CVE-2026-88879
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 4.0
- 5.3
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 - CVSS 3.1
- 8.2
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N - EPSS
- 0.3% chance of exploitation in the next 30 days; higher than 21% of CVEs (scores of 2026-10-25)
- CWE
- CWE-290
- Published
- Modified
- Record from
- the NVD, synced
Affected
- traefik:traefik
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-rf44-j88r-hh8c | Go | github.com/traefik/traefik/v2 | <2.11.56 | 2.11.56 |
| GHSA-rf44-j88r-hh8c | Go | github.com/traefik/traefik/v3 | >=3.0.0 <3.7.12 | 3.7.12 |
| GO-2026-6455 | Go | github.com/traefik/traefik | all versions | no fix published |
| GO-2026-6455 | Go | github.com/traefik/traefik/v2 | <2.11.56 | 2.11.56 |
| GO-2026-6455 | Go | github.com/traefik/traefik/v3 | >=3.0.0 <3.7.12 | 3.7.12 |