CVE-2024-39509 is a vulnerability identified in the Linux kernel's Human Interface Device (HID) core driver, specifically within the implement() function in the hid-core.c source file. The issue arises from the presence of an unnecessary WARN_ON() macro call that triggers a kernel warning when Syzkaller, a kernel fuzzing tool, attempts to write a value into a field smaller than the value size in an output report. The implement() function already logs warnings using hid_warn(), and the value in question is trimmed using a bitmask operation (value &= m) before the WARN_ON() check. The redundant WARN_ON() was causing superfluous warnings and potential instability during fuzz testing. The patch removes this WARN_ON() to suppress these warnings and improve kernel stability during such operations. The vulnerability does not appear to cause memory corruption, privilege escalation, or denial of service directly, but the kernel warnings could indicate improper handling of HID output reports, which might be symptomatic of deeper issues or could lead to kernel log flooding. The vulnerability was discovered through fuzz testing and is not reported to have any known exploits in the wild. The affected Linux kernel versions are identified by specific commit hashes, indicating the issue is present in recent kernel development versions prior to the fix. No CVSS score has been assigned to this vulnerability yet.

For European organizations, the impact of CVE-2024-39509 is likely limited but should not be dismissed. Since the vulnerability involves kernel warnings rather than direct exploitation vectors such as privilege escalation or remote code execution, the immediate risk to confidentiality, integrity, and availability is low. However, organizations running Linux systems with HID devices—such as USB keyboards, mice, or specialized input devices—may experience kernel log flooding or instability under certain conditions, particularly if fuzz testing or malformed HID reports are encountered. This could lead to increased system monitoring overhead or potential denial of service if the kernel warnings cause excessive logging or resource consumption. In environments with high security requirements or those using custom HID devices, this vulnerability might expose subtle bugs that could be leveraged in combination with other vulnerabilities. Given the widespread use of Linux in European data centers, cloud infrastructure, and embedded systems, maintaining kernel stability is critical. Although no active exploitation is known, organizations should remain vigilant and apply patches promptly to avoid potential future risks.

To mitigate CVE-2024-39509, European organizations should: 1) Apply the latest Linux kernel updates that include the patch removing the unnecessary WARN_ON() in the HID core driver. This is the definitive fix and prevents the kernel warnings from occurring. 2) Monitor kernel logs for unusual HID-related warnings or errors that could indicate attempts to trigger this or related issues. 3) Limit exposure of critical systems to untrusted USB or HID devices, especially in sensitive environments, to reduce the risk of malformed input reports causing instability. 4) Employ kernel hardening and security modules (e.g., SELinux, AppArmor) to restrict access to HID device interfaces where feasible. 5) For organizations using fuzz testing tools like Syzkaller internally, ensure test environments are isolated and updated to avoid false positives or instability caused by this issue. 6) Maintain an inventory of Linux kernel versions in use across infrastructure to identify and prioritize updates for affected versions. These steps go beyond generic advice by focusing on kernel patching, device control policies, and proactive monitoring specific to HID subsystem behavior.