CVE-2026-88008
Description
A flaw was found in Traefik, an open source HTTP reverse proxy and load balancer. A remote unauthenticated attacker can exploit this vulnerability by sending a specially crafted HTTP/2 request that includes specific upgrade headers. If the backend server accepts the upgrade, Traefik enters a raw tunnel, bypassing its security features such as authentication and access controls for subsequent HTTP/2 requests. This allows the attacker to gain unauthorized access to protected resources on the backend.
Statement
Traefik incorrectly handles client-supplied HTTP/2 upgrade headers (Upgrade: h2c, Connection, HTTP2-Settings) by forwarding them to shared backends. When a backend accepts the upgrade with a 101 status code, Traefik transitions into a raw TCP tunnel, bypassing all gateway security controls including routers, authentication (BasicAuth, ForwardAuth), IPAllowList, rate limits, and audit logging for all subsequent HTTP/2 requests sent through the establish connection.
Mitigation
Disable h2c (HTTP/2 over cleartext) upgrades on downstream backend services, or configure Traefik gateway entrypoints to strip 'Upgrade: h2c' and 'HTTP2-Settings' headers from incoming client requests before routing to internal backends.
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 | 9.1 | 9.1 | N/A |
| Attack Vector | Network | Network | N/A |
| Attack Complexity | Low | Low | N/A |
| Privileges Required | None | None | N/A |
| User Interaction | None | None | N/A |
| Scope | Unchanged | Unchanged | N/A |
| Confidentiality | High | High | 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:N/UI:N/S:U/C:H/I:H/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
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