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PatchSiren cyber security CVE debrief

CVE-2025-21835 Linux CVE debrief

This CVE addresses an incorrect length field in USB MIDI Streaming descriptors within the Linux kernel's USB gadget subsystem. The vulnerability exists in the f_midi driver, which is responsible for implementing USB MIDI functionality when a Linux system operates as a USB device (gadget mode). The descriptor length miscalculation could lead to out-of-bounds memory access or parsing errors when a host system interacts with the USB MIDI gadget. Siemens has identified this vulnerability as affecting the GNU/Linux subsystem of their SIMATIC S7-1500 TM MFP industrial control product. The vulnerability was published on April 9, 2024, and the advisory has been updated multiple times through September 2025 to include additional related CVEs. As of the latest advisory revision, no patch is available from Siemens for this specific product. The vulnerability requires local access with low privileges to exploit, and successful exploitation could result in high availability impact (denial of service) on the affected system.

Vendor
Linux
Product
SIMATIC S7-1500 TM MFP - GNU/Linux subsystem
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2024-04-09
Original CVE updated
2026-05-14
Advisory published
2024-04-09
Advisory updated
2026-05-14

Who should care

Organizations operating Siemens SIMATIC S7-1500 TM MFP systems with the GNU/Linux subsystem enabled should prioritize this vulnerability. System integrators and OT security teams responsible for industrial control system security should assess exposure, particularly where USB gadget functionality may be utilized. Asset owners in critical infrastructure sectors using affected Siemens products should implement recommended mitigations and monitor for patch availability.

Technical summary

The f_midi driver in the Linux USB gadget subsystem incorrectly calculates MIDI Streaming descriptor lengths. This miscalculation can cause memory safety issues when the USB gadget presents MIDI functionality to a host system. The vulnerability is classified as CWE-20 (Improper Input Validation). The CVSS 3.1 score of 5.5 reflects local attack requirements with high availability impact but no confidentiality or integrity impact. The affected product is the GNU/Linux subsystem within Siemens SIMATIC S7-1500 TM MFP, an industrial automation platform. No vendor patch is currently available; mitigation relies on access controls and trusted application execution.

Defensive priority

medium

Recommended defensive actions

  • Limit access to the interactive shell of the additional GNU/Linux subsystem to trusted personnel only
  • Only build and run applications from trusted sources
  • Monitor for future Siemens security advisories for patch availability
  • Apply defense-in-depth strategies for industrial control systems per CISA guidance
  • Review and implement ICS-CERT recommended practices for securing embedded Linux subsystems

Evidence notes

The vulnerability description indicates this is a Linux kernel USB gadget driver issue (f_midi) affecting MIDI Streaming descriptor lengths. The CVSS vector confirms local attack vector with low attack complexity and low privileges required. The advisory explicitly states 'Currently no fix is available' for the affected Siemens product. The revision history shows this CVE was added in 'Additional Release 6' on June 10, 2025, indicating it was among 63 CVEs added in that update.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-21835 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-21835

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

  • CVE-2025-21835 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-21835

    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/2024/icsa-24-102-01.json

    cisa_csaf

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/csaf/ssa-265688.json

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://cert-portal.siemens.com/productcert/html/ssa-265688.html

    Reference

  • Source reference

    Unverified legacy reference

    URL: https://www.cisa.gov/news-events/ics-advisories/icsa-24-102-01

    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.