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PatchSiren cyber security CVE debrief

CVE-2025-21959 Linux CVE debrief

The Linux kernel's netfilter component has a vulnerability due to incomplete initialization of the nf_conncount_tuple structure in the insert_tree function. This could potentially lead to uninitialized data being used, causing system instability or crashes. System administrators and security teams should assess their exposure and apply patches or updates as necessary to ensure the nf_conncount module is properly initialized. The vulnerability has a CVSS score of 5.5 and requires local access. Affected systems may experience potential issues if the nf_conncount module is not properly initialized.

Vendor
Linux
Product
SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0)
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-14
Original CVE updated
2026-07-28
Advisory published
2026-07-14
Advisory updated
2026-07-28

Who should care

System administrators and security teams responsible for Linux-based systems, especially those using the affected kernel versions, should assess their exposure and apply patches or updates as necessary.

Why it matters

CVE-2025-21959 is a medium-severity vulnerability in the Linux kernel's netfilter component. It requires local access and has a CVSS score of 5.5. System administrators and security teams should assess their exposure and apply patches or updates as necessary.

  • Verify and apply patches or updates to ensure the nf_conncount module is properly initialized.
  • Assess exposure and prioritize patching for Linux-based systems using the affected kernel versions.
  • Monitor system stability and potential crashes due to uninitialized data usage.

Technical summary

The Linux kernel's netfilter component has a vulnerability due to the incomplete initialization of the nf_conncount_tuple structure in the insert_tree function. This could potentially lead to uninitialized data being used, causing system instability or crashes. The vulnerability has a CVSS score of 5.5 and requires local access. The nf_conncount module did not fully initialize a data structure, leading to potential issues. System administrators and security teams should assess their exposure and apply patches or updates as necessary to ensure the nf_conncount module is properly initialized.

Defensive priority

Medium priority, as the vulnerability requires local access and has a CVSS score of 5.5.

Recommended defensive actions

  • Review and apply the provided patches or updates to ensure the nf_conncount module is properly initialized.
  • Limit access to the interactive shell of the additional GNU/Linux subsystem to trusted personnel only.
  • Only build and run applications from trusted sources.
  • Verify and apply patches or updates to ensure the nf_conncount module is properly initialized.
  • Assess exposure and prioritize patching for Linux-based systems using the affected kernel versions.
  • Monitor system stability and potential crashes due to uninitialized data usage.
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented.

Evidence notes

The vulnerability was introduced in the Linux kernel and affects the netfilter component. The issue arises from the incomplete initialization of the nf_conncount_tuple structure in the insert_tree function. This could potentially lead to uninitialized data being used, causing system instability or crashes. However, there is no evidence of exploitation or significant impact.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-21959 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-21959

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2025-21959 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-21959

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

  • CVE-2025-21959

    Unverified legacy reference

    URL: https://raw.githubusercontent.com/cisagov/CSAF/develop/csaf_files/OT/white/2026/icsa-26-209-04.json

    cisa_csaf

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/csaf/ssa-019113.json

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/html/ssa-019113.html

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-advisories/icsa-26-209-04

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-alerts/ics-alert-10-301-01

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/resources-tools/resources/ics-recommended-practices

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/topics/industrial-control-systems

    Supplemental source

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/sites/default/files/recommended_practices/NCCIC_ICS-CERT_Defense_in_Depth_2016_S508C.pdf

    Supplemental source

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.