CVE-2026-39858

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.

Weakness classification

• security-advisories@github.com: CWE-290
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c: CWE-289

Primary references

• security-advisories@github.com — Product, Release Notes
• security-advisories@github.com — Product, Release Notes
• security-advisories@github.com — Product, Release Notes
• security-advisories@github.com — Exploit, Mitigation, Patch, Vendor Advisory
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
• 0b0ca135-0b70-47e7-9f44-1890c2a1c46c