BadHost CVE-2026-48710: a single-character HTTP Host header injection bypasses Starlette/FastAPI authentication, hitting millions of MCP servers
X41 D-Sec found CVE-2026-48710 (BadHost): injecting a single character into the HTTP Host header of requests sent to an MCP server or AI agent harness built on Starlette makes the authentication middleware evaluate the wrong path from request.url.path, letting unauthorized access through.
Why it matters: This post breaks down how the BadHost vulnerability works, its blast radius, and the fixed versions, and explains what an authorization gate does and does not cover.