CVE-2026-9546: Haxx Curl

High severity, CVSS 7.5. EPSS: 0.7% chance of exploitation in the next 30 days.

A vulnerability in libcurl caused the HTTP `Referer:` header to persist even when explicitly cleared. While the documentation states that passing NULL to `CURLOPT_REFERER` suppresses the header, the option failed to clear the internal state. As a result, the previous referrer string was erroneously reused and sent in subsequent requests, potentially leaking sensitive information to unintended servers.

Affected products

• Haxx Curl: from 8.18.0, before 8.21.0 (fixed in 8.21.0)

Published 2026-07-03. Last modified 2026-09-15.