CVE-2026-27840

### Summary Opaque OIDC access tokens in v2 format, truncated to 80 characters are still considered valid. ZITADEL 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. The 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` ### Impact V1 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. V2 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. By 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. This issue is not considered exploitable, but may look awkward when reproduced. ### Affected Versions All versions within the following ranges, including release candidates (RCs), are affected: - **v4.x**: `4.0.0` through `4.10.1` - **3.x**: `3.0.0` through `3.4.6` - **2.x**: `2.31.0` through `2.71.19` ### Patches The 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 4.x: Upgrade to >=[4.11.0](https://github.com/zitadel/zitadel/releases/tag/v4.11.0) 3.x: Update to >=[3.4.7](https://github.com/zitadel/zitadel/releases/tag/v3.4.7) 2.x: Update to >=[3.4.7](https://github.com/zitadel/zitadel/releases/tag/v3.4.7) ### Workarounds The recommended solution is to update ZITADEL to a patched version. ### Questions If there any questions or comments about this advisory, please send an email to [security@zitadel.com](mailto:security@zitadel.com) ### Credits ZITADEL thanks Olivier Becker and Lucas Dodgson for reporting this vulnerability.

ZITADEL's truncated opaque tokens are still valid in github.com/zitadel/zitadel. NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions. (If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.) The additional affected modules and versions are: github.com/zitadel/zitadel from v3.0.0 before v3.4.7; github.com/zitadel/zitadel from v4.0.0 before v4.11.0.