CVE-2026-55568mediumCWE-311CWE-319CWE-636

CVE-2026-55568: medium-severity vulnerability in guzzle

Guzzle: Silent HTTPS-Proxy Downgrade to Cleartext

Published

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13Vexday Risk Score

No sign of exploitation. No public exploitation artifact known so far.

ssvc Trackcvss 5.9epss 0.1%
In short

Guzzle's HTTP client can accidentally send proxy traffic in plain text instead of encrypted, exposing passwords and sensitive data when using HTTPS proxies with older cURL versions. This happens silently without warning, putting users at risk.

Technical detail

CWE-311/319/636: Guzzle's cURL handlers accept https:// proxy URLs but libcurl <7.50.2 silently downgrades to unencrypted http://, transmitting Proxy-Authorization headers, proxy credentials, and CONNECT tunnel metadata in cleartext. Attack vector requires application to configure an https:// proxy with libcurl <7.50.2; impact includes proxy credential exposure and target host/port disclosure on the proxy hop.

Summary generated and translated by AI from the official 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.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
Affected products
guzzle · guzzle
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