GHSA-jp82-jpqv-5vv3

### Summary

In affected versions, the HTTP request path is not validated before being used to reconstruct `request.url`. Because `request.url` is rebuilt by concatenating `{scheme}://{host}{path}` and re-parsing the result, a path that does not begin with `/` (for example `@google.com`) moves the authority boundary during re-parsing, so `request.url.hostname` and `request.url.netloc` become attacker-controlled. Code that reads `request.url.hostname` (rather than the `Host` header or `scope`) can therefore be misled into trusting an attacker-supplied host.

### Details

When a client requests a path that does not start with `/`:

```http GET @google.com HTTP/1.1 Host: localhost ```

affected versions reconstruct the URL as `http://localhost@google.com`. Per [RFC 3986 §3.2.1](https://www.rfc-editor.org/rfc/rfc3986.html#section-3.2.1), the substring before `@` in the authority is `userinfo`, so re-parsing yields `username = "localhost"` and `hostname = "google.com"`, with an empty path:

```text request.url == "http://localhost@google.com" request.url.hostname == "google.com" request.url.path == "" ```

The root cause is that the path is concatenated directly after the host without a separating `/`, and without validating that it begins with one. Only the `Host` header was validated when constructing `request.url`; the path was not.

This requires an ASGI server that forwards a request-target lacking a leading `/` into `scope["path"]`.

### Impact

Any application running an affected version that uses `request.url`, `request.url.netloc`, or `request.url.hostname` for a security-sensitive decision (host-based authorization, redirect/callback base, SSRF target, cache key, audit log) may be affected, when no fronting proxy or load balancer rejects the malformed request-target first.

Note that this is less exploitable than [GHSA-86qp-5c8j-p5mr](https://github.com/Kludex/starlette/security/advisories/GHSA-86qp-5c8j-p5mr): there, the poison is carried in the `Host` header, so the real path still routes to a valid endpoint while `request.url.path` lies. Here, the poison must be carried in the path itself, and that path (`@google.com`) does not match any registered route, so routing returns `404` and no endpoint handler runs. The exposure is limited to code that reads `request.url` before routing - notably middleware - or in 404/exception handlers.

### Mitigation

Upgrade to a patched version, which prevents the request path from crossing into the URL authority. The request above instead yields `http://localhost/@google.com` with `request.url.hostname == "localhost"`.