CVE-2026-9545: Haxx Curl

High severity, CVSS 7.5. EPSS: 0.4% chance of exploitation in the next 30 days.

In this scenario, libcurl first uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate. When libcurl returns to the hostname the second time with a cached SSL session (`CURLOPT_SSL_SESSIONID_CACHE` is not disabled) and early data enabled (the `CURLSSLOPT_EARLYDATA` bit is set in `CURLOPT_SSL_OPTIONS`), libcurl might send off the second request's bytes on that new connection *before* enforcing the certificate verification failure. Potentially leaking sensitive information.

Affected products

• Haxx Curl: from 8.11.0, before 8.21.0 (fixed in 8.21.0)

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