guzzlehttp/psr7 has CRLF Injection via URI Host Component

Impact

guzzlehttp/psr7 did not reject ASCII control characters, whitespace, or DEL in first-party URI host components. The issue requires a PSR-7 request to be serialized into a raw HTTP/1.x message, for example with GuzzleHttp\Psr7\Message::toString() or an equivalent custom serializer. Creating a Uri, Request, or other PSR-7 object alone is not sufficient. The malformed host must be copied into the serialized Host header without further validation.

A vulnerable flow is:

1. An application accepts a user-controlled URL.
2. The URL is used to construct a PSR-7 Uri or Request.
3. The host component contains CRLF or another header-unsafe character.
4. The request is serialized into a raw HTTP/1.x message without an explicit Host header.
5. The host is copied into the serialized Host header.
6. The serialized request is written to the network or otherwise processed by software that does not independently reject the malformed host.

In that flow, an attacker can cause the serialized request to contain additional attacker-controlled header lines. For example, a host containing "\r\nX-Injected: yes" can cause the generated Host header to span multiple HTTP header lines.

This is not the normal request-sending path used by guzzlehttp/guzzle. Applications using only through Guzzle's standard HTTP client APIs are not expected to be affected. Applications are most likely to be affected when they manually serialize PSR-7 requests, forward raw HTTP messages, or use custom transports, proxying, crawling, webhook delivery, or similar request-dispatch code that serializes requests without independently validating URI hosts and header data. In deployments involving HTTP/1.1 connection reuse, proxies, gateways, or load balancers, this malformed serialized request may also contribute to request smuggling or cache poisoning, depending on how downstream components parse the request.