{"cve":"CVE-2026-39858","nvd":{"published":"2026-04-30T21:16:32.313","modified":"2026-07-15T02:20:53.433","status":"Modified","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.","cvss_v4":{"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","score":7.8},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N","score":10.0},"cwes":["CWE-290","CWE-306","CWE-289"],"references":["https://github.com/traefik/traefik/releases/tag/v2.11.43","https://github.com/traefik/traefik/releases/tag/v3.6.14","https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2","https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm","https://access.redhat.com/errata/RHSA-2026:21772"],"products":["traefik:traefik"],"source":"nvd","synced_at":"2026-10-19T22:33:09.003606135+00:00"},"epss":{"cve":"CVE-2026-39858","score":0.00479,"percentile":0.41388,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-5m6w-wvh7-57vm","url":"https://osv.dev/vulnerability/GHSA-5m6w-wvh7-57vm","summary":"Traefik: Pre-authentication decision bypass due to forwarded alias spoofing","modified":"2026-06-25T18:56:35.695416554Z","packages":[{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","purl":"pkg:golang/github.com/traefik/traefik/v3","ranges":[">=3.7.0-ea.1 <3.7.0-rc.2"],"fixed":["3.7.0-rc.2"]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","purl":"pkg:golang/github.com/traefik/traefik/v3","ranges":[">=3.0.0-beta1 <3.6.14"],"fixed":["3.6.14"]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v2","purl":"pkg:golang/github.com/traefik/traefik/v2","ranges":["<2.11.43"],"fixed":["2.11.43"]},{"ecosystem":"Go","name":"github.com/traefik/traefik","purl":"pkg:golang/github.com/traefik/traefik","ranges":["<=1.7.34"],"fixed":[]}]},{"id":"GO-2026-5149","url":"https://osv.dev/vulnerability/GO-2026-5149","summary":"Traefik: Pre-authentication decision bypass due to forwarded alias spoofing in github.com/traefik/traefik","modified":"2026-06-25T18:45:26.695617307Z","packages":[{"ecosystem":"Go","name":"github.com/traefik/traefik","purl":"pkg:golang/github.com/traefik/traefik","ranges":["all versions"],"fixed":[]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v2","purl":"pkg:golang/github.com/traefik/traefik/v2","ranges":["<2.11.43"],"fixed":["2.11.43"]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","purl":"pkg:golang/github.com/traefik/traefik/v3","ranges":[">=3.0.0-beta1 <3.6.14",">=3.7.0-ea.1 <3.7.0-rc.2"],"fixed":["3.6.14","3.7.0-rc.2"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}