CVE-2026-102269: PyJWT Non-canonical Signature Segments Enable Raw-token Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hxm8-2xgr-2p9m",
  "modified" : "2026-09-29T23:17:55Z",
  "published" : "2026-09-29T23:17:55Z",
  "aliases" : [ "CVE-2026-102269" ],
  "summary" : "PyJWT: Non-canonical signature segments enable raw-token revocation bypass",
  "details" : "## Summary\n\nPyJWT 2.13.0 accepts compact JWS signature segments containing characters that\nare not in the Base64URL alphabet. Appending `!!!!` to a valid signature does\nnot change the decoded signature bytes or authenticated claims, but it changes\nthe serialized token and its SHA-256 hash. An application that indexes logout\nor revocation state by the raw token therefore rejects the logged-out token and\naccepts its no-key alternate serialization.\n\nThis report separates two issues: permissive compact-JWS decoding is the\nlibrary behavior, while using raw serialized JWT text as a security identity is\na dangerous application composition. A `jti`-indexed control blocks both\nrepresentations.\n\n## Affected Version\n\nConfirmed with PyJWT 2.13.0 on CPython 3.13. The mutation changes only the\nsignature segment and requires no signing key.\n\n## End-to-End Reproduction\n\n```bash\n.venv-research/bin/python 06-practical-testing/revocation-bypass-lab.py\n```\n\nThe loopback-only lab performs:\n\n1. Issue a valid HS256 admin token.\n2. Access `/protected` successfully.\n3. Log out the canonical token and store `SHA256(raw_token)`.\n4. Confirm canonical replay returns HTTP 401.\n5. Append `!!!!` to only the signature segment.\n6. Confirm the mutated token has the same verified claims and signature bytes,\n   a different raw hash, and receives HTTP 200.\n7. Repeat with `jti`-indexed revocation and confirm both forms return HTTP 401.\n\nObserved statuses for raw-token revocation were `200 -> logout -> 401` for the\ncanonical token and `200` for the alternate serialization. The `jti` control\nreturned `401` for both replays after logout.\n\n## Security Boundaries and Severity\n\nThis is not signature forgery: the attacker starts with a valid token and\ncannot alter its claims. Impact requires a consumer to bind revocation, replay\nstate, rate limits, or cache identity to the raw compact string. Under that\ncomposition, an authenticated user can bypass logout or token-specific\nrevocation without the signing key. Medium is suggested for triage because the\nsecurity impact is real but application-dependent.\n\n## Recommended Fix\n\n- Reject signature segments containing characters outside `[A-Za-z0-9_-]`.\n- Reject padding and other non-canonical Base64URL encodings for compact JWS.\n- Add regression tests proving malformed signature encodings fail before or\n  during verification.\n- Document that applications must use a semantic identifier such as `jti`, not\n  the raw serialized token, for revocation and replay state.\n\n## Maintainer update — 2026-09-09\n\nWe reproduced the reported behavior on the verified `master` commit `8915570`: appending `!!!!` to a valid compact-JWS signature segment is accepted, produces the same decoded signature bytes and claims, and changes the serialized token hash. Existing JWS verification controls pass. The behavior is present in the tested release range represented by tags `2.4.0`, `2.8.0`, `2.10.1`, `2.12.1`, and `2.13.0`.\n\nThe security impact is conditional on an application using the raw serialized token as the identity for logout, revocation, replay, rate-limit, or cache state. Under that composition, an attacker who already holds a valid token can use an alternate serialization after the canonical string is revoked. This is not signature forgery or claim alteration; applications should use a semantic identifier such as `jti` for token identity.\n\nThe report is ready to be accepted as a draft for continued remediation. A narrow fix assessment recommends rejecting non-canonical Base64URL characters and padding in compact-JWS segments. No fix commit or release is claimed yet.\n\n### Maintainer update — 2026-09-09\n\nThe fix has been rebased onto the current `master` as `e6f48401001609a8f99e71fcaf355fb895d508a8` and is ready to be pushed. Compact-JWS header, payload, and signature segments are now required to use the unpadded Base64URL alphabet and canonical re-encoding; a signature segment containing non-canonical characters such as `!!!!` is rejected before verification. Valid detached `b64=false` handling remains supported.\n\nVerification on the rebased fix commit passed 372 tests with 4 intentional cryptography-environment skips; Ruff formatting/lint and `git diff --check` passed. The compressed-payload regression fixture was updated to valid canonical compact-JWS encoding. No release containing the fix has been published yet, so the patched version remains unset pending release planning.\n\nThe advisory remains medium severity and unpublished.\n\n## Classification update — 2026-09-10\n\nWe completed the advisory classification review. The proposed CVSS 3.1 vector is `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N` and the proposed CWE classification is CWE-180. The issue permits alternate non-canonical serializations of an already valid signed token to bypass applications that identify tokens by their raw serialization; it does not forge signatures or alter claims. CWE-180 captures validation/canonicalization ordering.\n\nThese metadata changes do not alter the affected range, fix status, or lifecycle state.\n\n## Maintainer update — 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "PyJWT"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.14.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.13.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-hxm8-2xgr-2p9m"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-102269"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/commit/e6f48401001609a8f99e71fcaf355fb895d508a8"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/jpadilla/pyjwt"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/jpadilla/pyjwt/releases/tag/2.14.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-180" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-29T23:17:55Z",
    "nvd_published_at" : "2026-09-28T21:17:14Z"
  }
}