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.

References

Reference URLs as listed by NVD, grouped by a mechanical match on the link's host/pattern. Labels describe the link type only.

• Patch https://github.com/traefik/traefik/releases/tag/v2.11.43
• Patch https://github.com/traefik/traefik/releases/tag/v3.6.14
• Patch https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2
• Vendor advisory https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm
• Distro https://access.redhat.com/errata/RHSA-2026:21772
• Distro https://access.redhat.com/security/cve/CVE-2026-39858
• Distro https://bugzilla.redhat.com/show_bug.cgi?id=2464234
• Distro https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json