{"id":"CVE-2026-39858","aliases":["GHSA-5m6w-wvh7-57vm","GO-2026-5149"],"url":"https://o3.security/vulnerability/CVE-2026-39858","summary":"Traefik: Forwarded alias spoofing top pre-auth decision bypass","details":"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.","published":"2026-04-30T20:26:26.300Z","modified":"2026-08-07T11:49:50.700302609Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","fixedVersion":"3.7.0-rc.2"},{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","fixedVersion":"3.6.14"},{"ecosystem":"Go","name":"github.com/traefik/traefik/v2","fixedVersion":"2.11.43"},{"ecosystem":"Go","name":"github.com/traefik/traefik","fixedVersion":null}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/traefik/traefik/releases/tag/v2.11.43"},{"type":"WEB","url":"https://github.com/traefik/traefik/releases/tag/v3.6.14"},{"type":"WEB","url":"https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2"},{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:21772"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-39858"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39858.json"},{"type":"ADVISORY","url":"https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-39858"},{"type":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2464234"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-07T11:49:50.700302609Z"}}