Guzzle: Cookies With a Dot-Only Domain Are Sent to Every Host

Published as "Dot-only cookie domains match all hosts"

medium Guzzle · Last reviewed Nov 7, 2026

CVE: CVE-2026-55767
GHSA: GHSA-cwxw-98qj-8qjx
Packagist: PKSA-93qv-9n9h-6k6p
Reported: Jun 18, 2026

Lets a malicious server plant a cookie that Guzzle then sends to every other host using the same cookie jar.

Affected and fixed versions

In guzzlehttp/guzzle. Find the row for the version you run, and update to the release next to it.

If you run	Update to	Support
Up to 7.12.0	7.12.1	Supported

Upgrade

Run the line for the branch you are on.

composer require guzzlehttp/guzzle:^7.12.1

What this means

Guzzle's cookie jar accepted cookies whose Domain attribute is made only of dots, and then treated them as matching every host. A server your app talks to could set a cookie that Guzzle later sends to unrelated hosts that use the same jar.

Depending on how those other services read the cookie, this can lead to cookie injection or session fixation (making a victim use a session the attacker chose). See the Guzzle advisory and the fix.

Who is affected

Apps that use Guzzle's cookie support and share one jar between hosts they trust and hosts an attacker could control, such as URLs users supply. That includes:

new Client(['cookies' => true]), which creates one shared jar for the client
one CookieJar instance passed to requests for different hosts

Apps that don't use cookies in Guzzle, or keep a separate jar for each origin or trust boundary, aren't affected.

What to do

Update guzzlehttp/guzzle to 7.12.1. If you can't update yet, Guzzle's advice is to use a separate cookie jar for each origin or trust boundary, or turn off cookie handling for requests to untrusted hosts. Avoid 'cookies' => true on clients that talk to hosts with different trust levels.

Reported by Ilia Alshanetsky.

Check your app with an agent

Paste this into a coding agent that can read your project, such as Claude Code, Codex or Amp. It checks your composer.lock against the versions above and tells you what to update, without changing anything.

Check whether the PHP project you have access to is affected by this security advisory, and tell me what to update. Do a read-only check: don't edit files, don't run composer update or composer require, and don't try to reproduce the flaw.

Advisory: Guzzle: Cookies With a Dot-Only Domain Are Sent to Every Host (CVE-2026-55767, GHSA-cwxw-98qj-8qjx)
Details: https://phpnews.net/security/guzzle/ghsa-cwxw-98qj-8qjx

Affected versions, as Composer constraints:
- guzzlehttp/guzzle: <7.12.1

Fixed releases on branches that still get security fixes:
- Up to 7.12.0: update to 7.12.1

1. Find every composer.lock in the project outside vendor/ (a monorepo can have one per app) and read the installed version of each package above. Packages installed as dependencies of other packages count too.
2. Compare each installed version with its constraint: "|" separates alternative ranges, and "," means both bounds apply.
3. For each affected install, read "What this means" at https://phpnews.net/security/guzzle/ghsa-cwxw-98qj-8qjx and check whether the project meets the conditions it describes (a feature in use, a config value, a driver). Say which conditions you could check and which you couldn't.
4. Report a table: lock file | package | installed version | affected (yes, no or unknown) | conditions met (yes, no or unknown) | update to | evidence. For each affected package, give the command that would fix it, without running it: composer update <package> --with-dependencies when it is only installed as a dependency of another package, or composer require with the fixed version when composer.json requires it directly. If the installed version is on a branch with no fixed release listed, say it needs a move to a newer branch.

Sources

← All Guzzle advisories