If you host VMs for anyone you don’t fully trust, this one’s for you. CVE-2026-99999, nicknamed PwnEscape by the researchers, is a guest-to-host escape. Someone who controls a guest VM can send crafted network packets that leak the host’s kernel memory, crash the host, or, in the researcher’s lab, run code as root on the host. The kernel CVE team scores it 9.3 Critical, and Red Hat rates it Critical too.
What broke
Despite the name, it isn’t a KVM bug. It’s in the host kernel’s IPv4 stack. Guests using virtio networking can hand the host checksum offload metadata, and the host trusted it without checking it against the IPv4 header. With bad metadata, skb_checksum_help() writes over a header the kernel already acted on, or as the patch puts it, it can “corrupt the network header and cause a bunch of mischief.” It’s a local-to-the-guest problem, not an open-internet remote exploit.
The fix
The upstream fix is commit 6785011f8b16, “ipv4: validate checksum_start before completing checksum.” On October 15, Greg Kroah-Hartman pushed it in seven stable kernels: 7.2.11, 6.18.57, 6.12.113, 6.6.159, 6.1.190, 5.15.223 and 5.10.272. AlmaLinux 8, 9 and 10 are affected and have patched kernels in testing. No separate workaround is advertised, so plan the reboot.
What to do
- Patch the host kernel first. That’s where the bug lives. “We updated the guests” isn’t the same as “the host is fixed.”
- Reboot and confirm the running kernel with
uname -r, not just the package log. - Watch your distro and cloud node images for the backports.
- If you rent a VPS, your provider has to patch their hosts. Keep your guest kernel updated anyway.
I’m not posting exploit details here. Read the write-ups below and patch.
Happy Hacking!