HTTP request smuggling via host-part path injection bypasses path-based authentication in Starlette (CVE-2026-48710)
CISA added this flaw to KEV on 2026-09-02 with a 2026-09-16 remediation deadline, and any Starlette or FastAPI service whose authentication decisions depend on the reconstructed URL path remains open to unauthenticated bypass until upgraded.
Starlette versions before 1.0.1 allow an attacker to inject path components into the host portion of a request so that they are prepended to the actual path when the URL is reconstructed. Where authentication or authorization logic keys off that reconstructed path, an unauthenticated attacker can reach protected routes; the issue can be chained with CVE-2026-42271.
Affected: Kludex starlette < 1.0.1; Red Hat AI Inference Server images prior to 3.3.5 (CUDA and ROCm)
How to Test
Run `pip show starlette` (or inspect your lockfile / container SBOM) in every Python service and FastAPI application; any version below 1.0.1 is vulnerable. Identify middleware or dependencies that make authentication decisions from the reconstructed request URL path. Review access logs for requests carrying path-like content in the Host header or unusual host/path combinations hitting authenticated routes.
How to Patch
Upgrade starlette to 1.0.1 or later and redeploy affected services; for Red Hat AI Inference Server, move to the 3.3.5 images published in RHSA-2026:30089 (CUDA) and RHSA-2026:30088 (ROCm). Until upgraded, enforce authentication at a reverse proxy or gateway that does not depend on the application's reconstructed URL, and reject requests with malformed Host headers at the edge.