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
Affected Versions
All versions within the following ranges, including release candidates (RCs), are affected:
- v4.x:
4.0.0through4.10.1 - 3.x:
3.0.0through3.4.6 - 2.x:
2.31.0through2.71.19
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 [email protected]
Credits
ZITADEL thanks Olivier Becker and Lucas Dodgson for reporting this vulnerability.
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.
CVE-2026-27840 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.11.0, 3.4.7, 1.80.0-v2.20.0.20260216092519-feab8e1fa371); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
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
3.x: Update to >=3.4.7
2.x: Update to >=3.4.7
Frequently Asked Questions
- What is CVE-2026-27840? CVE-2026-27840 is a medium-severity security vulnerability in github.com/zitadel/zitadel (go), affecting versions >= 4.0.0, < 4.11.0. It is fixed in 4.11.0, 3.4.7, 1.80.0-v2.20.0.20260216092519-feab8e1fa371.
- How severe is CVE-2026-27840? CVE-2026-27840 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/zitadel/zitadel are affected by CVE-2026-27840? github.com/zitadel/zitadel (go) versions >= 4.0.0, < 4.11.0 is affected.
- Is there a fix for CVE-2026-27840? Yes. CVE-2026-27840 is fixed in 4.11.0, 3.4.7, 1.80.0-v2.20.0.20260216092519-feab8e1fa371. Upgrade to this version or later.
- Is CVE-2026-27840 exploitable, and should I be worried? Whether CVE-2026-27840 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-27840 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-27840?
- Upgrade
github.com/zitadel/zitadelto 4.11.0 or later - Upgrade
github.com/zitadel/zitadelto 3.4.7 or later - Upgrade
github.com/zitadel/zitadelto 1.80.0-v2.20.0.20260216092519-feab8e1fa371 or later
- Upgrade