The kernel CNA published CVE-2026-99999 (CVSS 3.1 9.3, AV:L with scope change) on Oct 15 for "ipv4: validate checksum_start before completing checksum". This is the networking-stack bug Paulos Yibelo (pwn.ai) found in Vercel's $1M sandbox challenge. A guest that already runs code can send frames with crafted virtio-net checksum metadata. The host's TAP path feeds them to the IPv4 stack, where skb_checksum_help can corrupt the network header. Vercel confirmed the host-memory leak against its production sandboxes. Host crash and root on the host were shown only on the researcher's matching KVM/Firecracker lab. The fix (mainline commit 6785011f8b16) is described by its author as defence in depth, and fuller input validation is still being worked on. First fixed stable kernels are 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.57 and 7.2.11. Affected code dates back to 2.6.27. No in-the-wild use is reported and it is not in CISA KEV (catalog 2026.10.15). Who should care: anyone running untrusted guests on Linux hosts with tap/virtio-net (KVM, Firecracker, cloud sandboxes, CI runners). Patch the host kernel. Distros often backport without changing version numbers, so check your vendor advisory. Update Oct 16: AlmaLinux 8, 9 and 10 are affected; patched kernels are in its testing repo pending verification. Red Hat and the kernel CVE team both rate it Critical. Some hosting providers are already doing emergency host reboots to patch.