AIKIDO-2026-75039

TL;DR

Guzzle hands the request URI host to the transport as written and supplies the Host header separately, so libcurl and the stream handler percent-decode and canonicalize the authority differently than Guzzle does. A host that Guzzle and application checks read as one destination, such as a percent-escaped or noncanonical numeric spelling, can be resolved and connected by the transport as a different address like loopback. Before the fix this lets externally influenced URIs reach hosts excluded by denylists, private-range, or IP-literal checks, and it skews no_proxy, redirect credential stripping, and cookie storage decisions. The fix validates the request host in all built-in handlers, rejecting non-printable, percent-escaped, delimiter-bearing, and trailing-dot numeric hosts, and regenerates the derived Host header after URI rewrites.