CVE-2025-21756 Siemens CVE debrief

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.

Official resources