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PatchSiren cyber security CVE debrief

CVE-2025-21756 Siemens CVE debrief

CVE-2025-21756 is a medium-severity local availability issue associated in Siemens' CSAF advisory with specific SIMATIC S7-1500 CPU MFP and SIPLUS CPU variants. The CVE description points to a vsock binding-preservation flaw, while Siemens' remediation notes say no fix is currently available, so defenders should rely on access restriction and trusted-source controls for the affected GNU/Linux subsystem.

Vendor
Siemens
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
2025-06-10
Original CVE updated
2026-05-14
Advisory published
2025-06-10
Advisory updated
2026-05-14

Who should care

OT and ICS teams responsible for the listed Siemens SIMATIC S7-1500 CPU 1518/1518F MFP variants, especially environments that expose the additional GNU/Linux subsystem or allow local application execution on the device.

Technical summary

The supplied CVE description says vsock bindings should be preserved until socket destruction, including sockets that were explicitly bound or implicitly autobound during connect(). In the Siemens advisory context, the affected products are five SIMATIC S7-1500 CPU variants. The supplied CVSS 3.1 vector (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) indicates a local attacker with low privileges could trigger a high availability impact, with no confidentiality or integrity impact indicated. Siemens' advisory lists no available fix and recommends limiting interactive shell access of the additional GNU/Linux subsystem to trusted personnel and only building or running applications from trusted sources.

Defensive priority

Elevated for affected OT assets because Siemens lists no fix and the issue can cause high availability impact on devices in the advisory scope.

Recommended defensive actions

  • Confirm whether any of the five Siemens product variants listed in the advisory are deployed in your environment.
  • Restrict interactive shell access to the additional GNU/Linux subsystem to trusted personnel only.
  • Allow only applications from trusted sources to be built and run on affected systems.
  • Review Siemens advisory SSA-082556 and CISA advisory ICSA-25-162-05 for updates or future remediation guidance.
  • Apply local-access and privilege-minimization controls consistent with CISA ICS recommended practices while no vendor fix is available.
  • Validate backup, recovery, and operational fallback plans for availability-sensitive OT deployments.

Evidence notes

This debrief is based only on the supplied CISA CSAF source item and its referenced Siemens advisory. The source maps CVE-2025-21756 to five affected Siemens products: SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0), 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0), 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0), 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0), and SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0). The remediation section explicitly states that currently no fix is available and recommends restricting shell access and using trusted sources. No KEV entry is included in the supplied enrichment.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-21756 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-21756

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

  • CVE-2025-21756 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-21756

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

Supplemental references

  • Source item URL

    Unverified legacy reference

    URL: https://raw.githubusercontent.com/cisagov/CSAF/develop/csaf_files/OT/white/2025/icsa-25-162-05.json

    cisa_csaf

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/csaf/ssa-082556.json

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/html/ssa-082556.html

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-advisories/icsa-25-162-05

    Reference

  • Source reference

    Unverified legacy reference

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

    Reference

  • Source reference

    Unverified legacy reference

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

    Reference

  • Source reference

    Unverified legacy reference

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

    Reference

  • Source reference

    Unverified legacy reference

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

    Reference

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.