CVE-2025-39686 Cert Portal CVE debrief

Who should care

Administrators and operators of Siemens SIMATIC CN 4100 appliances covered by ICSA-26-134-10, especially systems running versions earlier than 5.0. Linux/embedded platform teams should also review any deployment that relies on the comedi subsystem’s INSN_BITS-to-INSN_READ/WRITE emulation path.

Technical summary

The vulnerable helper insn_rw_emulate_bits() is used as a fallback when a subdevice implements INSN_BITS but not INSN_READ or INSN_WRITE. The bug is that the helper only emulated one sample, while the instruction contract allows multiple samples through insn->n. For INSN_READ, the comedi core may copy insn->n samples back to user space, so under multi-sample requests the helper’s incomplete handling could surface uninitialized data handling problems and KASAN kernel-infoleak warnings. The advisory’s CVSS vector rates the issue as local, low-complexity, low-privilege, and availability-high.

Defensive priority

Moderate to high for affected Siemens/embedded Linux environments: plan patching in the next normal maintenance window, and accelerate if the device is exposed to routine local user access or if the comedi path is in active use.

Recommended defensive actions

• Update affected Siemens SIMATIC CN 4100 systems to V5.0 or later, per the Siemens remediation guidance.
• Verify whether the device or embedded image is actually affected by the advisory mapping before scheduling maintenance.
• Inventory Linux kernel builds and confirm whether the comedi subsystem is present and used on deployed systems.
• Treat any local-privilege access on affected systems as higher risk until remediation is complete.
• Use the official Siemens and CISA advisory references to validate product applicability and remediation steps.

Evidence notes

Primary evidence comes from the CISA CSAF advisory ICSA-26-134-10 and the linked Siemens ProductCERT advisory SSA-032379, both of which describe the comedi insn_rw_emulate_bits() sample-count mismatch and the vendor remediation to update to V5.0 or later. The supplied source also provides the CVSS v3.1 vector AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. The vendor/product mapping in the source is marked low confidence and needs review, so product applicability should be confirmed against the vendor advisory before broad rollout.

Official resources