GHSA-f7vp-7xgx-4w4r

Impact

SetCookie::matchesDomain() gives every subdomain of a cookie Domain that cookie unless it recognizes the Domain as an IP literal or a numeric host, and it decides that from the domain's own text, so two spellings a transport reads as an address keep subdomain scope. On 7.15, hexadecimal and mixed-base forms such as 0x7f000001 and 0177.0.0.0x1 go unrecognized while libcurl 8.21.0 reads both as 127.0.0.1, so closing them completes the rule GHSA-g446-98w2-8p5w (CVE-2026-59883) set out to establish, which cited the WHATWG IPv4 parser and so already admitted a 0x part. A percent-escaped Domain keeps that scope on both branches for a different reason: percent-decoding sits above numeric parsing, so 192.168.0.%31 and 127.0.0.1%2e are registered names in the URI grammar rather than address literals, and no numeric rule in any base classifies them, while libcurl decodes the host before it resolves and reads them as 192.168.0.1 and 127.0.0.1. The escape must fall in the rightmost label on 8.0.0, which already catches a decimal, octal or 0x part there, but not on 7.15, where %31%32%37.0.0.0x1 also reads as 127.0.0.1; 8.0.0 is affected in the percent spelling alone.

Both directions of the defect are reproduced over a real socket. A cookie stored for Domain=0x7f000001 is placed in the Cookie header of a request to evil.0x7f000001, disclosing a session identifier or token to a host that is not that address, and a response from evil.0x7f000001 setting Domain=0x7f000001 is accepted into the jar and replayed to the address, so a server answering for the look-alike name can fix a session or set application state.

Exploitation requires the application to enable cookie support, address an origin by one of these spellings, and contact a host whose name ends in it. No public DNS delegation provides such a name, so the exposure is to private, split-horizon and container zones. Applications that do not use Guzzle's cookies, that keep a separate jar for each host or trust boundary, or that address origins only by ordinary names are not affected.

Patches

This is a summary; the patches are the authority. The issue is fixed in 7.15.2 and 8.0.1, which hold a cookie Domain containing a % byte to an exact match against the request host. 7.15.2 additionally holds to an exact match a Domain whose every dot-separated part is decimal, octal or hexadecimal, a rule wide enough that 0x100000000, a name to libcurl, loses subdomain matching too; 8.0.0 already recognizes a 0x-prefixed rightmost label. One class stays open: an IDN-capable transport reads a fullwidth-digit Domain such as 127.0.0.１ as 127.0.0.1 while Guzzle matches it by suffix, and these patches do not close that. Versions before 7.15.2 and version 8.0.0 are affected, 8.0.0 in the percent spelling alone.

Workarounds

If you cannot upgrade, do not share one CookieJar between an origin addressed by a numeric or percent-escaped spelling and any other host ending in that spelling. Use a separate jar for each host or trust boundary, or address such an origin by its canonical dotted-decimal form, writing 127.0.0.1, which released versions already hold to an exact match, rather than 0x7f000001 or 127.0.0.%31. Rejecting a request URI host that contains a percent escape, before you hand the URI to Guzzle and on every redirect hop, closes that spelling, and rejecting a non-ASCII byte closes the fullwidth one, but nothing outside the jar closes the hexadecimal one.

Do not use filter_var() with FILTER_VALIDATE_IP to decide whether a domain is an address: it rejects 0x7f000001 and 127.0.0.%31, which a transport reads as 127.0.0.1.