{"id":"CVE-2026-88879","title":"Traefik is an HTTP reverse proxy and load balancer","summary":"Traefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three disti…","severity":"high","cvss":8.2,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N","cwe":["CWE-290","CWE-444"],"vendor":"traefik","product":"traefik","affected":["traefik >= 1.0, < 2.11.56","traefik >= 3.0.0, < 3.7.12"],"patched":["traefik 3.7.12"],"published":"2026-09-10","updated":"2026-09-14","sourceUpdated":"2026-09-14T19:58:33.037","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-88879","references":[{"url":"https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c","label":"disclosure@vulncheck.com"},{"url":"https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias","label":"disclosure@vulncheck.com"},{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-88879.json"},{"url":"https://access.redhat.com/security/cve/CVE-2026-88879"},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2531425"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-88879"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-88879"}],"tags":["nvd","cve.org","csaf","vex","red-hat","score-dispute"],"epss":0.00207,"epssPercentile":0.11065,"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-09-10T15:03:25.506535Z"},"scores":{"nvd":8.2,"cna":5.3,"vendor":9.6},"ingestedAt":"2026-09-10T13:50:37.334Z","slug":"CVE-2026-88879","body":"## Overview\n\nTraefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three distinct headers by Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX and others) collapse them into a single variable. A client can therefore smuggle a dot-form alias of a header that Traefik manages past the middleware managing it — for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware — causing such a backend to read the client-supplied value instead of the identity Traefik asserted. In the tested configuration (PHP 8.2 built-in SAPI over an HTTP/1 backend path), Go's lexical header ordering makes the attacker-supplied value win deterministically, so a client that ForwardAuth admits as a low-privilege identity can be treated by the backend as a different user or role. Any header Traefik sets is affected, not only ForwardAuth's. This is an incomplete fix for GHSA-x677-9fxg-v5c5, which blocked only the underscore form. Fixed in v2.11.56 and v3.7.12, which add the aliasHeadersStrategy entry-point option; because it defaults to 'keep' for backwards compatibility, it must be explicitly set to 'delete' or 'reject' for the fix to take effect. Unmaintained release lines will not receive a patch.\n\n## Affected\n\n- `traefik >= 1.0, < 2.11.56`\n- `traefik >= 3.0.0, < 3.7.12`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `traefik 3.7.12`\n\n## Vendor advisories\n\n- **Red Hat VEX** · Critical · affected: Red Hat OpenShift Dev Spaces · no fix planned: Red Hat OpenShift Dev Spaces · updated 2026-09-14 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-88879.json)","depth":"twilight","depthScore":45,"depthScoreParts":{"impact":45.1,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[{"seq":203221,"id":"CVE-2026-88879","ts":1789416896091,"field":"cvss","old":"5.3","new":"8.2"},{"seq":203220,"id":"CVE-2026-88879","ts":1789416896091,"field":"severity","old":"medium","new":"high"}]}