CVE-2025-21840 is a vulnerability identified in the Linux kernel's thermal subsystem, specifically related to the netlink interface used for communicating thermal events between kernel space and userspace. The issue originated from a kernel commit (1773572863c4) that introduced a new element into the middle of the enum thermal_genl_attr, which defines attributes for the thermal generic netlink interface. This change altered the raw values of existing attributes, notably THERMAL_GENL_ATTR_CPU_CAPABILITY, which is used by the intel-lpmd userspace tool to receive hardware feedback interface (HFI) events. Because intel-lpmd continued to use the old attribute values, it encountered segmentation faults due to the mismatch. The root cause is the insertion of a new enum element in the middle of the list, which shifted the values of subsequent attributes, breaking backward compatibility and causing userspace tools relying on the previous enum layout to malfunction. The vulnerability does not directly allow code execution or privilege escalation but can cause userspace applications that monitor thermal events to crash, potentially impacting system monitoring and management. The fix involved moving the newly introduced attribute to the end of the enum, preserving the original attribute values and maintaining compatibility with existing userspace tools. This change prevents segmentation faults by ensuring that userspace tools like intel-lpmd receive consistent attribute values as expected. No known exploits are reported in the wild, and the vulnerability primarily affects systems running the specific kernel commit before the fix was applied. The vulnerability highlights the risks of modifying kernel interface enums in a non-backward-compatible manner, which can lead to stability issues in dependent userspace applications.

For European organizations, the impact of CVE-2025-21840 is primarily related to system stability and reliability rather than direct security breaches. Organizations relying on Linux systems for critical infrastructure, data centers, or industrial control systems that use thermal monitoring tools like intel-lpmd may experience application crashes or failures in thermal event reporting. This can degrade the ability to monitor CPU thermal states effectively, potentially leading to overheating or hardware damage if thermal events are not properly handled or logged. In sectors such as manufacturing, energy, telecommunications, and cloud services, where Linux is widely deployed, such disruptions could affect operational continuity and hardware lifespan. However, since the vulnerability does not enable privilege escalation or remote code execution, the confidentiality and integrity of data are not directly threatened. The main risk is reduced availability of thermal monitoring tools and possible increased maintenance overhead to update or patch affected systems. Organizations with strict uptime requirements or those operating in harsh environments where thermal management is critical should prioritize addressing this vulnerability to avoid unexpected system behavior.

To mitigate CVE-2025-21840, European organizations should: 1) Ensure that Linux kernel versions deployed are updated to include the fix that moves the THERMAL_GENL_ATTR_TZ_PREV_TEMP attribute to the end of the enum thermal_genl_attr, preserving attribute value consistency. 2) Update userspace tools such as intel-lpmd to versions compatible with the fixed kernel interface, or verify that these tools correctly check the THERMAL_GENL_VERSION and adapt attribute usage accordingly. 3) Implement rigorous testing of kernel updates in staging environments to detect any regressions or compatibility issues with thermal monitoring tools before production deployment. 4) Monitor system logs for segmentation faults or crashes related to thermal netlink events to identify affected systems promptly. 5) For critical infrastructure, consider deploying fallback monitoring or alerting mechanisms independent of the thermal netlink interface to maintain thermal oversight during remediation. 6) Maintain close coordination between kernel and userspace package updates to avoid mismatches in interface expectations. These steps go beyond generic patching advice by emphasizing compatibility checks, proactive monitoring, and fallback strategies tailored to thermal event handling.