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

Impact

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 SERVERNAME or SERVERADDR 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 noproxy 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.

Patches

The issue is patched in 2.12.3 and later. Starting in that release, guzzlehttp/psr7 rejects URI host components that contain authority delimiters, an embedded port, or IPv6 brackets that do not frame the host, both when parsing a URI and via Uri::withHost(), so Uri::getHost() agrees with the URI authority. A framed bracketed literal whose contents are not a valid IPv6 address is preserved rather than rejected, so getHost() still agrees with the authority.

Workarounds

If you cannot upgrade immediately, validate and normalize host values before you build or modify a Uri from untrusted input, and do not treat Uri::getHost() as a trust boundary for a host you have not independently checked. Before passing a host to Uri, reject any value that contains /, ?, #, @, or \, that contains a colon outside a bracketed IPv6 literal, or that has unbalanced brackets. Applications that parse, forward, replay, or re-serialize raw HTTP messages, or that run with attacker-controlled $SERVER values, should validate the host before calling Message::parseRequest() or ServerRequest::fromGlobals(), and should avoid reparsing untrusted input.

References

https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.2 https://datatracker.ietf.org/doc/html/rfc3986#section-3.2.3 https://datatracker.ietf.org/doc/html/rfc6265#section-5.1.3