CVE-2026-27840 is a medium-severity (CVSS 4.3) CWE-302 vulnerability in github.com/zitadel/zitadel. A fix is available for github.com/zitadel/zitadel — see the affected versions and patch details below.
ZITADEL's truncated opaque tokens are still valid
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-27840 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/zitadel/zitadel🐹github.com/zitadel/zitadel🐹github.com/zitadel/zitadel🐹github.com/zitadel/zitadelReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
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:
4.0.0through4.10.1 - 3.x:
3.0.0through3.4.6 - 2.x:
2.31.0through2.71.19
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 3.x: Update to >=3.4.7 2.x: Update to >=3.4.7
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 [email protected]
Credits
ZITADEL thanks Olivier Becker and Lucas Dodgson for reporting this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/zitadel/zitadel | ≥ 4.0.0&&< 4.11.0 | 4.11.0go get github.com/zitadel/zitadel@v4.11.0 |
| 🐹Go | github.com/zitadel/zitadel | ≥ 3.0.0&&< 3.4.7 | 3.4.7go get github.com/zitadel/zitadel@v3.4.7 |
| 🐹Go | github.com/zitadel/zitadel | ≥ 2.31.0 | No fix |
| 🐹Go | github.com/zitadel/zitadel | all versions | 1.80.0-v2.20.0.20260216092519-feab8e1fa371go get github.com/zitadel/zitadel@v1.80.0-v2.20.0.20260216092519-feab8e1fa371 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/zitadel/zitadel, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/zitadel/zitadel to 4.11.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27840 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27840 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27840. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-27840 in your dependencies?
O3 Security finds CVE-2026-27840 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.