{"cve":"CVE-2026-88879","nvd":{"published":"2026-09-10T14:17:16.337","modified":"2026-10-08T16:17:58.963","status":"Analyzed","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.","cvss_v4":{"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},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N","score":8.2},"cwes":["CWE-290"],"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"],"products":["traefik:traefik"],"source":"nvd","synced_at":"2026-10-19T22:52:53.108888569+00:00"},"epss":{"cve":"CVE-2026-88879","score":0.00295,"percentile":0.219,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-rf44-j88r-hh8c","url":"https://osv.dev/vulnerability/GHSA-rf44-j88r-hh8c","summary":"Traefik: ForwardAuth identity spoofing via dot-form header alias","modified":"2026-09-17T17:40:34.771461848Z","packages":[{"ecosystem":"Go","name":"github.com/traefik/traefik/v2","purl":"pkg:golang/github.com/traefik/traefik/v2","ranges":["<2.11.56"],"fixed":["2.11.56"]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","purl":"pkg:golang/github.com/traefik/traefik/v3","ranges":[">=3.0.0 <3.7.12"],"fixed":["3.7.12"]}]},{"id":"GO-2026-6455","url":"https://osv.dev/vulnerability/GO-2026-6455","summary":"Traefik: ForwardAuth identity spoofing via dot-form header alias in github.com/traefik/traefik","modified":"2026-09-17T17:40:34.771461848Z","packages":[{"ecosystem":"Go","name":"github.com/traefik/traefik","purl":"pkg:golang/github.com/traefik/traefik","ranges":["all versions"],"fixed":[]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v2","purl":"pkg:golang/github.com/traefik/traefik/v2","ranges":["<2.11.56"],"fixed":["2.11.56"]},{"ecosystem":"Go","name":"github.com/traefik/traefik/v3","purl":"pkg:golang/github.com/traefik/traefik/v3","ranges":[">=3.0.0 <3.7.12"],"fixed":["3.7.12"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}