CVE-2026-55568
Description
Guzzle is an extensible PHP HTTP client. Prior to 7.12.1, in certain configurations, traffic expected to be protected by TLS on the hop to the proxy is transmitted in cleartext. Proxy authentication credentials (the Proxy-Authorization header, proxy userinfo in the proxy URL, or CURLOPT_PROXYUSERPWD) are sent without encryption, and the CONNECT target host and port for tunneled HTTPS requests are exposed. The built-in cURL handlers (GuzzleHttp\Handler\CurlHandler and GuzzleHttp\Handler\CurlMultiHandler, used by default whenever the PHP cURL extension is available) accept an https:// proxy. libcurl older than 7.50.2 silently treats an https:// proxy as a plaintext http:// proxy. The TLS connection to the proxy is never established, and the proxy leg is cleartext with no error or warning. An application is affected when it sends requests through one of the built-in cURL handlers, configures an https:// proxy expecting the proxy connection itself to be encrypted, and runs with libcurl older than 7.50.2. This vulnerability is fixed in 7.12.1.
Summary dbcve.org
Guzzle's built-in cURL handlers accept https:// proxy URLs, but libcurl versions older than 7.50.2 silently treat them as plaintext http:// proxies. This causes all proxy traffic including authentication credentials (Proxy-Authorization header, proxy userinfo, CURLOPT_PROXYUSERPWD) and CONNECT tunnel target host/port information to be transmitted in cleartext without any error or warning.
Mitigation
Upgrade Guzzle to version 7.12.1 or later, or ensure libcurl 7.50.2 or newer is installed; verify proxy connections are actually encrypted using network traffic analysis.