CVE-2025-21923 is a high-severity use-after-free vulnerability in the Linux kernel's hid-steam driver, which handles Human Interface Devices (HID) related to Steam hardware. The flaw arises during the removal of a hid-steam device, where the cleanup process for the client_hdev structure—used to intercept hidraw access—is improperly ordered. Specifically, deferred work intended to reattach the input device is scheduled before the client_hdev is fully cleaned up. Although the deferred work is canceled, this cancellation occurs prior to the cleanup of client_hdev, resulting in the deferred work being rescheduled and causing a use-after-free condition. This vulnerability is classified under CWE-416 (Use After Free), which can lead to memory corruption. Exploiting this flaw could allow an attacker with local privileges to execute arbitrary code with kernel-level permissions, potentially compromising system confidentiality, integrity, and availability. The vulnerability requires low privileges (local access) but no user interaction, and the attack vector is local (AV:L). The affected Linux kernel versions include specific commits identified by their hashes, indicating that multiple recent kernel versions are impacted. No known exploits are currently reported in the wild, but the high CVSS score (7.8) reflects the serious risk posed by this vulnerability if exploited. The patch corrects the order of cleanup operations to ensure deferred work is properly canceled after client_hdev is cleaned up, preventing the use-after-free condition.

For European organizations, this vulnerability poses a significant risk, especially for those relying on Linux-based systems with the hid-steam driver enabled. Potential impacts include unauthorized privilege escalation, allowing attackers to gain kernel-level control, which can lead to full system compromise, data theft, or disruption of critical services. Sectors such as finance, government, healthcare, and critical infrastructure that use Linux servers or workstations could face severe operational and reputational damage. Since the vulnerability requires local access, insider threats or attackers who have gained initial footholds could leverage this flaw to escalate privileges. The compromise of confidentiality, integrity, and availability could affect sensitive personal data protected under GDPR, leading to regulatory penalties. Additionally, Linux is widely used in embedded systems and IoT devices across Europe, which may also be affected if they include the vulnerable driver, potentially impacting industrial control systems and other critical environments.

European organizations should promptly apply the official Linux kernel patches that fix this vulnerability by correcting the cleanup order in the hid-steam driver. System administrators must verify that all affected Linux kernel versions are updated to patched releases. For environments where immediate patching is not feasible, disabling or unloading the hid-steam driver can serve as a temporary mitigation to prevent exploitation. Organizations should also enforce strict local access controls and monitor for unusual activity indicative of privilege escalation attempts. Employing kernel integrity monitoring and leveraging security modules like SELinux or AppArmor can help detect and prevent exploitation. Regular vulnerability scanning and maintaining an up-to-date asset inventory to identify affected systems are critical. Finally, educating users and administrators about the risks of local privilege escalation vulnerabilities and enforcing the principle of least privilege can reduce the attack surface.