CVE-2026-48710

Starlette has missing Host header validation that poisons request.url.path, bypassing path-based security checks

### Summary In affected versions, the HTTP `Host` request header was not validated before being used to reconstruct `request.url`. Because the routing algorithm relies on the raw HTTP path while `request.url` is rebuilt from the `Host` header, a malformed header could make `request.url.path` differ from the path that was actually requested. Middleware and endpoints that apply security restrictions based on `request.url` (rather than the raw `scope` path) could therefore be bypassed. ### Details When a client requests `http://example.com/foo`, it sends: ```http GET /foo HTTP/1.1 Host: example.com ``` Affected versions reconstructed the URL by concatenating `http://{host}{path}` and re-parsing the result. The `Host` value is only valid as a `uri-host [ ":" port ]` per [RFC 9112 §3.2](https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2-6), where `uri-host` follows the restricted `host` grammar of [RFC 3986 §3.2.2](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.2.2). When it contains characters outside that grammar - notably `/`, `?`, or `#` - those characters move the path/query/fragment boundaries during re-parsing, so the parsed `request.url.path` no longer matches the path the server actually received. For example: ```http GET /foo HTTP/1.1 Host: example.com/abc?bar= ``` reconstructs to `http://example.com/abc?bar=/foo`, whose parsed `path` is `/abc` - even though routing used the real path `/foo`. The router still dispatches to `/foo` and the endpoint executes, but any middleware or code that reads `request.url.path` sees `/abc`, so path-based authorization checks can be bypassed. ### Impact Any application running an affected version that relies on `request.url` (or `request.url.path`) for security-sensitive decisions is affected. The most common case is middleware that gates access to certain path prefixes based on `request.url.path`. Deployments fronted by a proxy or load balancer are mitigated only if that proxy rejects or normalizes the malformed `Host` header before forwarding and the application does not trust attacker-controlled host headers (e.g. `X-Forwarded-Host`) elsewhere. ### Mitigation Upgrade to a patched version, which validates the `Host` header against the grammar of [RFC 9112 §3.2](https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2-6) / [RFC 3986 §3.2.2](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.2.2) when constructing `request.url` and falls back to `scope["server"]` for malformed values.