USN-8486-1: libssh2 vulnerabilities

It was discovered that libssh2 incorrectly handled the sftp_symlink()
function. A malicious SSH server or machine-in-the-middle attacker could
possibly use this issue to obtain sensitive information or cause a denial
of service. (CVE-2025-15661)

It was discovered that libssh2 had a pre-authentication denial of service
vulnerability in the SSH_MSG_EXT_INFO handler. A malicious SSH server could
possibly use this issue to cause a client CPU exhaustion loop, resulting in
a denial of service. (CVE-2026-55199)

It was discovered that libssh2 incorrectly handled packet length fields. A
remote attacker could possibly use this issue to execute arbitrary code.
This issue only affected Ubuntu 25.10 and Ubuntu 26.04 LTS.
(CVE-2026-55200)