CVE-2025-21656 is a vulnerability identified in the Linux kernel, specifically within the hardware monitoring (hwmon) subsystem's drivetemp driver. The issue arises from improper handling of error codes returned by the scsi_execute_cmd() function, which is responsible for executing SCSI commands to storage devices. This function can return both negative Linux error codes and positive error codes embedded in the scsi_cmnd result field. The vulnerable driver incorrectly passes these error codes directly to the hwmon core, which only checks for negative error codes. Consequently, when a SCSI error occurs—such as a disk drive disconnection—the driver may report uninitialized or garbage data to userspace. This can lead to misleading or incorrect hardware monitoring information being presented to system administrators or automated monitoring tools. The patch for this vulnerability modifies the driver to check the output of scsi_execute_cmd() and return a standardized negative error code (-EIO) if the error code is positive, thereby preventing the propagation of invalid data. The vulnerability does not appear to have known exploits in the wild and affects specific Linux kernel versions identified by commit hashes. The root cause is a logic flaw in error code handling within the hwmon drivetemp driver, which could impact the reliability of hardware monitoring data but does not directly allow code execution or privilege escalation.

For European organizations, the primary impact of CVE-2025-21656 lies in the potential degradation of hardware monitoring accuracy on Linux systems using the affected kernel versions. Organizations relying on Linux servers for critical infrastructure, data centers, or cloud services may receive incorrect temperature or hardware status readings, which could delay detection of actual hardware failures or lead to unnecessary maintenance actions. This could indirectly affect system availability and operational efficiency. While the vulnerability does not directly compromise confidentiality or integrity, the reliability of monitoring data is crucial for proactive system management. In sectors such as finance, healthcare, manufacturing, and telecommunications—where Linux servers are prevalent—misleading hardware status could increase operational risks. However, since exploitation does not appear to allow arbitrary code execution or privilege escalation, the threat is primarily to system monitoring fidelity rather than direct system compromise.

To mitigate this vulnerability, European organizations should promptly apply the Linux kernel patch that corrects error code handling in the hwmon drivetemp driver. Specifically, updating to a kernel version that includes the fix or recompiling the kernel with the patch is recommended. Organizations should audit their Linux systems to identify those running affected kernel versions (noted by the provided commit hashes) and prioritize patching on critical infrastructure. Additionally, monitoring tools that rely on hwmon data should be configured to validate sensor readings and flag anomalous or out-of-range values, reducing the risk of acting on corrupted data. Implementing redundancy in hardware monitoring, such as cross-verifying temperature data with alternative sensors or external monitoring solutions, can further enhance reliability. Finally, maintaining robust incident response procedures to investigate unusual hardware alerts will help mitigate operational impacts stemming from this vulnerability.