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PatchSiren cyber security CVE debrief

CVE-2025-21703 Siemens CVE debrief

CISA’s CSAF advisory for Siemens SIMATIC S7-1500 TM MFP - BIOS was published on 2025-03-11 and later revised on 2025-09-09. The supplied vulnerability description says the child qdisc backlog must be reduced before qdisc_tree_reduce_backlog() is called; otherwise parent notification can be missed, and in the DRR case that could lead to a use-after-free because qlen_notify() maintains the active list. At publication, the advisory listed no fix and only a limited workaround.

Vendor
Siemens
Product
SIMATIC S7-1500 TM MFP - BIOS
CVSS
HIGH 7
CISA KEV
Not listed in stored evidence
Original CVE published
2025-03-11
Original CVE updated
2025-09-09
Advisory published
2025-03-11
Advisory updated
2025-09-09

Who should care

Siemens customers using the affected SIMATIC S7-1500 TM MFP - BIOS, along with OT/ICS defenders responsible for tracking vendor advisories and compensating controls, should prioritize this issue.

Technical summary

The core issue described in the source is an ordering bug in traffic-control backlog handling: qdisc_tree_reduce_backlog() only notifies a parent qdisc when the child becomes empty, so the child backlog needs to be reduced first. If that order is wrong, qlen_notify() may not run; the advisory states that in DRR this disrupted maintenance of the active list and resulted in a use-after-free.

Defensive priority

High

Recommended defensive actions

  • Review Siemens advisory SSA-503939 and CISA ICSA-25-072-03 for the full affected-scope and status details.
  • Treat the affected product as a high-priority maintenance item and plan for vendor guidance updates, since the advisory states no fix was available at publication.
  • Apply the advisory’s stated workaround only from trusted sources, and validate any compensating controls during approved OT change windows.
  • Monitor for revised advisory information after 2025-09-09 and re-check the product’s exposure in your asset inventory.

Evidence notes

The source corpus ties CVE-2025-21703 to Siemens SIMATIC S7-1500 TM MFP - BIOS and gives a netem/qdisc description involving qdisc_tree_reduce_backlog(), qlen_notify(), DRR, and a resulting UAF. The advisory metadata states publication on 2025-03-11, revision on 2025-09-09, CVSS 3.1 vector AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H, no fix available, and a workaround limited to building/running applications from trusted sources. The corpus also contains a product/advisory context that appears broader than the technical description; this debrief follows the supplied sources without inferring beyond them.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-21703 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-21703

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

  • CVE-2025-21703 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-21703

    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-25-072-03.json

    cisa_csaf

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/csaf/ssa-503939.json

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/html/ssa-503939.html

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-advisories/icsa-25-072-03

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/uscert/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://us-cert.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.