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    <lastBuildDate>Mon, 28 Sep 2026 21:28:37 +0000</lastBuildDate>
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      <title>CVE-2026-39858 — Traefik: Forwarded alias spoofing top pre-auth decision bypass</title>
      <link>https://vulnerability.circl.lu/vuln/cve-2026-39858</link>
      <description>&lt;p&gt;&lt;strong&gt;Affected:&lt;/strong&gt; traefik, Red Hat OpenShift Dev Spaces 3.28&lt;/p&gt;
&lt;p&gt;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&amp;#39;s ForwardAuth and snippet-based authentication middleware. Traefik&amp;#39;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.&lt;/p&gt;</description>
      <content:encoded>&lt;p&gt;&lt;strong&gt;Affected:&lt;/strong&gt; traefik, Red Hat OpenShift Dev Spaces 3.28&lt;/p&gt;
&lt;p&gt;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&amp;#39;s ForwardAuth and snippet-based authentication middleware. Traefik&amp;#39;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.&lt;/p&gt;</content:encoded>
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