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Linux Kernel
2 articles on linux kernel.
PwnEscape: How Untrusted Checksum Metadata Rewrote a Validated Packet Header — and Leaked Host Memory
The Critical bug from Vercel’s $1M Sandbox challenge — nicknamed PwnEscape by researcher Paulos Yibelo (PWNAI, pwn.ai) — is a Linux-kernel IPv4 networking flaw, not a KVM break. Deferred checksum completion (CHECKSUM_PARTIAL / csum_start) rewrites an IPv4 header AFTER the kernel validated it — a time-of-check/time-of-use on packet metadata. Guest-controlled checksum data mutates the IHL field (0x45→0x4f), so the fragmenter copies roughly 32 bytes of host memory beyond the logical packet into outgoing IP options → host-memory disclosure. Confirmed leaking host memory on Vercel production; host crash and full guest-to-host code execution were demonstrated only in the researcher’s own KVM/Firecracker lab. Fixed upstream in commit 6785011f8b16 and backported across stable kernels; CVE pending. The mechanism, why it matters for multi-tenant isolation, and what defenders should do.
October 17, 2026
The Vercel Sandbox Escape, One Week On: a Linux Kernel Zero-Day, Not a Hypervisor Break
Vercel’s first-party findings from its $1M Sandbox challenge (14 Oct 2026) put the most severe bug in the Linux kernel’s networking stack — triggered by crafted packets from inside the microVM — not in the KVM/Firecracker hypervisor, which an independent Trail of Bits review called architecturally sound. A researcher leaked host memory on Vercel production and, in a matching lab, crashed the host with two packets and ran commands with full administrator privileges. CVEs are pending and no customer data was reported accessed. What changed from the early “KVM escape” framing, what is confirmed vs claimed, and what a security leader should do.
October 17, 2026