authentik's XML Signature Wrapping in SAML Source ACS allows authentication as arbitrary federated user
Date: 2026-05
Affected: go/goauthentik.io
Attack vector: auth-bypass
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-47201
CWE: CWE-20
CWE-287
CWE-347
CVSS: 8.5 (CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H)

### Summary authentik's SAML Source ACS endpoint is vulnerable to XML Signature Wrapping when validating upstream SAML responses. An attacker with any account at the upstream IdP can reuse a valid signed assertion to authenticate as another federated user. ### Patches authentik 2026.5.1, 2026.2.4 and 2025.12.6 fix this issue. ### Impact Affected: authentik deployments using a SAML Source for upstream SAML federation with signed assertions, or signed responses without signed assertions. Not affected: deployments that do not use SAML Source for upstream SAML federation. The SAML Source trusts that the verified XML signature belongs to the assertion or response that authentik later consumes. A crafted SAML response can make signature verification succeed against the attacker's original signed assertion while authentik reads identity data from a different forged assertion. An attacker first completes a legitimate login to the upstream IdP and captures the signed SAML response sent through their browser. They then submit a modified response to the ACS endpoint where the valid signature still verifies, but the consumed assertion contains a victim identifier or attacker-chosen attributes. The attacker can authenticate as a victim who has previously used the SAML Source, or as a local user matched by forged email or username when those matching modes are enabled. ### Workarounds Disable affected SAML Sources, or block access to their ACS endpoints. ##


Mitigations:

Architectural layers (MAESTRO):