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PatchSiren cyber security CVE debrief

CVE-2024-35910 Siemens CVE debrief

A use-after-free vulnerability exists in the Linux kernel's TCP timer handling for kernel sockets. When TCP sockets are closed, the inet_csk_clear_xmit_timers() function uses del_timer() to stop timers, which allows ongoing timers to complete asynchronously. For user sockets, this is safe because timers hold references on the socket and the socket holds a reference on the network namespace. However, kernel sockets do not hold references on the network namespace, creating a race condition where the namespace may be freed before timer completion. This vulnerability affects Siemens SIMATIC S7-1500 TM MFP devices through their GNU/Linux subsystem. The issue was resolved by introducing inet_csk_clear_xmit_timers_sync() using sk_stop_timer_sync() to ensure synchronous timer termination before kernel socket release.

Vendor
Siemens
Product
SIMATIC S7-1500 TM MFP - GNU/Linux subsystem
CVSS
MEDIUM 5.8
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 devices with the GNU/Linux subsystem enabled, particularly in industrial control system (ICS/OT) environments where network namespace manipulation or kernel socket usage occurs. System administrators responsible for Linux kernel security in embedded industrial devices should prioritize access controls until patches are available.

Technical summary

The vulnerability stems from asynchronous timer termination in the Linux kernel's TCP implementation. The inet_csk_clear_xmit_timers() function uses sk_stop_timer() (del_timer()) which does not wait for running timers to complete. For kernel sockets, which lack network namespace references, this creates a window where the netns can be dismantled while timers are still executing. The fix introduces inet_csk_clear_xmit_timers_sync() using sk_stop_timer_sync() to block until timer completion, ensuring proper synchronization during kernel socket teardown in netns exit handlers.

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 vendor security updates from Siemens for patch availability

Evidence notes

The vulnerability description indicates this was discovered through syzbot reports and reproduced by Josef Bacik. The fix was implemented in the Linux kernel TCP subsystem. Siemens has identified this as affecting the GNU/Linux subsystem of SIMATIC S7-1500 TM MFP devices. The CVSS 3.1 vector (AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H) indicates local attack vector, high attack complexity, low privileges required, no user interaction, with impacts to confidentiality, integrity, and high availability impact.

Official resources

2024-04-09