CVE-2026-27840 ZITADEL's truncated opaque tokens are still valid

What is CVE-2026-27840?

CVE-2026-27840 is a medium-severity authentication bypass vulnerability in zitadel, published on 26 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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.

Weakness: CWE-302

Which versions are affected by CVE-2026-27840?

10 open source packages ship an affected version, including github.com/zitadel/zitadel. It affects 1 product.

Products include Zitadel Zitadel.

How do I fix CVE-2026-27840?

Upgrade github.com/zitadel/zitadel to a fixed version for your release line: 1.80.0-v2.20.0.20260216092519-feab8e1fa371, 3.4.7 or 4.11.0.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-27840 being exploited?

There is no known exploitation of CVE-2026-27840 in the wild and no public exploit code.

Other vulnerabilities in zitadel

• CVE-2026-29191 ZITADEL: 1-Click Account Takeover via XSS in /saml-post Endpoint Critical 9.3
• CVE-2025-67494 ZITADEL Vulnerable to Unauthenticated Full-Read SSRF via V2 Login Critical 9.3
• CVE-2025-27507 IDOR Vulnerabilities in ZITADEL's Admin API that Primarily Impact LDAP Configurations Critical 9.0
• CVE-2025-64103 Zitadel Bypass Second Authentication Factor High 8.7
• CVE-2025-31123 Zitadel Expired JWT Keys Usable for Authorization Grants High 8.7

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-6mq3-xmgp-pjm5, GO-2026-4573.