Guzzle: HTTPS Proxy Silently Falls Back to Cleartext on Old libcurl

Published as "Silent HTTPS proxy downgrade to cleartext"

medium Guzzle · Last reviewed Nov 4, 2026

CVE: CVE-2026-55568
GHSA: GHSA-wpwq-4j6v-78m3
Packagist: PKSA-k22t-f949-t9g6
Reported: Jun 18, 2026

Can send proxy traffic unencrypted when Guzzle is set to use an https:// proxy and the server's libcurl is older than 7.50.2.

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

If you told Guzzle to use an https:// proxy, very old libcurl versions could quietly talk to that proxy in plain text instead. Anyone on the network between you and the proxy could then read your proxy username and password. They could also see which hosts you connect to, and the full contents of plain HTTP requests.

HTTPS requests tunneled through the proxy keep their own encryption, so their contents stay protected.

Who is affected

Apps on guzzlehttp/guzzle before 7.12.1 that use the default cURL handlers (CurlHandler or CurlMultiHandler) with an https:// proxy, and run on libcurl older than 7.50.2. The proxy can come from the proxy option or from variables like https_proxy and HTTPS_PROXY. The Guzzle advisory says apps without an https:// proxy, and servers with newer libcurl, don't get the silent downgrade. Guzzle's stream handler isn't affected.

You can check your libcurl version with curl_version() in PHP.

What to do

Update to 7.12.1 or later. Guzzle now refuses an https:// proxy that libcurl can't support, before sending anything. Until you can update, don't use an https:// proxy on a server with old libcurl, or upgrade libcurl to 7.52.0 or newer with HTTPS proxy support.

Reported by GrahamCampbell.

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: HTTPS Proxy Silently Falls Back to Cleartext on Old libcurl (CVE-2026-55568, GHSA-wpwq-4j6v-78m3)
Details: https://phpnews.net/security/guzzle/ghsa-wpwq-4j6v-78m3

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-wpwq-4j6v-78m3 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