On October 15, 2026, Next.ink reported the public release of the CVE record and corrected stable kernel versions for PwnEscape (CVE-2026-99999). The flaw sits in Linux’s host-side IPv4 checksum processing. Vercel and PWN.AI reported host-memory disclosure in Vercel production sandboxes; Vercel placed host crashes and code-execution demonstrations in researchers’ lab environments. (Next.ink; Vercel; PWN.AI)
How PwnEscape affects Linux networking
The vulnerability is in the Linux host’s IPv4 network path, not in KVM itself. Guest-controlled checksum metadata can trigger a deferred write that changes an IPv4 header after the kernel has parsed it. That mismatch between the header’s earlier interpretation and its later contents creates the vulnerable path, as PWN.AI’s technical disclosure explains.
In PWN.AI’s bounded test case, a packet had 28 bytes of logical linear data but was later interpreted as having a 60-byte IPv4 header. That produced a 32-byte read beyond the packet’s logical data, not necessarily beyond its physical allocation. It is a result from that controlled case, not a per-packet yield. (PWN.AI)
What the demonstrations reached
Vercel and PWN.AI reported that host-memory disclosure was demonstrated against Vercel production sandboxes. Vercel said the researcher began from a standard sandbox without special access. The reported production result was memory disclosure; the accounts placed host crashes and host-code execution in researcher-owned lab systems. (Vercel; PWN.AI)
PWN.AI also described selected 64-byte host-kernel writes and a bounded host-root chain on its own KVM test setup. Those were laboratory results, distinct from the production memory-disclosure demonstration. (PWN.AI)
What the upstream fix guards
The accepted Linux change checks the checksum start before checksum completion. It requires csum_start to be at or beyond network_header + ip_hdrlen(skb), keeping checksum completion from targeting the parsed IPv4 header. The upstream commit contains the guard.
PWN.AI describes the check as defense in depth: it blocks the demonstrated IPv4-header overwrite path, but does not validate that the checksum target is the correct TCP or UDP field or redesign the wider checksum interface. PWN.AI says the fix entered Linux’s networking tree on October 8, 2026; Next.ink reported that it reached the mainline kernel on October 14, before the October 15 public release of the CVE record and corrected stable versions. (PWN.AI; Next.ink)
Reported kernel versions and distribution backports
In its October 19, 2026 report, Next.ink listed these corrected kernel versions: 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.57 and 7.2.11. It also reported that 7.3 was unaffected. (Next.ink)
Linux distributions can backport security fixes, so a version reported for an upstream branch may not match the package version on a particular system. Check the security advisory for the distribution and package you use; uname -r by itself may not reveal whether a backport is present, as Next.ink noted.
