapping across verification modes. The focused tests pass, the full suite passes with 369 tests and 4 intentional cryptography-environment skips, and lint/format/type checks pass.\n\nThe advisory remains in triage while the fix goes through release planning. The independently verified range in this investigation is 2.13.0; reports of earlier affected versions should include the exact version and reproduction details.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "PyJWT"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.11.0"
      }, {
        "last_affected" : "2.13.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-gvp8-978c-rx2q"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/commit/0c87c8c8b1a74cac99ad8115f3050efcb7fbed35"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jpadilla/pyjwt"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-471" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T23:53:17Z",
    "nvd_published_at" : null
  }
}