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CVE-2026-54763 – Authentication Bypass & Identity Spoofing via Header Injection – Traefik prior t…

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Summary

CVE-2026-54763 is a high-severity authentication bypass and identity spoofing vulnerability affecting Traefik, a widely deployed cloud-native HTTP reverse proxy and load balancer. The flaw resides in Traefik’s BasicAuth, DigestAuth, and ForwardAuth middlewares, which correctly strip canonical-cased identity headers (e.g., X-Forwarded-User) but fail to strip their underscore-variant equivalents (e.g., X_Forwarded_User). Because many backend frameworks — including CGI, WSGI, PHP, and NGINX — normalize both forms identically, attackers can inject spoofed identity context that survives Traefik’s sanitization and reaches backend services; on the unauthenticated ForwardAuth path, this enables complete authentication bypass without any credentials.

Technical details

  • Root cause: Traefik’s authentication middlewares invoke Go’s Header.Del() to remove identity headers prior to writing Traefik’s own values. Go’s textproto.CanonicalMIMEHeaderKey normalizes ASCII case and dash separators but does not treat underscores as equivalent separators. As a result, underscore-variant headers are not matched by the deletion call and pass through to the backend unmodified alongside — or instead of — the value Traefik intended to set.
  • Trigger conditions: The flaw affects four distinct code paths across the BasicAuth, DigestAuth, and ForwardAuth middlewares. The highest-severity path is ForwardAuth’s authResponseHeaders handling, where the route is unauthenticated and exploitation requires no credentials whatsoever.
  • Attack vector: Remotely exploitable over the network; no authentication or user interaction required on the ForwardAuth path. An attacker sends an HTTP request containing an underscore-variant identity header (e.g., X_Forwarded_User: admin) to a Traefik-protected route.
  • Impact: Injection of arbitrary identity or authorization context into backend applications. On the ForwardAuth path, a remote unauthenticated attacker can impersonate any user — including privileged accounts — to downstream services, resulting in high impact to the confidentiality and integrity of systems behind Traefik.

Affected software

  • Traefik v2.x: all versions prior to v2.11.51 (i.e., ≤ v2.11.50)
  • Traefik v3.6.x: all versions prior to v3.6.22 (i.e., ≤ v3.6.21)
  • Traefik v3.7.x: all versions prior to v3.7.6 (i.e., ≤ v3.7.5)

Severity

The official CVE record carries a CVSS v4.0 score of 7.8 (HIGH), vector: 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. The vendor advisory (GHSA-x677-9fxg-v5c5) assigns separate CVSS v3.1 scores per affected code path:

  • ForwardAuth authResponseHeaders path (no credentials required): 9.1 (CRITICAL) — AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
  • BasicAuth / DigestAuth path (low-privilege attacker): 7.5 (HIGH) — AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate — upgrade to the patched release for your active branch:
    • Traefik v2.x → v2.11.51
    • Traefik v3.6.x → v3.6.22
    • Traefik v3.7.x → v3.7.6
  • If immediate patching is not feasible: Enable the allowHeadersWithUnderscores: false entry point option introduced in the patched releases. This configuration causes Traefik to strip any request header whose name contains an underscore before routing, blocking the injection vector at the entry point level.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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