Traefik up to 2.11.42/3.6.13/3.7.0-rc.1 X_Forwarded_Proto authentication spoofing

Summaryinfo

A vulnerability marked as critical has been reported in Traefik up to 2.11.42/3.6.13/3.7.0-rc.1. This vulnerability affects unknown code. This manipulation of the argument X_Forwarded_Proto causes authentication spoofing. This vulnerability is registered as CVE-2026-39858. Remote exploitation of the attack is possible. No exploit is available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in Traefik up to 2.11.42/3.6.13/3.7.0-rc.1. It has been declared as critical. Affected by this vulnerability is an unknown code. The manipulation of the argument X_Forwarded_Proto with an unknown input leads to a authentication spoofing vulnerability. The CWE definition for the vulnerability is CWE-290. This attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-39858 since 04/07/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details are known, but no exploit is available.

Upgrading to version 2.11.43, 3.6.14 or 3.7.0-rc.2 eliminates this vulnerability.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Name

• Traefik

Version

• 2.11.0
• 2.11.1
• 2.11.2
• 2.11.3
• 2.11.4
• 2.11.5
• 2.11.6
• 2.11.7
• 2.11.8
• 2.11.9
• 2.11.10
• 2.11.11
• 2.11.12
• 2.11.13
• 2.11.14
• 2.11.15
• 2.11.16
• 2.11.17
• 2.11.18
• 2.11.19
• 2.11.20
• 2.11.21
• 2.11.22
• 2.11.23
• 2.11.24
• 2.11.25
• 2.11.26
• 2.11.27
• 2.11.28
• 2.11.29
• 2.11.30
• 2.11.31
• 2.11.32
• 2.11.33
• 2.11.34
• 2.11.35
• 2.11.36
• 2.11.37
• 2.11.38
• 2.11.39
• 2.11.40
• 2.11.41
• 2.11.42
• 3.6.0
• 3.6.1
• 3.6.2
• 3.6.3
• 3.6.4
• 3.6.5
• 3.6.6
• 3.6.7
• 3.6.8
• 3.6.9
• 3.6.10
• 3.6.11
• 3.6.12
• 3.6.13
• 3.7.0-rc.0
• 3.7.0-rc.1

Website

• Product: https://github.com/traefik/traefik/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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