CVE-2026-39858: Traefik: Forwarded alias spoofing top pre-auth decision bypass

Description

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.

Affected products

Version statuses are supplied by the assigning organization. Consult the original record for detailed ranges and platform conditions.

traefik traefik

< 2.11.43affected
>= 3.0.0-beta1, < 3.6.14affected
>= 3.7.0-ea.1, < 3.7.0-rc.2affected

CVE-2026-39858

Traefik: Forwarded alias spoofing top pre-auth decision bypass

Description

Affected products

traefik traefik

References