Traefik: Forwarded alias spoofing top pre-auth decision bypass

Summary

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.

Timeline

1. Apr 30, 2026, 8:26 PM UTCCVE published
2. May 4, 2026, 4:57 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Jul 15, 2026, 1:01 AM UTCRecord last updated

Affected versions

• traefik traefik

  fixed in 2.11.43; From 3.0.0-beta1 · fixed in 3.6.14; From 3.7.0-ea.1 · fixed in 3.7.0-rc.2

• github.com/traefik/traefik/v3Go

  From 3.7.0-ea.1 · fixed in 3.7.0-rc.2

• github.com/traefik/traefik/v3Go

  From 3.0.0-beta1 · fixed in 3.6.14

• github.com/traefik/traefik/v2Go

  fixed in 2.11.43

• github.com/traefik/traefikGo

  through 1.7.34

• github.com/traefik/traefikGo

  All versions

• github.com/traefik/traefik/v3Go

  From 3.0.0-beta1 · fixed in 3.6.14; From 3.7.0-ea.1 · fixed in 3.7.0-rc.2

References 4