### 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`.

### 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`.