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

Source: https://junglewise.ai/threats/cve-2026-31665-linux-kernel-use-after-free-in-netfilter-nft-ct-timeout-object  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-24. Updated: 2026-07-14  
License: CC BY 4.0 with a link back

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](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). Vendors: [Linux](https://junglewise.ai/threats/vendors/linux), [Siemens](https://junglewise.ai/threats/vendors/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

- [https://git.kernel.org/stable/c/070abdf1b04325b21a20a2a0c39a2208af107275](https://git.kernel.org/stable/c/070abdf1b04325b21a20a2a0c39a2208af107275)
- [https://git.kernel.org/stable/c/aa7cfa16f98f8ec3e6d47c34e1a8c1ae4b9b8b77](https://git.kernel.org/stable/c/aa7cfa16f98f8ec3e6d47c34e1a8c1ae4b9b8b77)
- [https://git.kernel.org/stable/c/b42aca3660dc2627a29a38131597ca610dc451f9](https://git.kernel.org/stable/c/b42aca3660dc2627a29a38131597ca610dc451f9)
- [https://git.kernel.org/stable/c/c458fc1c278a65ad5381083121d39a479973ebed](https://git.kernel.org/stable/c/c458fc1c278a65ad5381083121d39a479973ebed)
- [https://git.kernel.org/stable/c/c581e5c8f2b59158f62efe61c1a3dc36189081ff](https://git.kernel.org/stable/c/c581e5c8f2b59158f62efe61c1a3dc36189081ff)
- [https://git.kernel.org/stable/c/d0983b48c10d1509fd795c155f8b1e832e1369ff](https://git.kernel.org/stable/c/d0983b48c10d1509fd795c155f8b1e832e1369ff)
- [https://git.kernel.org/stable/c/f16fe84879a5280f05ebbcea593a189ba0f3e79a](https://git.kernel.org/stable/c/f16fe84879a5280f05ebbcea593a189ba0f3e79a)

## Related threats

- [Linux IOMMU page table logic error in arm_lpae_split_blk_unmap](https://junglewise.ai/threats/cve-2026-56934-in-arm-lpae-split-blk-unmap-of-drivers-iommu-io-pgtable-arm)
- [Linux kernel null pointer dereference in TCP established lookup](https://junglewise.ai/threats/cve-2026-99288-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel xen-netback race condition in XenBus disconnect](https://junglewise.ai/threats/cve-2026-99286-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel shared-frag flag loss in skb_zerocopy](https://junglewise.ai/threats/cve-2026-99284-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)
- [Linux kernel netfilter conntrack TCP RST timeout handling](https://junglewise.ai/threats/cve-2026-99283-in-the-linux-kernel-the-following-vulnerability-has-been-resolved)