CVE-2026-59882: guzzlehttp/psr7: Host Confusion via Weak URI Host Validation

guzzlehttp/psr7 did not reject URI host components containing authority delimiters (/, ?, #, @, or \), an embedded port such as host:8080, or IPv6 brackets that do not frame the host (such as [::1 or ::1]). Uri::assertValidHost() rejected only control characters, so these malformed hosts were accepted both when parsing a URI string — including through new Uri(), an absolute-form request target passed to Message::parseRequest(), and the synthetic SERVER_NAME or SERVER_ADDR fallback used by ServerRequest::fromGlobals() — and when setting a host with Uri::withHost(). Because the stored host was not validated, Uri::getHost() could return a value that does not agree with the URI’s authority or with the host that is actually used on the wire. For example, Uri::withHost('good.com@evil.com') is accepted, and new Uri('http://[gggg::1]/') produces a getHost() of '[gggg:' while getAuthority() returns '[gggg::1'.

You are affected if your application builds a Uri or calls Uri::withHost() with host data that is wholly or partly attacker-controlled and then relies on Uri::getHost() for a security or routing decision, such as host allowlisting or denylisting, an SSRF guard, cookie scoping, or proxy-bypass selection. You are also affected if you parse, forward, proxy, or re-serialize URIs that come from untrusted raw HTTP messages or from synthetic $_SERVER values. Because getHost() can then disagree with the real connection target, the concrete impact depends on how your application and any downstream HTTP/1.x component use the URI, and may include host-based access-control bypass, server-side request forgery, routing to an unintended host, or Host header confusion. You are not affected if every host you place into a Uri is already trusted and well formed.

Applications using guzzlehttp/guzzle are affected in the same way through Guzzle’s use of PSR-7. Guzzle derives the request Host header from the request URI, and its cookie matching and no_proxy proxy-bypass matching both read Uri::getHost(). An application that builds a Guzzle request URI from attacker-controlled host input can therefore have its Host header, cookie scope, or proxy-bypass decision computed against a host that differs from the one Guzzle connects to. This is not the normal request-sending path, and an application that passes only trusted, well-formed URIs to Guzzle and never builds a request URI from attacker-controlled host input is not affected through Guzzle’s standard client APIs.