{"id":"CVE-2026-88879","revision":"e29838ce65674e2ab19fc6b34dbcfa8d","records":[{"revision":"e29838ce65674e2ab19fc6b34dbcfa8d","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-88879","aliases":[],"title":"Traefik before v2.11.56 Identity Spoofing via Header Alias","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":"2026-09-10T13:05:30.872+00:00","modifiedAt":"2026-10-08T15:25:59.454+00:00","affected":[{"ecosystem":"Vendor product","name":"traefik","ranges":[],"versions":[],"rawVersionEvidence":"{\n                    \"vendor\": \"traefik\",\n                    \"product\": \"traefik\",\n                    \"defaultStatus\": \"unaffected\",\n                    \"packageURL\": \"pkg:golang/github.com/traefik/traefik/v2\",\n                    \"versions\": [\n                        {\n                            \"version\": \"0\",\n                            \"status\": \"affected\",\n                            \"versionType\": \"semver\",\n                            \"lessThan\": \"2.11.56\"\n                        },\n                        {\n                            \"version\": \"2.11.56\",\n                            \"status\": \"unaffected\",\n                            \"versionType\": \"semver\"\n                        }\n                    ]\n                }"},{"ecosystem":"Vendor product","name":"traefik","ranges":[],"versions":[],"rawVersionEvidence":"{\n                    \"vendor\": \"traefik\",\n                    \"product\": \"traefik\",\n                    \"defaultStatus\": \"unaffected\",\n                    \"packageURL\": \"pkg:golang/github.com/traefik/traefik/v3\",\n                    \"versions\": [\n                        {\n                            \"version\": \"3.0.0\",\n                            \"status\": \"affected\",\n                            \"versionType\": \"semver\",\n                            \"lessThanOrEqual\": \"3.7.13\"\n                        }\n                    ]\n                }"}],"metrics":[{"system":"CVSS 4.0","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","score":5.3,"severity":"MEDIUM"}],"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://www.cve.org/CVERecord?id=CVE-2026-88879","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-88879","identifiers":["CVE-2026-88879"]},"retained":false,"historicalAffected":[]}],"coverage":[]}