{"id":"CVE-2026-27840","aliases":["GHSA-6mq3-xmgp-pjm5","GO-2026-4573"],"url":"https://o3.security/vulnerability/CVE-2026-27840","summary":"ZITADEL's truncated opaque tokens are still valid","details":"### Summary\n\nOpaque OIDC access tokens in v2 format, truncated to 80 characters are still considered valid. \n\nZITADEL uses a symmetric AES encryption for opaque tokens. The cleartext payload is a concatenation of a couple of identifiers, such as a token ID and user ID. Internally Zitadel has 2 different versions of token payloads. v1 tokens are no longer created, but are still verified as to not invalidate existing session after upgrade.\n\nThe cleartext payload has a format of `<token_id>:<user_id>`. v2 tokens distinguished further where the `token_id` is of the format `v2_<oidc_session_id>-at_<access_token_id>`. This is an example of such a cleartext: `V2_354201447279099906-at_354201447279165442:354201364702363650`\n\n### Impact\n\nV1 token authZ/N session data is retrieved from the database using the (simple) `token_id` value and `user_id` value. The `user_id` (called `subject` in some parts of our code) was used as being the trusted user ID.\n\nV2 token authZ/N session data is retrieved from the database using the `oidc_session_id` and `access_token_id` and in this case the `user_id` from the token is ignored and taken from the session data in the database.\n\nBy truncating the token to 80 chars, the user_id is now missing from the cleartext of the v2 token: `V2_354201447279099906-at_354201447279165442:`  The back-end still accepts this for above reasons.\n\nThis issue is not considered exploitable, but may look awkward when reproduced.\n\n### Affected Versions\n\nAll versions within the following ranges, including release candidates (RCs), are affected:\n- **v4.x**: `4.0.0` through `4.10.1`\n- **3.x**: `3.0.0` through `3.4.6`\n- **2.x**: `2.31.0` through `2.71.19`\n\n### Patches\n\nThe vulnerability has been addressed in the latest releases. The patch resolves the issue by verifying the `user_id` from the token against the session data from the database\n\n4.x: Upgrade to >=[4.11.0](https://github.com/zitadel/zitadel/releases/tag/v4.11.0)\n3.x: Update to >=[3.4.7](https://github.com/zitadel/zitadel/releases/tag/v3.4.7)\n2.x: Update to >=[3.4.7](https://github.com/zitadel/zitadel/releases/tag/v3.4.7)\n\n### Workarounds\n\nThe recommended solution is to update ZITADEL to a patched version.\n\n### Questions\n\nIf there any questions or comments about this advisory, please send an email to [security@zitadel.com](mailto:security@zitadel.com)\n\n### Credits\n\nZITADEL thanks Olivier Becker and Lucas Dodgson for reporting this vulnerability.","published":"2026-02-26T00:27:08.933Z","modified":"2026-08-12T03:51:38.896704630Z","cvss":{"score":4.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N"},"epss":{"score":0.00142,"percentile":0.03974,"asOf":"2026-08-23"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/zitadel/zitadel","fixedVersion":"4.11.0"},{"ecosystem":"Go","name":"github.com/zitadel/zitadel","fixedVersion":"3.4.7"},{"ecosystem":"Go","name":"github.com/zitadel/zitadel","fixedVersion":null},{"ecosystem":"Go","name":"github.com/zitadel/zitadel","fixedVersion":"1.80.0-v2.20.0.20260216092519-feab8e1fa371"}],"fix":{"url":"https://github.com/zitadel/zitadel/commit/feab8e1fa371f3ad654640fc869b2c14f2fdb602","label":"zitadel/zitadel@feab8e1"},"references":[{"type":"WEB","url":"https://github.com/zitadel/zitadel/releases/tag/v3.4.7"},{"type":"WEB","url":"https://github.com/zitadel/zitadel/releases/tag/v4.11.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27840.json"},{"type":"ADVISORY","url":"https://github.com/zitadel/zitadel/security/advisories/GHSA-6mq3-xmgp-pjm5"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27840"},{"type":"WEB","url":"https://github.com/zitadel/zitadel/commit/feab8e1fa371f3ad654640fc869b2c14f2fdb602"},{"type":"PACKAGE","url":"https://github.com/zitadel/zitadel"},{"type":"WEB","url":"https://github.com/zitadel/zitadel/releases/tag/v2.71.19"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:38.896704630Z"}}