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 guzzlehttp/psr7 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.
Patches
The issue is patched in 2.10.2 and later. 1.x is end-of-life and will not receive a patch.
Workarounds
If you cannot upgrade immediately, validate and reject all untrusted URI strings before constructing PSR-7 Uri or Request instances. Reject input containing ASCII control characters, whitespace, or DEL, including CRLF, tab, space, NUL, or DEL characters:
php if (pregmatch('/[\x00-\x20\x7F]/', $untrustedUrl)) { throw new \InvalidArgumentException('Insecure URL detected'); }
Applications that manually serialize or forward requests should also ensure the final HTTP client, transport, or serializer rejects invalid URI and header data before writing requests to the network.
References
https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2 https://www.rfc-editor.org/rfc/rfc9112.html#section-5 https://www.rfc-editor.org/rfc/rfc9112.html#section-11.2 https://www.rfc-editor.org/rfc/rfc9110.html#section-7.2
Impact
guzzlehttp/psr7 improperly interpreted malformed Host header values when constructing request URIs from inbound request data. This issue concerns inbound request parsing and server request construction. It does not require serializing a PSR-7 request, and it is not part of the normal outbound request-sending path used by guzzlehttp/guzzle.
A vulnerable flow is:
1. An attacker controls a raw HTTP request or server variable containing a Host value. 2. The Host value contains URI authority delimiters, such as trusted.example@evil.example. 3. guzzlehttp/psr7 uses that value to construct a URI. 4. The URI parser treats the portion before @ as userinfo and the portion after @ as the URI host. 5. The resulting PSR-7 request URI host differs from the original Host header value.
For example, Host: trusted.example@evil.example can result in a PSR-7 URI whose host is evil.example, while the original Host header value remains trusted.example@evil.example.
Applications are affected if they parse attacker-controlled raw HTTP requests with GuzzleHttp\Psr7\Message::parseRequest() or the legacy 1.x GuzzleHttp\Psr7\parserequest() function, or if they build server requests from attacker-controlled server variables with GuzzleHttp\Psr7\ServerRequest::fromGlobals() or GuzzleHttp\Psr7\ServerRequest::getUriFromGlobals(), and then rely on the resulting URI host for routing, allow-list checks, credential selection, or forwarding decisions. Applications using guzzlehttp/psr7 only through Guzzle's standard HTTP client APIs are not expected to be affected. In affected forwarding or gateway scenarios, this may cause requests or credentials to be sent to an unintended host.
Patches
The issue is patched in 2.10.2 and later. 1.x is end-of-life and will not receive a patch.
Workarounds
If you cannot upgrade immediately, validate Host values before passing untrusted request data to Message::parseRequest(), legacy 1.x parserequest(), ServerRequest::fromGlobals(), or ServerRequest::getUriFromGlobals().
Accept only uri-host [ ":" port ]. Reject values containing whitespace, control characters, userinfo (@), path (/ or \), query (?), fragment (#), malformed IP literals or bracket syntax, or invalid port syntax.
Do not validate Host by prefixing it with http:// and passing it to parseurl(), because that can reinterpret malformed values as URI userinfo and host.
References
https://www.rfc-editor.org/rfc/rfc9112.html#section-3.2 https://www.rfc-editor.org/rfc/rfc9112.html#section-3.3 https://www.rfc-editor.org/rfc/rfc9110.html#section-4.2.4 https://www.rfc-editor.org/rfc/rfc9110.html#section-7.2