CVE-2026-25748 is an improper authentication vulnerability (CWE-287) discovered in the open-source identity provider authentik, specifically affecting versions from 2025.10.0-rc1 up to but not including 2025.10.4 and 2025.12.4. The vulnerability arises when authentik is configured to use forward authentication in its Proxy Provider component, particularly when deployed behind reverse proxies such as Traefik or Caddy. An attacker can craft a malformed cookie that causes authentik to bypass the normal authentication process. When this malformed cookie is presented, authentik fails to set the expected X-Authentik-* HTTP headers that downstream applications rely on to verify authenticated sessions. As a result, depending on the application's trust model, the attacker may gain unauthorized access without valid credentials. The vulnerability is remotely exploitable without requiring any privileges or user interaction, increasing its risk profile. The CVSS v3.1 base score is 8.6, indicating a high severity due to the network attack vector, low attack complexity, no privileges required, no user interaction, and a complete confidentiality breach with scope change. The flaw does not impact integrity or availability directly but compromises authentication, which is critical for access control. The issue was addressed in authentik versions 2025.10.4 and 2025.12.4 by correcting the handling of malformed cookies and ensuring proper header setting. No public exploits have been reported yet, but the vulnerability's nature and ease of exploitation make it a significant threat to deployments using these versions.

For European organizations, this vulnerability poses a significant risk to the confidentiality of sensitive data and internal resources protected by authentik. Since authentik is an identity provider, unauthorized access could lead to data breaches, lateral movement within networks, and exposure of critical systems. Organizations using Traefik or Caddy as reverse proxies in conjunction with authentik are particularly vulnerable, as the exploit depends on this deployment configuration. The impact is heightened for sectors with stringent data protection requirements such as finance, healthcare, and government agencies across Europe. Compromise could result in regulatory non-compliance with GDPR due to unauthorized data access. Additionally, the breach of authentication mechanisms undermines trust in identity and access management infrastructure, potentially disrupting business operations and leading to reputational damage. The vulnerability's remote exploitability without authentication or user interaction increases the likelihood of automated attacks or scanning by threat actors targeting European enterprises adopting modern cloud-native architectures.

European organizations should immediately upgrade authentik to versions 2025.10.4 or 2025.12.4, which contain the official patches for this vulnerability. Until patching is complete, administrators should audit and harden reverse proxy configurations, particularly for Traefik and Caddy, to ensure that malformed or unexpected cookies are rejected or sanitized before reaching authentik. Implement strict validation of X-Authentik-* headers in downstream applications to verify their presence and integrity, denying access if headers are missing or malformed. Employ network-level controls such as Web Application Firewalls (WAFs) to detect and block suspicious cookie patterns indicative of exploitation attempts. Conduct thorough access reviews and monitor authentication logs for anomalies that could signal exploitation attempts. Additionally, consider deploying multi-factor authentication (MFA) to add an extra layer of security, mitigating the impact of authentication bypass. Finally, maintain an incident response plan tailored to identity provider compromises to rapidly contain and remediate any potential breaches.