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Vulnerabilities / CVE-2026-39858

critical

CVE-2026-39858

traefik: Traefik: Authentication bypass via unsanitized alias headers

Published Modified CWE 290, 306, 289
In the Trivy database since at least 24 Oct 2026, 07:11 UTCWhen it arrived before that build is not recorded.

Severity by source

NVD10.0CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
GHSA7.8CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
Red Hat8.2CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

Trivy's own label is CRITICAL. Rated critical by NVD. Rated lower by GHSA (high), Red Hat (high).

Description

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's ForwardAuth and snippet-based authentication middleware. Traefik'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.

Affected packages and fixed versions

SourcePackageFixed in
go::GitHub Security Advisory Gogithub.com/traefik/traefik, github.com/traefik/traefik/v2, github.com/traefik/traefik/v32.11.43 and 1 more
minimostraefik-3, traefik-fips-33.6.14-r0
rapidfort alpine 3.20traefik3.7.0-r0
rapidfort alpine 3.21traefik3.7.0-r0
rapidfort alpine 3.22traefik3.7.0-r0
rapidfort alpine 3.23traefik3.7.0-r0
Show all 9 entries from 6 sources
SourcePackageFixed in
go::GitHub Security Advisory Gogithub.com/traefik/traefikno fix listed
go::GitHub Security Advisory Gogithub.com/traefik/traefik/v22.11.43
go::GitHub Security Advisory Gogithub.com/traefik/traefik/v33.7.0-rc.2, 3.6.14
minimostraefik-33.6.14-r0
minimostraefik-fips-33.6.14-r0
rapidfort alpine 3.20traefik3.7.0-r0
rapidfort alpine 3.21traefik3.7.0-r0
rapidfort alpine 3.22traefik3.7.0-r0
rapidfort alpine 3.23traefik3.7.0-r0

References

All 11 references

Vulnerability data compiled from the Trivy vulnerability database (trivy-db, Aqua Security, Apache License 2.0), which aggregates third-party advisories. CVE records are published by The MITRE Corporation under the CVE Terms of Use; CVE is a trademark of MITRE. This product uses data from the NVD API but is not endorsed or certified by the NVD. Severity, titles and advisory data from the GitHub Advisory Database (CC BY 4.0), Red Hat security data (CC BY 4.0) and the other sources linked on each entry. Entries may be summarised or re-formatted; the linked original is the authoritative record.