CVE-2026-88879
Description
A flaw was found in Traefik, an HTTP reverse proxy and load balancer. Due to a discrepancy in how Traefik and certain backend applications process HTTP header names, a remote attacker can smuggle specially crafted headers. This allows the attacker to bypass identity assertions made by Traefik, leading to identity spoofing and potentially gaining unauthorized access or elevated privileges within the backend application.
Statement
A flaw was found in Traefik's header canonicalization logic, where header names are canonicalized only on dashes. A remote authenticated client can supply dot-form header aliases (e.g., X.Auth.User alongside X-Auth-User) to bypass Traefik middleware assertions. When proxied to backends that collapse non-alphanumeric characters into singular variables (e.g., CGI, WSGI, PHP, NGINX), the backend resolves the attacker-controlled header value over Traefik's asserted identity. This leads to privilege escalation or identity spoofing within the downstream application context.
Mitigation
Upgrade to fixed Traefik versions (v2.11.56, v3.7.12, or later) and explicitly configure the entrypoint option aliasHeadersStrategy to 'delete' or 'reject'. Alternatively, configure downstream backend web servers or WSGI/CGI gateways to strip or reject headers containing non-canonical characters before processing.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 9.6 | 8.2 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | Low | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Changed | Unchanged | N/A |
| Confidentiality | High | Low | N/A |
| Integrity Impact | High | High | N/A |
| Availability Impact | None | None | N/A |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N
Understanding the Weakness (CWE)
Integrity,Non-Repudiation,Access Control
Technical Impact: Unexpected State; Hide Activities; Bypass Protection Mechanism
An attacker could create HTTP messages to exploit a number of weaknesses including 1) the message can trick the web server to associate a URL with another URL's webpage and caching the contents of the webpage (web cache poisoning attack), 2) the message can be structured to bypass the firewall protection mechanisms and gain unauthorized access to a web application, and 3) the message can invoke a script or a page that returns client credentials (similar to a Cross Site Scripting attack).
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