CVE-2026-31665: Linux Kernel use-after-free in Netfilter nft_ct timeout object

CVE-2026-31665 · Severity: high · CVSS 7.8 · Published 2026-04-24

A vulnerability exists in the Linux kernel's networking subsystem (Netfilter) that could allow a local attacker to crash the system or potentially execute unauthorized code. The issue occurs when the system incorrectly handles the deletion of network timeout rules, leading to a memory error. This affects various Linux-based systems, including certain industrial controllers from Siemens, and could impact system stability and availability.

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

Quick answers

Is CVE-2026-31665 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-31665 being exploited in the wild.
How severe is CVE-2026-31665?
High severity, with a CVSS score of 7.8, published 24 April 2026.
Which products and versions are affected?
Linux Kernel Fixed in various stable branches including 5.10.y, 5.15.y, 6.1.y, 6.6.y, 6.8.y, 6.9.y; Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6.
Is there a fix for CVE-2026-31665?
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-2026-31665 attacked?
It is attacked with local access, in an operating system.
Am I affected by CVE-2026-31665?
Check whether you run Linux Kernel or Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP 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 use-after-free vulnerability exists in the nft_ct_timeout_obj_destroy() function within the Linux kernel's Netfilter subsystem. The root cause is that the timeout object is freed using kfree() immediately after nf_ct_untimeout() without waiting for an RCU grace period, while concurrent CPUs may still hold RCU-protected references. An attacker with local access can trigger this race condition during concurrent packet processing, leading to memory corruption (as seen in KASAN reports in nf_conntrack_tcp_packet). This can result in a kernel crash or arbitrary code execution. The fix involves adding an rcu_head to the nf_ct_timeout structure and using kfree_rcu() to defer freeing.

Affected products

  • Linux Linux Kernel Fixed in various stable branches including 5.10.y, 5.15.y, 6.1.y, 6.6.y, 6.8.y, 6.9.y
  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5, V3.1.6

Timeline

  • 2026-04-03: other: Patch authored
  • 2026-04-24: advisory: NVD publication date
  • 2026-07-14: other: Siemens advisory enrichment added to CVE record

References

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