libvirt version before 4.2.0-rc1 is vulnerable to a resource exhaustion as a result of an incomplete fix for CVE-2018-5748 that affects QEMU monitor but now also triggered via QEMU guest agent.
libvirt version 2.3.0 and later is vulnerable to a bad default configuration of "verify-peer=no" passed to QEMU by libvirt resulting in a failure to validate SSL/TLS certificates by default.