PwnEscape (CVE-2026-99999): Patched kernels available in testing

Key points

A Linux kernel vulnerability nicknamed PwnEscape (CVE-2026-99999) lets anyone who controls a guest virtual machine attack the host through crafted network packets: leaking host kernel memory, crashing the host or, in the researcher's own lab, running code as root on it. Red Hat and the kernel CVE team both rate it Critical. AlmaLinux 8, 9 and 10 are affected, and patched kernels went into the testing repository on October 16, 2026, ahead of Red Hat.

What the bug is

• Despite the name, it is not a KVM bug. It sits in the host kernel's IPv4 networking stack.
• A guest sending through a virtio network device can pass checksum offload metadata saying where a checksum still has to be calculated and written. The host trusted that location without checking it against the packet's IPv4 header.
• With bad metadata, the host's checksum code overwrites an IPv4 header the kernel has already parsed. In the researcher's example the header length changed from 5 to 15, so IP fragmentation treated a 20-byte header as 60 bytes.
• The code dates from virtio header support for tun/tap (Linux 2.6.27, 2008) and checksum offload for packet sockets (2010): about 18 years, and every kernel AlmaLinux has shipped.

What was demonstrated

• A repeatable leak of host kernel memory for as long as the guest keeps sending packets. Vercel reports it was shown against its production sandboxes.
• A host crash that takes down every VM on the machine.
• Writes into host kernel memory chained into running a program as root on the host, shown on the researcher's KVM and Firecracker machines. Full details are held back for a later write-up.

Discovery and upstream fix

• Paulos Yibelo of PWNAI Research found it during Vercel's public sandbox security challenge and reported it privately on 2026-09-04.
• The fix by Michael S. Tsirkin (Red Hat), "ipv4: validate checksum_start before completing checksum" (commit 6785011f8b16), was merged into mainline on 2026-10-14.
• Upstream calls it defense in depth: such packets should not be accepted at all, and fuller validation of incoming packets is still being worked on.

Who should update

• Most at risk: virtualization hosts running guests for others, such as public or private clouds, VPS providers and code sandbox platforms. One guest can attack the host and every other guest on it.
• Red Hat notes a local attacker can also trigger it, and the code is in every AlmaLinux kernel whether or not the system runs VMs. AlmaLinux recommends updating every system.
• No workaround has been published by the researcher, upstream or Red Hat. The patched kernel is the fix.

Shipping ahead of RHEL

• At the time of writing Red Hat lists RHEL 8, 9 and 10 as affected with no fix released.
• The AlmaLinux core team backported the fix to every affected branch, and ALESCo, the technical steering committee, approved shipping before a CentOS Stream / RHEL update.
• The kernels move to production once the community has helped verify them, and the post will be updated then.

Patched versions

• AlmaLinux 8: kernel-4.18.0-553.174.2.el8_10
• AlmaLinux 9: kernel-5.14.0-687.57.2.el9_8
• AlmaLinux 10: kernel-6.12.0-211.66.2.el10_2
• AlmaLinux Kitten 10: no separate build; the fix comes with the next regular kernel update.
• Install the listed version for your release, or higher.

How to test

• Install from the testing repository and reboot. On a virtualization host, migrate or shut down guests first, since the reboot interrupts all of them.

sudo dnf install -y almalinux-release-testing
sudo dnf update 'kernel*' --enablerepo=almalinux-testing
sudo reboot

• Confirm with uname -r or rpm -q kernel. On production machines, disable the testing repository after rebooting:

sudo dnf config-manager --disable almalinux-testing

• Problems go to AlmaLinux chat or bugs.almalinux.org.

Summary by DistroNews, based on the original post.