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

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.

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

Quick answers

Is CVE-2026-33433 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-33433 being exploited in the wild.
How severe is CVE-2026-33433?
High severity, with a CVSS score of 8.8, published 27 March 2026.
Which products and versions are affected?
github.com/traefik/traefik (Go) SEMVER:introduced=0; github.com/traefik/traefik/v2 (Go) < 2.11.42, SEMVER:introduced=0,fixed=2.11.42; github.com/traefik/traefik/v3 (Go) >= 3.0.0-beta1, < 3.6.11, SEMVER:introduced=3.0.0-beta1,fixed=3.6.12, SEMVER:introduced=3.0.0-beta1,fixed=3.6.12,introduced=3.7.0-ea.1,fixed=3.7.0-ea.3; Traefik Labs Traefik Proxy < 2.11.42, >= 3.0.0-beta1 < 3.6.11, >= 3.7.0-ea.1 < 3.7.0-ea.3.
Is there a fix for CVE-2026-33433?
Yes. github.com/traefik/traefik/v2 (Go): upgrade to 2.11.42 or later; github.com/traefik/traefik/v3 (Go): upgrade to 3.6.12, 3.7.0-ea.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-33433 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-33433?
Check whether you run github.com/traefik/traefik (Go), github.com/traefik/traefik/v2 (Go), github.com/traefik/traefik/v3 (Go) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • github.com/traefik/traefik/v2 (Go): upgrade to 2.11.42 or later
    go get github.com/traefik/traefik/v2@v2.11.42
  • github.com/traefik/traefik/v3 (Go): upgrade to 3.6.12, 3.7.0-ea.3 or later
    go get github.com/traefik/traefik/v3@v3.7.0-ea.3

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

Identifiers

  • CVE-2026-33433
  • GHSA-qr99-7898-vr7c
  • GO-2026-4893

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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