CVE-2026-27840: ZITADEL Auth Bypass Vulnerability

CVE-2026-27840 Overview

CVE-2026-27840 is an authentication bypass vulnerability in ZITADEL, an open source identity management platform. The vulnerability exists in the handling of opaque OIDC access tokens in the v2 format, where tokens truncated to 80 characters are still incorrectly considered valid by the backend system. This flaw stems from differences in how v1 and v2 token formats handle user identification during authorization verification.

Critical Impact

Truncated OIDC access tokens missing user_id values are still accepted as valid, potentially allowing authentication verification to proceed without proper user identity validation.

Affected Products

• ZITADEL versions 2.31.0 through 3.4.6
• ZITADEL versions 4.x prior to 4.11.0
• ZITADEL deployments using opaque OIDC access tokens (v2 format)

Discovery Timeline

• 2026-02-26 - CVE CVE-2026-27840 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-27840

Vulnerability Analysis

ZITADEL utilizes symmetric AES encryption for opaque OIDC tokens. The cleartext payload concatenates multiple identifiers including a token ID and user ID. The platform maintains two token payload versions: v1 tokens with a format of <token_id>:<user_id>, and v2 tokens where the token_id follows the format v2_<oidc_session_id>-at_<access_token_id>.

The vulnerability arises from an inconsistency in how user identity is validated between token versions. For v1 tokens, authorization and authentication session data is retrieved from the database using both the token_id and user_id values, with the user_id (referred to as subject in portions of the codebase) being trusted directly from the token. However, for v2 tokens, session data is retrieved using the oidc_session_id and access_token_id, and the user_id from the token payload is ignored in favor of session data from the database.

When a v2 token is truncated to 80 characters, the user_id portion is removed from the cleartext payload. Despite this, the backend continues to accept the truncated token as valid because it relies solely on session data for v2 token validation rather than verifying the token's embedded user identity.

Root Cause

The root cause is classified under CWE-302 (Authentication Bypass by Assumed-Immutable Data). The vulnerability exists because the v2 token validation logic does not verify that the user_id embedded in the token matches the user identity stored in the session database. This assumption that the token payload would always contain the complete and intact user_id creates a validation gap when tokens are truncated.

Attack Vector

The vulnerability is exploitable over the network and requires user interaction. An attacker would need to intercept or obtain a valid v2 OIDC access token and truncate it to 80 characters. The truncated token, missing the user_id component, would still be accepted by the ZITADEL backend for authorization purposes.

The attack flow involves:

1. Obtaining a valid v2 format OIDC access token from ZITADEL
2. Truncating the token to exactly 80 characters, removing the user_id portion
3. Presenting the truncated token to ZITADEL services for authentication
4. The backend retrieves session data using oidc_session_id and access_token_id without validating the missing user_id

While the vulnerability is confirmed to exist, the ZITADEL security team notes this issue is not considered practically exploitable in most scenarios, though it represents an authentication verification weakness.

Detection Methods for CVE-2026-27840

Indicators of Compromise

• OIDC access tokens with exactly 80 characters being processed by authentication endpoints
• Authentication logs showing successful token validations without corresponding user_id verification
• Unusual patterns of truncated token submissions in identity provider logs

Detection Strategies

• Implement log monitoring for OIDC token validation events, specifically flagging tokens at or below 80 characters
• Enable detailed authentication logging to capture full token metadata during verification processes
• Deploy anomaly detection for authentication patterns that deviate from normal token length distributions
• Monitor ZITADEL audit logs for sessions where token-based user_id and database user_id have discrepancies

Monitoring Recommendations

• Enable comprehensive OIDC authentication logging in ZITADEL deployments
• Configure alerts for authentication attempts using abnormally short access tokens
• Review authentication success logs for sessions that may have bypassed full user_id validation
• Implement token length validation monitoring at network edge or WAF level

How to Mitigate CVE-2026-27840

Immediate Actions Required

• Upgrade ZITADEL to version 3.4.7 or later for 3.x deployments
• Upgrade ZITADEL to version 4.11.0 or later for 4.x deployments
• Review recent authentication logs for potential exploitation attempts
• Invalidate and rotate OIDC sessions created prior to patching

Patch Information

ZITADEL has released security patches that resolve this vulnerability by implementing verification of the user_id from the token against the session data stored in the database. The fix ensures that even if a token is truncated, the backend will detect the missing or mismatched user identity and reject the authentication attempt.

Patched versions are available:

• ZITADEL v3.4.7 for 3.x release branch
• ZITADEL v4.11.0 for 4.x release branch

For complete technical details, refer to the GitHub Security Advisory GHSA-6mq3-xmgp-pjm5.

Workarounds

• No known workarounds are available for this vulnerability according to the vendor advisory
• Organizations unable to immediately patch should implement additional authentication monitoring
• Consider implementing network-level token length validation as a defense-in-depth measure
• Enable detailed audit logging to detect any potential exploitation attempts pending upgrade