Every cloud tenant lives behind one promise: your virtual machine cannot touch the host, and the host cannot be reached by the tenant next door. CVE-2026-99999, nicknamed PwnEscape , breaks that promise, and the break is not where anyone was looking. The flaw is not in KVM. It is in the host kernel's IPv4 networking code, in a checksum optimization that trusted metadata supplied by the guest.

TL;DR

What: A Linux kernel flaw in IPv4 checksum offload handling. A guest VM sends packets with crafted checksum metadata; the host kernel's skb_checksum_help() then writes out of bounds and rewrites an IPv4 header it had already validated (CWE-787).

Impact: Host memory disclosure, host crash, and, demonstrated in the researcher's lab, controlled host-kernel writes and root-level code execution on the host. A guest-to-host escape primitive.

Severity: 9.3 Critical on the CVE.org record; Red Hat scores it 8.8.…