CVE-2026-41577: authentik SAML Auth Bypass Vulnerability

CVE-2026-41577 Overview

CVE-2026-41577 affects authentik, an open-source identity provider, in its SAML source response processor. The ResponseProcessor.parse() function fails to validate the Conditions element on SAML assertions. Specifically, the NotBefore, NotOnOrAfter, and AudienceRestriction constraints are ignored during processing. This allows attackers to replay expired SAML assertions and accept assertions intended for other service providers. The flaw is categorized as insufficient verification of data authenticity [CWE-345]. Versions prior to 2025.12.5 and 2026.2.3 are affected, and patches are available in those releases.

Critical Impact

Attackers can replay expired SAML assertions or reuse assertions issued for different service providers to gain unauthorized access through authentik-protected identity flows.

Affected Products

• authentik versions prior to 2025.12.5
• authentik versions prior to 2026.2.3
• Deployments using SAML source authentication

Discovery Timeline

• 2026-06-02 - CVE-2026-41577 published to NVD
• 2026-06-02 - Last updated in NVD database

Technical Details for CVE-2026-41577

Vulnerability Analysis

The vulnerability resides in authentik's SAML source response processor. When a SAML Identity Provider returns a signed assertion, the ResponseProcessor.parse() method processes the response but skips validation of the Conditions XML element. The Security Assertion Markup Language (SAML) specification requires service providers to enforce three Conditions sub-elements: NotBefore, NotOnOrAfter, and AudienceRestriction. Ignoring these turns time-bound, audience-scoped assertions into reusable credentials.

An attacker who obtains a previously valid SAML assertion can submit it to authentik after expiration. The processor accepts the assertion as long as the signature is valid. The same flaw permits cross-audience replay: an assertion issued for service provider A can be presented to authentik acting as service provider B.

Root Cause

The root cause is missing input validation on the SAML Conditions element during assertion parsing. The processor evaluates signature and structural integrity but does not enforce temporal and audience-binding constraints required by the SAML 2.0 core specification.

Attack Vector

Exploitation requires network access to the authentik SAML endpoint and possession of a SAML assertion signed by a trusted Identity Provider. No authentication or user interaction is needed at the time of replay. An attacker who captures or extracts a SAML assertion through log exposure, browser history, network interception, or assertions issued for a different relying party can submit it to authentik to authenticate as the subject.

No verified proof-of-concept code is available. See the GitHub Security Advisory for technical details.

Detection Methods for CVE-2026-41577

Indicators of Compromise

• SAML assertions processed by authentik containing NotOnOrAfter timestamps in the past relative to processing time
• Successful authentications where the assertion AudienceRestriction value does not match the authentik service provider entity ID
• Repeated authentication events using assertions with identical AssertionID values
• Unusual login activity from accounts that have not recently interacted with the upstream Identity Provider

Detection Strategies

• Parse authentik audit logs for SAML source authentications and correlate assertion timestamps against acceptance time
• Alert on any SAML assertion accepted by authentik where the Audience value does not match the configured service provider identifier
• Monitor for reuse of SAML AssertionID values across multiple authentication events
• Compare upstream Identity Provider issuance logs against authentik consumption logs to detect divergence

Monitoring Recommendations

• Enable verbose logging on the authentik SAML source provider and forward events to a centralized SIEM
• Track authentication patterns per identity to baseline normal Identity Provider session frequency
• Inspect captured SAML responses at the network or proxy layer for stale Conditions values
• Review session creation events tied to SAML sources for anomalies in geographic or device context

How to Mitigate CVE-2026-41577

Immediate Actions Required

• Upgrade authentik to version 2025.12.5 or 2026.2.3 without delay
• Inventory all authentik SAML source configurations and confirm the upstream Identity Provider trust chain
• Rotate or invalidate active SAML sessions established before the patch is applied
• Audit authentication logs for the past 90 days to identify potential replay activity

Patch Information

The authentik maintainers fixed the vulnerability in versions 2025.12.5 and 2026.2.3. The patches add validation logic in ResponseProcessor.parse() that enforces the NotBefore, NotOnOrAfter, and AudienceRestriction elements per the SAML 2.0 specification. Refer to the GitHub Security Advisory GHSA-4v4x-x5pr-8gp2 for upgrade instructions.

Workarounds

• If immediate patching is not possible, disable SAML source authentication and route users through alternative authentication flows
• Restrict network access to the authentik SAML assertion consumer endpoint to known Identity Provider egress addresses
• Reduce the trust window by configuring upstream Identity Providers to issue short-lived assertions and tightly scoped audiences
• Require an additional authentication factor on the authentik side for sessions established through SAML sources

# Upgrade authentik via Docker Compose
docker compose pull
docker compose up -d

# Verify the running version
docker compose exec server ak version