Vulnerability 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.
CVSS Score
CRITICAL
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Traefik | Traefik | < 2.11.43 |
Related Weaknesses (CWE)
References
- https://github.com/traefik/traefik/releases/tag/v2.11.43ProductRelease Notes
- https://github.com/traefik/traefik/releases/tag/v3.6.14ProductRelease Notes
- https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2ProductRelease Notes
- https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vmExploitMitigationPatch
FAQ
What is CVE-2026-39858?
CVE-2026-39858 is a vulnerability with a CVSS score of 10.0 (CRITICAL). 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...
How severe is CVE-2026-39858?
CVE-2026-39858 has been rated CRITICAL with a CVSS base score of 10.0/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2026-39858?
Check the references section above for vendor advisories and patch information. Affected products include: Traefik Traefik.