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

Quick assessment

Affected
traefik traefik
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Traefik是Traefik开源的一款反向代理与负载均衡工具。 Traefik存在安全漏洞，该漏洞源于转发标头清理逻辑仅针对规范标头名称，未处理使用下划线替代破折号的别名变体，可能导致身份验证绕过。以下版本受到影响：2.11.43之前版本、3.6.14之前版本和3.7.0-rc.2之前版本。

AI Predicted 9.1 Difficulty: Easy EPSS 0.60% · P47

Possible ATT&CK Techniques 1 AI

T1556.001 · Domain Controller Authentication

Affected Version Matrix 3

VendorProduct Version RangeStatus
traefik traefik < 2.11.43 affected
>= 3.0.0-beta1, < 3.6.14 affected
>= 3.7.0-ea.1, < 3.7.0-rc.2 affected
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I. Basic Information for CVE-2026-39858

Vulnerability Information
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Vulnerability Title
Traefik: Forwarded alias spoofing top pre-auth decision bypass
Source: CVE Program / CVE List V5
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
使用欺骗进行的认证绕过
Source: CVE Program / CVE List V5
Vulnerability Title
Traefik 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Traefik是Traefik开源的一款反向代理与负载均衡工具。 Traefik存在安全漏洞，该漏洞源于转发标头清理逻辑仅针对规范标头名称，未处理使用下划线替代破折号的别名变体，可能导致身份验证绕过。以下版本受到影响：2.11.43之前版本、3.6.14之前版本和3.7.0-rc.2之前版本。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
traefik traefik < 2.11.43 -

II. Public POCs for CVE-2026-39858

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III. Intelligence Information for CVE-2026-39858

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Vendor Advisories for CVE-2026-39858 (1)

Vendor Pages for CVE-2026-39858 (3)

Same Patch Batch · traefik · 2026-04-30 · 5 CVEs total

CVE-2026-41263 Traefik: BasicAuth middleware: timing side-channel vulnerability
CVE-2026-41174 Traefik Kubernetes CRD allows unauthorized cross-namespace middleware binding
CVE-2026-35051 Traefik: ForwardAuth trustForwardHeader=false allows spoofed X-Forwarded-Prefix to bypass
CVE-2026-40912 Traefik: StripPrefixRegex auth bypass via Path/RawPath desync

IV. Related Vulnerabilities

Same product: traefik
Same vendor: traefik
Same weakness: CWE-290

V. Comments for CVE-2026-39858

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