Authentication Bypass by Spoofing in Traefik - CVE-2026-39858
Published: May 5, 2026
Vulnerability identifier: #VU129676
CSH Severity: High
CVSS v4: 8.8 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-39858
CWE-ID: CWE-290
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to bypass authentication and access protected endpoints.
The vulnerability exists due to authentication bypass by spoofing in ForwardAuth and snippet-based authentication middleware when forwarding client-supplied alias forwarded headers to the authentication backend. A remote attacker can send a specially crafted request with spoofed forwarded-header aliases to bypass authentication and access protected endpoints.
Exploitation requires an authentication backend that normalizes underscore and dash header forms equivalently.
Affected software
Traefik
How to mitigate CVE-2026-39858
Install security update from vendor's website.
Traefik - addressed in versions 2.11.43, 3.6.14, 3.7.0 rc.2