# CVE-2026-31495: Linux Kernel undefined behavior in netfilter ctnetlink

Source: https://junglewise.ai/threats/cve-2026-31495-linux-kernel-undefined-behavior-in-netfilter-ctnetlink  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-22. Updated: 2026-07-14  
License: CC BY 4.0 with a link back

CVE-2026-31495 · Severity: medium · CVSS 5.5 · Published 2026-04-22

A vulnerability in the Linux kernel's networking subsystem could allow a local user to cause undefined system behavior or a crash. The issue exists in the component responsible for managing network connection states (conntrack), where certain configuration values were not properly restricted. This could lead to a denial-of-service, impacting the availability of affected systems, including certain Siemens industrial controllers that utilize the Linux kernel.

Technologies: [Linux Kernel](https://junglewise.ai/threats/technologies/kernel), [Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP](https://junglewise.ai/threats/technologies/simatic-s7-1500-cpu-1518-4-pn-dp-mfp), [Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP](https://junglewise.ai/threats/technologies/simatic-s7-1500-cpu-1518f-4-pn-dp-mfp), [Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP](https://junglewise.ai/threats/technologies/siplus-s7-1500-cpu-1518-4-pn-dp-mfp). Vendors: [Linux](https://junglewise.ai/threats/vendors/linux), [Siemens](https://junglewise.ai/threats/vendors/siemens).

## Quick answers

**Is CVE-2026-31495 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-31495 being exploited in the wild.

**How severe is CVE-2026-31495?** Medium severity, with a CVSS score of 5.5, published 22 April 2026.

**Which products and versions are affected?** Linux Kernel All versions prior to the April 2026 patches; Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6; Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP V3.1.5, V3.1.6; Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP V3.1.6.

**Is there a fix for CVE-2026-31495?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-31495 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2026-31495?** Check whether you run Linux Kernel, Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP, Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability in the ctnetlink component of the Linux kernel netfilter subsystem stems from insufficient validation of Netlink attributes. Specifically, CTA_PROTOINFO_TCP_WSCALE_ORIGINAL and REPLY attributes accepted values between 0-255, whereas the TCP protocol maximum is 14. When an out-of-range value is used as a shift count for a u32 variable, it results in undefined behavior. Additionally, CTA_PROTOINFO_TCP_STATE lacked strict range checking at the policy level. An attacker with local access to Netlink sockets could exploit this to cause kernel instability or a denial-of-service. The fix implements NLA_POLICY_MAX and NLA_POLICY_MASK annotations to ensure the Netlink core rejects invalid values before they reach the conntrack logic.

## Affected products

- Linux Linux Kernel All versions prior to the April 2026 patches
- Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6
- Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP V3.1.5, V3.1.6
- Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP V3.1.6

## Timeline

- 2026-03-25: other: Patch authored
- 2026-04-22: disclosed: CVE published
- 2026-07-14: advisory: Siemens advisory updated with affected industrial products

## References

- [https://git.kernel.org/stable/c/2ef71307c86a9f866d6e28f1a0c06e2e9d794474](https://git.kernel.org/stable/c/2ef71307c86a9f866d6e28f1a0c06e2e9d794474)
- [https://git.kernel.org/stable/c/435b576cd2faa75154777868f8cbb73bf71644d3](https://git.kernel.org/stable/c/435b576cd2faa75154777868f8cbb73bf71644d3)
- [https://git.kernel.org/stable/c/45c33e79ae705b7af97e3117672b6cd258dd0b1b](https://git.kernel.org/stable/c/45c33e79ae705b7af97e3117672b6cd258dd0b1b)
- [https://git.kernel.org/stable/c/4f7d25f3f0786402ba48ff7d13b6241d77d975f5](https://git.kernel.org/stable/c/4f7d25f3f0786402ba48ff7d13b6241d77d975f5)
- [https://git.kernel.org/stable/c/675c913b940488a84effdeeac5a1cfb657b59804](https://git.kernel.org/stable/c/675c913b940488a84effdeeac5a1cfb657b59804)
- [https://git.kernel.org/stable/c/8f15b5071b4548b0aafc03b366eb45c9c6566704](https://git.kernel.org/stable/c/8f15b5071b4548b0aafc03b366eb45c9c6566704)
- [https://git.kernel.org/stable/c/c6cb41eaae875501eaaa487b8db6539feb092292](https://git.kernel.org/stable/c/c6cb41eaae875501eaaa487b8db6539feb092292)

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