Linux KVM: guest-to-host escape via malformed IPv4 checksum metadata lets guest VMs compromise the host (CVE-2026-99999)
Severity: critical · Patch status: patched · Since 2026-10-19 · CVE-2026-99999
Researcher Paulos Yibelo disclosed 'PwnEscape', a critical (CVSS 9.3) guest-to-host escape in Linux KVM, tracked as CVE-2026-99999. A guest can send malformed IPv4 packets whose checksum metadata causes skbchecksumhelp to corrupt the network header, letting untrusted guest-controlled metadata rewrite security-sensitive packet fields after kernel validation. Red Hat says impact may include arbitrary code execution on the host, denial of service, or sensitive memory disclosure, breaking guest-host isolation. Yibelo describes it as a novel attack class in Linux networking. AWS EC2 says it is not at risk and Google Cloud is likely unaffected. An upstream kernel fix is rolling into Debian, RHEL, Ubuntu and KVM-based products like Nutanix and Proxmox.
What to do
- Patch KVM hosts with the fixed kernel as it reaches your distro (Debian, RHEL, Ubuntu) and restart/evacuate workloads as required
- Prioritise hosts running untrusted or multi-tenant guest workloads
- Verify virtualization products built on KVM (Proxmox, Nutanix, custom stacks) have shipped the fix before exposing new tenants
- Review whether guest workloads can send arbitrary network traffic to the host and apply compensating network filtering where patching is delayed
- Monitor vendor advisories for exploitation reports
Sources
All live incidents on Threat Watch
