# CVE-2026-31671: Linux Kernel information leak in xfrm_user build_report

Source: https://junglewise.ai/threats/cve-2026-31671-linux-kernel-information-leak-in-xfrm-user-build-report  
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-2026-31671 · Severity: medium · CVSS 5.5 · Published 2026-04-24

A security vulnerability has been identified in the Linux kernel's networking subsystem, specifically affecting how it handles IPsec (XFRM) reports. This flaw allows small amounts of sensitive kernel memory to leak to user-level applications because certain internal data structures were not properly cleared before being shared. While this specific bug is primarily noted for causing potential stability issues or minor data exposure, it represents a weakness in how the system protects private kernel information from ordinary users.

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-2026-31671 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-31671 being exploited in the wild.

**How severe is CVE-2026-31671?** Medium severity, with a CVSS score of 5.5, published 24 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 V3.1.5, V3.1.6.

**Is there a fix for CVE-2026-31671?** A patch was published on 2026-04-06: Initial patch authored by Greg Kroah-Hartman Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-31671 attacked?** It is attacked with local access, in an operating system.

**Am I affected by CVE-2026-31671?** 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

An information leak vulnerability exists in the Linux kernel's net/xfrm/xfrm_user.c component. The 'xfrm_user_report' structure contains a single-byte 'proto' field followed by an 'xfrm_selector' structure, resulting in three bytes of uninitialized padding. Because this structure is copied to userspace without being zeroed (e.g., via memset), sensitive kernel stack or heap data residing in those padding bytes can be disclosed to local users. The fix involves explicitly zeroing the structure before populating its members. This affects various Linux stable branches and integrated products like Siemens SIMATIC S7-1500 MFP CPUs that utilize the Linux kernel.

## 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 V3.1.5, V3.1.6

## Timeline

- 2026-04-06: patched: Initial patch authored by Greg Kroah-Hartman
- 2026-04-24: advisory: CVE published to NVD
- 2026-07-14: other: Siemens advisory added to record

## References

- [https://git.kernel.org/stable/c/0616314b3b34f24cbb91da8c6bd8bcdc4c8592f9](https://git.kernel.org/stable/c/0616314b3b34f24cbb91da8c6bd8bcdc4c8592f9)
- [https://git.kernel.org/stable/c/0a30dceb0e1f0c480d2482e6d7cebf8aebb6eb72](https://git.kernel.org/stable/c/0a30dceb0e1f0c480d2482e6d7cebf8aebb6eb72)
- [https://git.kernel.org/stable/c/6c55714c931051cd7f4839c19ce0867179fd22fe](https://git.kernel.org/stable/c/6c55714c931051cd7f4839c19ce0867179fd22fe)
- [https://git.kernel.org/stable/c/716c546e88cfe49d841658240e10cb57bc50a2cc](https://git.kernel.org/stable/c/716c546e88cfe49d841658240e10cb57bc50a2cc)
- [https://git.kernel.org/stable/c/d10119968d0e1f2b669604baf2a8b5fdb72fa6b4](https://git.kernel.org/stable/c/d10119968d0e1f2b669604baf2a8b5fdb72fa6b4)
- [https://git.kernel.org/stable/c/d27c02eec529f78055a46a5c9e6c62684382b2d8](https://git.kernel.org/stable/c/d27c02eec529f78055a46a5c9e6c62684382b2d8)
- [https://git.kernel.org/stable/c/e0c8542c3d097ed4205ded51868195d5d6ddac62](https://git.kernel.org/stable/c/e0c8542c3d097ed4205ded51868195d5d6ddac62)

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