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

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, 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-2026-23391 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

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