Junglewise Threat Intelligence

CVE-2026-33433: Traefik identity spoofing via non-canonical headerField in Auth middleware

CVE-2026-33433 · Severity: high · CVSS 8.8 · Published 2026-03-27

Technologies: Traefik Labs Traefik Proxy, github.com/traefik/traefik/v3 (Go), github.com/traefik/traefik/v2 (Go), github.com/traefik/traefik (Go). Vendors: Traefik Labs, Red Hat, Go.

Executive brief

Traefik is a popular open-source tool used to route and manage web traffic to different backend services. A security flaw exists where an attacker with valid, low-level login credentials can trick the system into identifying them as a different, potentially more powerful user (such as an administrator). This occurs when the system is configured to pass user identity information using specific header names that do not follow standard capitalization rules, allowing an attacker to bypass security checks and gain unauthorized access to sensitive data or administrative functions.

Technical details

A vulnerability exists in Traefik's BasicAuth and DigestAuth middlewares due to improper handling of HTTP header canonicalization. When the 'headerField' is configured with a non-canonical name (e.g., 'x-auth-user' instead of 'X-Auth-User'), Traefik performs a direct map assignment in Go's http.Header map rather than using the Set() method. Because Go's http.Header map is keyed by canonical names, this creates a duplicate entry. An authenticated attacker can provide a canonical version of the header in their request, which Traefik fails to overwrite; most backend frameworks will then prioritize the attacker-controlled canonical header over the one set by Traefik. This allows for identity spoofing and privilege escalation. The issue is patched in versions 2.11.42, 3.6.11 (and 3.6.12), and 3.7.0-ea.3.

Affected products

  • Traefik Labs Traefik < 2.11.42, >= 3.0.0-beta1 < 3.6.11, >= 3.7.0-ea.1 < 3.7.0-ea.3
  • Red Hat Red Hat OpenShift Dev Spaces 3.27 3.27

Timeline

  • 2026-03-19: patched: Version 3.6.11 released
  • 2026-03-26: patched: Versions 2.11.42 and 3.7.0-ea.3 released
  • 2026-03-27: advisory: Initial disclosure of CVE-2026-33433

References

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