CVE-2026-99999

ipv4: validate checksum_start before completing checksum

No known exploitation. EPSS puts it in the 24th percentile. Fixed versions are out.

CVSS 3.19.3Critical EPSS percentile24th0.3% chance in 30 days CISA KEVNot listedNo confirmed exploitation
No public exploit known Fix1 product4 fixed builds listed below

Published 2026-10-15Updated 2026-10-19Sources: CVE.org, NVD

NVD CVE.org

What to do

Find your product and version, then upgrade to the build on the right.

LinuxAffected: ≥ f43798c27684ab925adde7d8acc34c78c6e50df8, < a464d950f31c89565cbaae038cc60db04fe01956Upgrade to: a464d950f31c89565cbaae038cc60db04fe01956Affected: ≥ f43798c27684ab925adde7d8acc34c78c6e50df8, < 0fab003ef4698e0628060a45a48f14b43ba2a40aUpgrade to: 0fab003ef4698e0628060a45a48f14b43ba2a40aAffected: ≥ f43798c27684ab925adde7d8acc34c78c6e50df8, < 94524b7b32691de2e2ce162c04d6cbc9106e5cbfUpgrade to: 94524b7b32691de2e2ce162c04d6cbc9106e5cbfAffected: ≥ f43798c27684ab925adde7d8acc34c78c6e50df8, < a65f69e0f38ffbed1b128029e09097436416dc8bUpgrade to: a65f69e0f38ffbed1b128029e09097436416dc8b + 4 more branchesStated by: CVE record

Versions as the sources state them. Full version matrix after sign-in

Steps

Written by AI from the record

Ask whoever manages your servers, cloud instances, or Linux-based appliances to check the current Linux kernel version in use.
Confirm whether that kernel includes the fix identified by commit a464d950f31c89565cbaae038cc60db04fe01956 (and related companion commits for other kernel branches).
If not, schedule an update to a kernel build that includes the fix as soon as possible, prioritizing any multi-user or shared systems.
Reboot affected systems after the update, since kernel fixes only take effect after a restart.
Restrict who has local shell or login access to these systems in the meantime, since the attack requires local access.

Usually a quick update

Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

Coverage timeline

2 publishers

A Saudi national employed at OpenAI was arrested for allegedly planting a concealed recording device in a San Francisco office conference room, charging with theft of trade secrets and unauthorized surveillance.
as reported · Oct 22, 2026·Risky Business News summary
The Linux kernel team patched PwnEscape (CVE-2026-99999), a critical vulnerability allowing virtual machine guests to escape KVM and execute code on the host, discovered using AI research tools.
as reported · Oct 19, 2026·Risky Business News summary
Researcher Paulos Yibelo demonstrated CVE-2026-99999 ('PwnEscape'), a Linux kernel flaw allowing guest VMs to read host memory, with patches released for stable kernels.
as reported · Oct 15, 2026·The Hacker News summary

Sourced from public security reporting — each date is when it was published, not when it happened. 1 further article named this CVE in wider roundups without reporting it as a distinct event.

What it is

From the CVE record

In the Linux kernel, the following vulnerability has been resolved: ipv4: validate checksum_start before completing checksum If a packet with bad checksum metadata gets into the ipv4 stack, skb_checksum_help can corrupt the network header and cause a bunch of mischief. This was discovered and reported by Paulos, and has been reporoduced by others independently since. We really shouldn't allow such packets in, but as a defence in depth measure, let's also check before we complete the checksum. A more complete validation at input is forthcoming, but needs more work.

In plain language

Written by AI from the record

A flaw in the core networking code of the Linux operating system can let someone who already has access to a machine corrupt network data and potentially crash or take over the system, and because it needs a kernel update (not an app update), you should act on it without delay.

CWE-20 (improper input validation) in the IPv4 checksum-handling path allows a local user to submit a packet with malformed checksum metadata, causing skb_checksum_help to corrupt the network header with memory-safety consequences up to system compromise.

If you're affected

System crash / denial of service
Full system compromise
Data corruption
Operational disruption

What is it

Think of this like a mail sorting room where a worker accepts envelopes with garbled sender information and ends up jamming the sorting machine. Here, the Linux operating system's network-handling code accepts a malformed data packet and lets it corrupt internal memory, which can crash the system or let an attacker who's already logged in gain deeper control. It doesn't require anyone to click anything — just someone with existing access on the machine.

Who is affected

This matters to you if you run any servers, cloud instances, internal tools, or network appliances built on the Linux kernel — which is most Linux-based infrastructure, since this is in core kernel networking code, not a specific app. It is only exploitable by someone who already has local access (an account or shell) on the affected machine, so externally-facing attackers without any foothold cannot use it directly.

How urgent is it

This is a critical-severity flaw (rated 9.3 out of 10) in widely-used core Linux networking code, and it has drawn sustained press attention tied to a reported incident. While there is no confirmed evidence yet of it being exploited in real-world attacks, the severity, the ease of triggering it with local access, and the ongoing public attention mean you should treat this as high priority and patch promptly rather than wait.

What to do — in detail

Identify all Linux hosts and their running kernel versions, especially multi-tenant or shared-access systems where local-user attacks are most relevant.
Cross-reference against the fix commits: a464d950f31c89565cbaae038cc60db04fe01956, 0fab003ef4698e0628060a45a48f14b43ba2a40a, 94524b7b32691de2e2ce162c04d6cbc9106e5cbf, a65f69e0f38ffbed1b128029e09097436416dc8b, 32a8b0adaa18e52aaa0b246b5ffe683075354dbb, f3e835af6226f1dd12160cbd696081e9b6155de0, c3e33464746166264334641dc850a36eef7e6326, and 6785011f8b16abf40b1e9d8b2e332232b3e68822 — these correspond to different stable/LTS branches, so match the right one to your kernel version.
Update to the vendor-provided kernel package or distro release that backports these commits; there is no userspace workaround since this is a kernel-level defense-in-depth check on checksum_start validation.
Reboot after applying the kernel update — it will not take effect otherwise.
As a temporary mitigation while patching is scheduled, tighten local account access and restrict who can log in to or run processes on affected hosts, since exploitation requires local access.
No CISA KEV listing exists currently, so there is no mandated remediation deadline, but given the CVSS 9.3 score and sustained press/incident attention, treat this on an expedited internal patch timeline.
Monitor vendor kernel advisories and the referenced upstream fix commits for any follow-up hardening, since the patch notes mention 'a more complete validation at input is forthcoming.'

Technical context

CVSS 3.1 score 9.3 (AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H) reflects local attack vector, low complexity, no privileges or user interaction required, and high impact to confidentiality, integrity, and availability with a scope change. Weakness class is CWE-20 (improper input validation): skb_checksum_help fails to validate checksum_start before completing the checksum, allowing a crafted packet with bad checksum metadata to corrupt the network header. Not listed in CISA KEV, and EPSS places predicted exploitation likelihood in the low range (24th percentile) with a flat trend, consistent with no public exploit code and no confirmed in-the-wild activity reported in news coverage, though a Vercel-hosted demonstration/challenge environment reproduced the issue. Press attention is sustained due to an associated incident reference (actor: Armory Castle), but this is attention, not confirmed exploitation. Fix is upstream across multiple stable kernel branches via the listed commits; no public exploit maturity beyond disclosure/PoC-style reproduction exists at this time.

This is a general assessment based on public vulnerability data. It does not account for your specific infrastructure — when in doubt, consult a security specialist.

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS24th

CISA KEV

Not in the catalog. CISA has not confirmed exploitation.

Public exploits

No public exploit or proof of concept found in the sources we track.

EPSS

0.3% chance of exploitation activity in the next 30 days, which ranks it in the 24th percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Attention now

Sustained, driven by wide press coverage.

Lifecycle

9 events over 7 days, from the signal feeds we watch.

2026-10-22OpenVAS check added
2026-10-19Record updatedrecord updated
2026-10-16Record updated
2026-10-15Publishedweakness classified, att&ck mapped, record updated, record updated

Affected products

Linuxoss-projectaka the linux kernel
- LinuxOSOn-premOperating Systems / linux-distro

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Scored 9.3 by NVD.

How it is reached

Attack Vector LocalRequires local access to the vulnerable system (e.g. local login, malicious file)
Attack Complexity LowNo special conditions — the attack can be reliably reproduced
Privileges Required NoneNo authentication required — anyone can exploit this
User Interaction NoneNo user interaction needed — fully automated exploitation

Scope

Scope ChangedThe exploit can affect other components (e.g. sandbox escape, host compromise from VM)

Impact if exploited

Confidentiality HighTotal information disclosure — all data in the component is compromised
Integrity HighTotal loss of integrity — attacker can modify any data in the component
Availability HighTotal denial of service — the component is completely unavailable