CVE-2026-39858
CriticalCVSS 10.0Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
A high severity authentication bypass vulnerability was found in Traefik's ForwardAuth and snippet-based authentication middleware. The issue stems from improper sanitization of header aliases using underscores instead of dashes, allowing an attacker to inject spoofed trust context and bypass authentication without valid credentials.
Risk Assessment
An attacker can gain unauthorized access to protected routes, potentially leading to data breaches, integrity violations, and privilege escalation in production environments.
Recommendation
Upgrade Traefik to version 2.11.43, 3.6.14, or 3.7.0-rc.2 immediately. After upgrading, review ForwardAuth and middleware configurations for non-standard headers.
Original NVD description (English source)
Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., X-Forwarded-Proto) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., X_Forwarded_Proto). These unsanitized alias headers are forwarded intact to the authentication backend. When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.