Starlette: Unvalidated request path concatenated into authority poisons request.url.hostname

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 /:

GET @google.com HTTP/1.1
Host: localhost

affected versions reconstruct the URL as http://localhost@google.com. Per RFC 3986 §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:

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: 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".