{"id":"EUVD-2026-75429","enisaUuid":"79dec7f3-0449-3f32-9e3b-dc6e10e69fb8","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\u0027s 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\u0027s. 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 \u0027keep\u0027 for backwards compatibility, it must be explicitly set to \u0027delete\u0027 or \u0027reject\u0027 for the fix to take effect. Unmaintained release lines will not receive a patch.","datePublished":"Sep 10, 2026, 1:05:30 PM","dateUpdated":"Oct 8, 2026, 3:25:59 PM","baseScore":5.3,"baseScoreVersion":"4.0","baseScoreVector":"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","references":"https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c\nhttps://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias\n","aliases":"CVE-2026-88879\nGHSA-cwr6-3wm2-9xhw\n","assigner":"VulnCheck","epss":0.3,"enisaIdProduct":[{"id":"a5f90b66-c81b-30b1-b37c-9dbbf7f36857","product":{"name":"traefik","vendor":{"name":"traefik"}},"product_version":"3.0.0 ≤3.7.13"},{"id":"c635f715-b188-3e6b-99e2-0dd1fa62c0ab","product":{"name":"traefik","vendor":{"name":"traefik"}},"product_version":"0 \u003c2.11.56"}],"enisaIdVendor":[{"id":"0f081ffd-a83f-3c0b-b171-5cf6fff7b44e","vendor":{"name":"traefik"}}],"enisaIdVulnerability":[{"id":"11b779e7-2c93-316a-aa61-5d27429e7d44","vulnerability":{"id":"CVE-2026-88879","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\u0027s 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\u0027s. 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 \u0027keep\u0027 for backwards compatibility, it must be explicitly set to \u0027delete\u0027 or \u0027reject\u0027 for the fix to take effect. Unmaintained release lines will not receive a patch.","datePublished":"Sep 10, 2026, 1:05:30 PM","dateUpdated":"Oct 8, 2026, 3:25:59 PM","status":"PUBLISHED","baseScore":5.3,"baseScoreVersion":"4.0","baseScoreVector":"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","references":"https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c\nhttps://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias\n","enisa_id":"EUVD-2026-75429\n","assigner":"VulnCheck","epss":0.3,"dataProcessed":"Oct 30, 2026, 7:14:26 AM","vulnerabilityProduct":[{"id":"01c9f455-1b7d-3d41-8914-5b3f51c29107","product":{"name":"traefik","vendor":{"name":"traefik"}},"product_version":"0 \u003c2.11.56"},{"id":"0cc09530-dc3b-3700-b01a-40d8486201e0","product":{"name":"traefik","vendor":{"name":"traefik"}},"product_version":"3.0.0 ≤3.7.13"}],"vulnerabilityVendor":[{"id":"36f1ee79-48e4-3ea8-a25b-b095b1e866ac","vendor":{"name":"traefik"}}]}},{"id":"85a09261-c870-3c70-b603-13566c0d4bd6","vulnerability":{"id":"GHSA-cwr6-3wm2-9xhw","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\u0027s 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\u0027s. 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 \u0027keep\u0027 for backwards compatibility, it must be explicitly set to \u0027delete\u0027 or \u0027reject\u0027 for the fix to take effect. Unmaintained release lines will not receive a patch.","datePublished":"Sep 10, 2026, 3:33:18 PM","dateUpdated":"Sep 14, 2026, 9:31:25 PM","baseScore":0.0,"references":"https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-88879\nhttps://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias\n","enisa_id":"EUVD-2026-75429\n","aliases":"GHSA-cwr6-3wm2-9xhw\nCVE-2026-88879\n","epss":0.0,"dataProcessed":"Oct 25, 2026, 2:01:07 AM","vulnerabilityProduct":[],"vulnerabilityVendor":[]}}],"enisaIdAdvisory":[]}