CVE-2025-39684 Cert Portal CVE debrief

Who should care

Linux kernel maintainers, downstream distributors, and operators of products that include the affected comedi code path. Because the source corpus also associates the CVE with Siemens SIMATIC CN 4100 <5.0, Siemens customers should verify the advisory applicability and update guidance in their environment.

Technical summary

The vulnerability is an uninitialized-memory information leak in comedi ioctl handling, specifically do_insn_ioctl() and do_insnlist_ioctl(). A kernel buffer is allocated for insn->n unsigned-int samples, but some instruction handlers initialize fewer than insn->n entries before the data is copied back to user space. The advisory calls out insn_rw_emulate_bits() and vm80xx_ai_insn_read() as examples of handlers that may leave trailing buffer contents untouched. The fix is to ensure the unused portions of the allocated buffer are zeroed before each instruction is processed, rather than relying on every handler to fully populate the buffer.

Defensive priority

Medium. The supplied CVSS is 5.5/Medium and the vector indicates local access with low privileges. Prioritize patching during normal maintenance, sooner on shared systems, multi-user appliances, or any environment where local users are not fully trusted.

Recommended defensive actions

• Apply the vendor fix or upstream kernel patch that zeros uninitialized buffer regions before do_insn_ioctl() and do_insnlist_ioctl() processing.
• If you manage Siemens products covered by the advisory, update to V5.0 or later as directed in the supplier remediation.
• Inventory systems that include the Linux comedi subsystem or the affected vendor firmware/software branch and confirm whether the advisory applies.
• Restrict local access and enforce least privilege until remediation is complete, since exploitation requires local user access per the supplied CVSS vector.
• Track the linked Siemens and CISA advisories for any clarification of affected product scope or follow-on updates.

Evidence notes

Source timing comes from the supplied CVE record and CISA CSAF: published 2026-05-12, modified 2026-05-14. The advisory text states that syzbot reported a KMSAN kernel-infoleak in do_insn_ioctl(), with a similar report for do_insnlist_ioctl(). The corpus explicitly says the fix is to zero uninitialized parts of the allocated buffer before handling each instruction. The supplied metadata also lists Siemens SIMATIC CN 4100 vers:intdot/<5.0 and remediation to update to V5.0 or later, but the vulnerability description itself is for Linux kernel comedi; treat that product mapping as low-confidence and review against the vendor advisory.

Official resources