{"id":"CVE-2026-88879","revision":"14c12da8ef4d492bb37e229d32f34681","records":[{"revision":"14c12da8ef4d492bb37e229d32f34681","observedAt":"2026-10-25T16:19:22.228+00:00","statement":{"source":"nvd","id":"CVE-2026-88879","aliases":[],"title":"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 distinct…","description":"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 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.","state":0,"publishedAt":"2026-09-10T14:17:16.337+00:00","modifiedAt":"2026-10-08T16:17:58.963+00:00","affected":[],"metrics":[{"system":"CVSS 4.0 · disclosure@vulncheck.com · Secondary","vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","score":5.3,"severity":"MEDIUM"},{"system":"CVSS 3.1 · nvd@nist.gov · Primary","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N","score":8.2,"severity":"HIGH"}],"references":["https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c","https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias"],"sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-88879","license":"NIST NVD usage terms; underlying source attribution retained","enrichment":{"sourceStatus":"Analyzed","weaknesses":["CWE-290"],"productCriteria":["cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:*"],"sourceDates":{"published":"2026-09-10T14:17:16.337","lastModified":"2026-10-08T16:17:58.963"},"relatedAdvisories":[],"note":"NVD analysis and CPE criteria are source context, not proof that a package or your environment is affected. Complete affected data, logical configurations and SSVC are retained in the original JSON."},"key":"nvd:CVE-2026-88879","identifiers":["CVE-2026-88879"]},"retained":false,"historicalAffected":[]}],"coverage":[]}