CVE-2026-48710

Er is een kwetsbaarheid verholpen in Starlette, een Python-library voor het ontwikkelen van webservices. Starlette wordt door verschillende producten gebruikt, waaronder FastAPI.

### 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.

Starlette reconstructs the requested URL based on the HTTP Host request header and requested path, but does not perform any validation of the Host header value. This allows attackers to inject paths into the host part, prepending the actual path. However, routing in Starlette is based on the actual request path. This inconsistent interpretation of HTTP requests may lead to issues such as authentication bypass when the authentication depends on the reconstructed URL’s path.