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PatchSiren cyber security CVE debrief

CVE-2025-21728 Siemens CVE debrief

CVE-2025-21728 is a medium-severity availability issue affecting Siemens SIMATIC S7-1500 CPU MFP variants that include an additional GNU/Linux subsystem. The source advisory maps the issue to Linux BPF signal handling: if a non-preemptible BPF program uses bpf_send_signal(), the call can sleep and trigger problems in that execution context. Siemens/CISA list no fix at publication time, so defenders should focus on restricting local access, limiting who can use the subsystem shell, and ensuring only trusted software is built and run there.

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/ICS operators, Siemens SIMATIC S7-1500 owners, plant engineers, and security teams responsible for devices running the additional GNU/Linux subsystem, especially where local users or custom applications are permitted.

Technical summary

The advisory describes a Linux BPF kfunc issue: bpf_send_signal() may sleep, which is incompatible with non-preemptible execution contexts. On the affected Siemens SIMATIC S7-1500 CPU 1518-family MFP products, the practical impact is availability-focused and locally reachable according to the supplied CVSS vector (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H). The source corpus does not identify a confidentiality or integrity impact, and it states that no fix is currently available for the listed products.

Defensive priority

Medium. Prioritize if the affected CPU runs third-party or custom applications in the GNU/Linux subsystem, or if multiple local users have access to the shell or application deployment path.

Recommended defensive actions

  • Inventory the listed affected Siemens products and confirm whether the additional GNU/Linux subsystem is enabled and in use.
  • Limit access to the interactive shell of the additional GNU/Linux subsystem to trusted personnel only.
  • Only build and run applications from trusted sources on the affected devices.
  • Apply Siemens and CISA advisory updates as they are released, since the source corpus states that no fix is currently available.
  • Reduce local privilege exposure and review who can execute or deploy BPF-related workloads in the subsystem.
  • Monitor the Siemens ProductCERT and CISA advisory pages for any new remediation guidance or firmware/software updates.

Evidence notes

The supplied CISA CSAF source item for ICSA-25-162-05 and Siemens SSA-082556 identify five affected SIMATIC S7-1500 CPU MFP product variants and state 'Currently no fix is available.' The advisory remediation guidance specifically recommends limiting interactive shell access and only running trusted-source applications. The CVSS vector is AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H, supporting a local, availability-focused impact assessment.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-21728 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-21728

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

  • CVE-2025-21728 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-21728

    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.