CVE-2026-27840 in Zitadel
Summary
by MITRE • 02/26/2026
ZITADEL is an open source identity management platform. Starting in version 2.31.0 and prior to versions 3.4.7 and 4.11.0, opaque OIDC access tokens in the 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>`. 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. The back-end still accepts this for above reasons. This issue is not considered exploitable, but may look awkward when reproduced. The patch in versions 4.11.0 and 3.4.7 resolves the issue by verifying the `user_id` from the token against the session data from the database. No known workarounds are available.
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Analysis
by VulDB Data Team • 03/05/2026
The vulnerability described in CVE-2026-27840 affects ZITADEL, an open source identity management platform, where opaque OIDC access tokens in v2 format are susceptible to truncation issues that compromise authentication integrity. This weakness exists in versions starting from 2.31.0 through 3.4.6 and 4.10.9, where tokens exceeding 80 characters are still accepted despite the truncation of critical user identification data. The platform employs symmetric AES encryption for opaque tokens, with cleartext payloads containing concatenated identifiers including token IDs and user IDs. The system maintains two token payload versions internally, with v1 tokens no longer being generated but still validated to prevent session invalidation during upgrades. V1 token format consists of `<token_id>:<user_id>` while v2 tokens utilize `v2_<oidc_session_id>-at_<access_token_id>` structure for token IDs, creating distinct verification mechanisms for each version.
The technical flaw stems from the truncation of v2 tokens to 80 characters, which removes the user ID component from the cleartext payload, while the backend continues to accept these truncated tokens for compatibility reasons. This behavior creates a potential security gap where authentication decisions rely on incomplete token data, as the user ID field becomes inaccessible during validation. In v1 token verification, the system retrieves authorization data using both token_id and user_id values directly from the token, whereas v2 token verification uses oidc_session_id and access_token_id from the token while sourcing the user_id from database session data. The mismatch between token structure and validation logic creates an inconsistency where truncated tokens can bypass certain validation checks, though the vulnerability is not considered exploitable based on current analysis.
The operational impact of this vulnerability manifests primarily in authentication integrity concerns rather than direct exploitation capabilities, as the system continues to accept truncated tokens despite their incomplete data structure. This issue could potentially lead to confusion during token validation processes and may create inconsistencies in session management where user identification becomes unreliable. The vulnerability aligns with CWE-200 (Information Exposure) and CWE-222 (Truncation of Security-relevant Data) categories, as it involves incomplete data handling that could expose system behavior patterns. From an ATT&CK perspective, this vulnerability relates to T1550.001 (Use of Cloud Administration APIs) and T1550.002 (Use of Remote Services) as it affects authentication mechanisms that could impact cloud-based identity management operations. The security implications are mitigated by the patch implemented in versions 3.4.7 and 4.11.0, which enforces verification of user_id from the token against database session data, ensuring complete authentication validation. The fix addresses the root cause by requiring complete token data validation rather than accepting truncated formats, thereby maintaining consistency between token structure and validation requirements.