# CVE-2026-99999: ipv4: validate checksum_start before completing checksum

> A flaw in the Linux kernel IPv4 stack allows corrupting network headers via bad checksum metadata, potentially leading to memory corruption or denial of service.

- **Severity:** Critical 9.3 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** AZL-108243, BELL-CVE-2026-99999, DEBIAN-CVE-2026-99999, UBUNTU-CVE-2026-99999

## What is CVE-2026-99999?

CVE-2026-99999 is a critical memory corruption vulnerability in Linux, published on 15 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2026-99999?


## How do I fix CVE-2026-99999?

Apply the vendor's patch.

## Is CVE-2026-99999 being exploited?

There is no known exploitation of CVE-2026-99999 in the wild and no public exploit code.

Public exploit code:

- PoC for CVE-2026-99999 (PwnEscape) on a prepared TAP. (github_poc): https://github.com/PeronGH/CVE-2026-99999
- CVE-2026-99999 PwnEscape PoC - Linux kernel IPv4 csum_start validation bug guest-to-host memory leak (CVSS 9.3) (github_poc): https://github.com/01luyicheng/CVE-2026-99999
- Restricted eBPF TC ingress guard, CO-RE detector, verified loader, and C11 VMM launch supervisor for CVE-2026-99999 (github_poc): https://github.com/MChorfa/csumguard
- cve-2026-99999-audit script (PwnEscape) (github_poc): https://github.com/Quaerendir/cve-2026-99999-audit
- CVE-2026-99999 (PwnEscape) reproduction: IPv4 csum_start validation bug — IHL rewrite and 32-byte kernel heap disclosure (github_poc): https://github.com/gandli/CVE-2026-99999-pwnescape
- kpatch livepatch build and load test for CVE-2026-99999 on Rocky Linux 8.10 (4.18.0-553.144.1.el8_10) (github_poc): https://github.com/nsrg-fmlorg/kpatch-rocky-cve-2026-99999

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99999 and fix it if it is affected.

What it is: CVE-2026-99999 is a critical memory corruption vulnerability in Linux, published on 15 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-99999 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-99999/ (Markdown: https://insights.sbomify.com/CVE-2026-99999/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99999
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99999
- OSV: https://osv.dev/vulnerability/CVE-2026-99999

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99999/
Summaries are generated and may contain mistakes; the official description above is the record.