{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "extracted_events": [
              {
                "introduced": "0"
              },
              {
                "fixed": "2.11.56"
              },
              {
                "introduced": "3.0.0"
              },
              {
                "last_affected": "3.7.13"
              }
            ],
            "source": "AFFECTED_FIELD"
          },
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "d87731461f0c7beb6fdbcc5a5655a44ce56e537a"
            },
            {
              "introduced": "d99d2f95e66a28ee8180634dc80fdf13ba68c62a"
            },
            {
              "last_affected": "fc92cc118a0557a029c7019d5ee06665127b0f13"
            }
          ],
          "repo": "https://github.com/traefik/traefik",
          "type": "GIT"
        }
      ]
    }
  ],
  "aliases": [
    "GHSA-rf44-j88r-hh8c"
  ],
  "database_specific": {
    "cna_assigner": "VulnCheck",
    "cwe_ids": [
      "CWE-290"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/88xxx/CVE-2026-88879.json"
  },
  "details": "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.",
  "id": "CVE-2026-88879",
  "modified": "2026-09-12T03:30:57.136236484Z",
  "published": "2026-09-10T13:05:30.872Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/88xxx/CVE-2026-88879.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-88879"
    },
    {
      "type": "ADVISORY",
      "url": "https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias"
    }
  ],
  "schema_version": "1.9.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Traefik before v2.11.56 Identity Spoofing via Header Alias"
}