CVE-2026-27840
Zitadel 2.31.0 – 2.71.19
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:NSummary
CVE-2026-27840 is a medium-severity Authentication Bypass by Assumed-Immutable Data (CWE-302) vulnerability in Zitadel Zitadel. Its CVSS base score is 4.3 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to IA-2 (Identification and Authentication (Organizational Users)) and AC-3 (Access Enforcement) — see the control section below for these in your framework.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-8789
Vulnerability Data
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…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V7.2.4
Mitigating Controls (NIST 800-53 r5) AI
Mandates proper unique identification and authentication of users, precluding reliance on attacker-controlled immutable assumptions.
Requires server-side enforcement of authorizations instead of trusting client-supplied mutable data for authentication decisions.
Mandates proper identification and authentication for non-organizational users, precluding reliance on attacker-controlled immutable assumptions.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Strong authentication mechanisms directly avoid reliance on attacker-controlled immutable data.
Protecting and verifying identity assertions prevents tampering with data assumed immutable during auth.
Least-privilege authorization policies reduce impact of bypassed authentication but do not address the root flaw.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can discover and block authentication bypasses that rely on mutable data.
Access-control policy can mandate validation of all identity data, reducing reliance on assumed-immutable fields.
Identity-management processes can require verification of mutable attributes, mitigating the root cause.
Proper management of authentication information prevents use of client-controlled tokens or cookies as sole proof of identity.
Access-rights reviews can detect and revoke rights granted via tampered immutable data.
Secure SDLC practices include threat modeling and input-validation requirements that catch assumed-immutable data flaws.