A newly disclosed critical flaw in the Linux kernel allows code inside a virtual machine to read memory on the host computer it runs on. A researcher demonstrated the leak against Vercel's live cloud sandboxes, and Vercel confirmed it.

The flaw, now tracked as CVE-2026-99999 and nicknamed PwnEscape, is in the Linux kernel's networking code, not in any one cloud's setup, so it reaches the shared layer that cloud providers use to keep customers' workloads apart.

It was found in a Vercel security challenge and reported privately before any public release, and there is no sign it has been used in a real attack.

Paulos Yibelo of the security firm pwn.ai found that specially made network packets sent from inside a virtual machine could do three things, he explained in a technical write-up: read the host's memory, crash the host, and run code on it. All three come from the same kernel flaw.

Vercel said in its report on the challenge that the memory leak was demonstrated against its production sandboxes. Each packet the guest sent out could carry back about 32 bytes of the host's memory, and the guest could keep going as long as it kept sending packets.

The crash and code execution were observed only on the researcher's own machines, which were set up to match Vercel's. Two specially crafted packets were enough to crash a host and take down every sandbox running on it.

Using the same flaw, the researcher then reached administrator control of a host, breaking out of the virtual machine.

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