# CVE-2026-23391: Linux Kernel netfilter use-after-free in xt_CT template removal

Source: https://junglewise.ai/threats/cve-2026-23391-linux-kernel-netfilter-use-after-free-in-xt-ct-template-removal  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-25. Updated: 2026-07-14  
License: CC BY 4.0 with a link back

CVE-2026-23391 · Severity: high · CVSS 7.8 · Published 2026-03-25

A vulnerability exists in the Linux kernel's networking subsystem, specifically within the netfilter component used for firewalling and connection tracking. Under certain conditions, network packets held in a queue can reference internal objects that have been removed, potentially leading to system instability or unauthorized access. This issue affects Linux-based systems and specific Siemens industrial controllers that utilize the affected kernel versions.

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-2026-23391 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-23391 being exploited in the wild.

**How severe is CVE-2026-23391?** High severity, with a CVSS score of 7.8, published 25 March 2026.

**Which products and versions are affected?** 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 V3.1.5, V3.1.6.

**Is there a fix for CVE-2026-23391?** 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-2026-23391 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2026-23391?** 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 Linux kernel netfilter xt_CT target arises from improper handling of connection tracking templates. When a template rule is removed, packets sitting in the nfqueue may still hold references to objects that are being destroyed, such as helpers (during module removal) or timeout policies (via nfnetlink_cttimeout). This creates a use-after-free condition. The fix involves flushing enqueued packets when the template rule is destroyed. An attacker with local access could potentially exploit this to achieve privilege escalation or cause a denial of service. The vulnerability is addressed in multiple stable kernel branches.

## Affected products

- Linux Linux Kernel
- 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 V3.1.5, V3.1.6

## Timeline

- 2026-03-12: disclosed: Initial patch authored by Pablo Neira Ayuso
- 2026-03-25: advisory: CVE published and patches committed to stable branches

## References

- [https://git.kernel.org/stable/c/19a230dec6bb8928e3f96387f9085cf2c79bcef9](https://git.kernel.org/stable/c/19a230dec6bb8928e3f96387f9085cf2c79bcef9)
- [https://git.kernel.org/stable/c/55445134d42b84cb0a272e42c98d233ca65eca83](https://git.kernel.org/stable/c/55445134d42b84cb0a272e42c98d233ca65eca83)
- [https://git.kernel.org/stable/c/63b8097cea1923fe82cd598068d0796da8c015ec](https://git.kernel.org/stable/c/63b8097cea1923fe82cd598068d0796da8c015ec)
- [https://git.kernel.org/stable/c/777d02efe3d630cca4c1b63962cec17c57711325](https://git.kernel.org/stable/c/777d02efe3d630cca4c1b63962cec17c57711325)
- [https://git.kernel.org/stable/c/cb549925875fa06dd155e49db4ac2c5044c30f9c](https://git.kernel.org/stable/c/cb549925875fa06dd155e49db4ac2c5044c30f9c)
- [https://git.kernel.org/stable/c/cc57506dd66555899560b9c0f24e813f034e12ec](https://git.kernel.org/stable/c/cc57506dd66555899560b9c0f24e813f034e12ec)
- [https://git.kernel.org/stable/c/d2d0bae0c9a2a17b6990a2966f5cdce0813d6256](https://git.kernel.org/stable/c/d2d0bae0c9a2a17b6990a2966f5cdce0813d6256)

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