# PwnEscape: Linux networking guest-to-host escape (CVE-2026-99999) · pwn.watch > Paulos Yibelo / pwn.ai found a Critical Linux networking flaw (CVE-2026-99999, CVSS 9.3) during Vercel’s $1M Sandbox challenge. Guest-controlled checksum metadata can rewrite an IPv4 header after validation, enabling host-memory disclosure and, in lab conditions, guest-to-host code execution on KVM/Firecracker. Upstream fix merged Oct 2026; host kernels need patch + reboot. Affects multi-tenant VPS / sandbox providers more than end-user desktops. Fly.io published an Oct 16 2026 postmortem of a ~10-hour fleet-wide host reboot after the CVE, saying customer data exposure was unlikely but advising rotation of credentials sent over HTTP or unencrypted 6PN between Oct 15 and 17. Page: https://pwn.watch/i/pwnescape-cve-2026-99999 · generated 2026-11-06T20:56:54.285Z ## Status - Severity: critical · status: live · patch: patched - Product: Linux kernel networking (host side of KVM/Firecracker and similar) - Started: 2026-10-08 - CVEs: CVE-2026-99999 - Affected: Kernels before stable backports such as 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.57, 7.2.11 (verify vendor packages) - Fixed in: Upstream commit 6785011f8b16 and matching stable backports; vendor kernels as listed by distros ## Follow this incident - RSS (one item per update): https://pwn.watch/i/pwnescape-cve-2026-99999/feed.xml - JSON Feed: https://pwn.watch/i/pwnescape-cve-2026-99999/feed.json - Full data (timeline, sources, posts, agentBrief): https://pwn.watch/i/pwnescape-cve-2026-99999.json - This file: https://pwn.watch/i/pwnescape-cve-2026-99999/llms.txt ## Updates (newest first) - 2026-11-06: Hacker News discusses Fly.io's PwnEscape postmortem (Well, That Wasn't Fun). (https://news.ycombinator.com/item?id=50397310) - 2026-10-16: Fly.io begins an emergency fleet-wide host reboot for CVE-2026-99999 (PwnEscape), completing the patched-kernel rollout in about 10 hours; later postmortem says customer data exposure was unlikely but advises rotating credentials sent over HTTP or unencrypted 6PN Oct 15–17. (https://fly.io/blog/well-that-wasnt-fun/) - 2026-10-14/15: Mainline merge / public write-ups. - 2026-10-07/08: Upstream IPv4 checksum guard accepted into net.git. - 2026-08-20: Initial host-memory disclosure reported. - 2026-08-18: Vercel Sandbox challenge opens. ## Fix brief # Security brief: PwnEscape: Linux networking guest-to-host escape (CVE-2026-99999) Source: https://pwn.watch/i/pwnescape-cve-2026-99999 · status: live · severity: critical · updated Nov 6, 2026, 20:52 UTC ## Patch status **PATCHED: fixed releases exist. Upgrade to a fixed version.** ## What it is Paulos Yibelo / pwn.ai found a Critical Linux networking flaw (CVE-2026-99999, CVSS 9.3) during Vercel’s $1M Sandbox challenge. Guest-controlled checksum metadata can rewrite an IPv4 header after validation, enabling host-memory disclosure and, in lab conditions, guest-to-host code execution on KVM/Firecracker. Upstream fix merged Oct 2026; host kernels need patch + reboot. Affects multi-tenant VPS / sandbox providers more than end-user desktops. Fly.io published an Oct 16 2026 postmortem of a ~10-hour fleet-wide host reboot after the CVE, saying customer data exposure was unlikely but advising rotation of credentials sent over HTTP or unencrypted 6PN between Oct 15 and 17. Severity detail: CVSS 9.3 (Critical) - Product: Linux kernel networking (host side of KVM/Firecracker and similar) - CVE / advisory ids: CVE-2026-99999 ## Affected versions - Kernels before stable backports such as 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.57, 7.2.11 (verify vendor packages) ## Check whether you're affected Run these where the product is installed or in the project that depends on it, then compare with the affected/fixed versions: ```sh uname -r ``` ## Fix Upgrade to one of these fixed versions (or later on the same branch): - Upstream commit 6785011f8b16 and matching stable backports; vendor kernels as listed by distros After upgrading, re-run the checks above and redeploy/restart the affected services. ## Secrets to rotate This incident involves possible credential exposure. Rotate any credentials (API keys, tokens, env vars, SSH keys) that were present on affected systems or in affected builds while vulnerable/compromised versions were installed, and review access logs. ## Latest updates - Nov 6, 2026: Hacker News discusses Fly.io's PwnEscape postmortem (Well, That Wasn't Fun). - Oct 16, 2026: Fly.io begins an emergency fleet-wide host reboot for CVE-2026-99999 (PwnEscape), completing the patched-kernel rollout in about 10 hours; later postmortem says customer data exposure was unlikely but advises rotating credentials sent over HTTP or unencrypted 6PN Oct 15–17. - Oct 14/15, 2026: Mainline merge / public write-ups. - Oct 7/08, 2026: Upstream IPv4 checksum guard accepted into net.git. - Aug 20, 2026: Initial host-memory disclosure reported. - Aug 18, 2026: Vercel Sandbox challenge opens. ## Advisories - [CVE-2026-99999: ipv4: validate checksum_start before completing checksum](https://lists.openwall.net/linux-cve-announce/2026/10/15/2) (Linux CVE announce (Openwall)) ## Write-ups & news - [PwnEscape write-up (pwn.ai)](https://pwn.ai/blog/pwnescape) - [Vercel: Findings from the $1M Sandbox challenge](https://vercel.com/blog/findings-from-the-1-million-dollar-vercel-sandbox-challenge) - [Well, That Wasn't Fun: October 16 '26](https://fly.io/blog/well-that-wasnt-fun/) - [Fly.io Got Breached](https://news.ycombinator.com/item?id=50397310) ## Caveats No public claim of in-the-wild exploitation. Fixed-version list is indicative — always confirm with your distro’s advisory. CVE id CVE-2026-99999 is the identifier used across pwn.ai / Vercel / community posts. --- Machine-readable: https://pwn.watch/i/pwnescape-cve-2026-99999.json · all incidents: https://pwn.watch/incidents.json. Verify against the vendor advisory before changing production systems.