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

Severity: critical (CVSS 10.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-04-30 · updated 2026-07-15
Weakness: CWE-290

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

Traefik: before 2.11.43; from 3.0.0 before 3.6.14; 3.7.0-ea1, 3.7.0-ea2, 3.7.0-ea3, 3.7.0-rc1 — fixed in 2.11.43, 3.6.14

How to fix

Upgrade to a fixed version: Traefik 2.11.43, 3.6.14. Check the vendor advisory for mitigations.

References

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