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

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, Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP, Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP, Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP. Vendors: Linux, 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

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