CVE-2026-11856: Haxx Curl

Critical severity, CVSS 9.8. EPSS: 0.7% chance of exploitation in the next 30 days.

Successfully using libcurl to do a transfer to a specific HTTP origin (`hostA`) with **Digest** authentication and then changing the origin to a different one (`hostB`) for a second transfer, reusing the same handle, makes libcurl wrongly pass on the `Authorization:` header field meant for `hostA`, to `hostB`.

Affected products

• Haxx Curl: from 7.10.6, before 8.21.0 (fixed in 8.21.0)

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