CVE-2025-21916 is a vulnerability identified in the Linux kernel specifically affecting the USB ATM (Asynchronous Transfer Mode) driver component, cxacru. The issue arises from improper endpoint verification during USB device binding in the cxacru_bind() function. The vulnerability was detected by Syzbot, an automated kernel fuzzing tool, which reported a mismatch in USB Request Block (URB) transfer pipe types, indicating a flaw in how USB endpoints were checked. The root cause is that the existing endpoint validation relied on usb_find_common_endpoints(), which only verifies the presence and type of endpoints but does not sufficiently validate the endpoint addresses. Since cxacru_bind() fills URBs with a specific endpoint address (CXACRU_EP_CMD), the lack of address verification could lead to incorrect URB submissions to inappropriate endpoints. This flaw could cause kernel warnings, potential memory corruption, or undefined behavior in the USB subsystem, possibly leading to system instability or denial of service. The fix involved switching to usb_check_XXX_endpoints(), which performs more thorough endpoint address verification, ensuring that only valid endpoints are used for URB submissions. The vulnerability affects multiple Linux kernel versions as identified by various commit hashes, and it was publicly disclosed in April 2025. No known exploits are reported in the wild yet, and no CVSS score has been assigned.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with USB ATM cxacru drivers enabled. The impact includes potential system crashes or denial of service due to kernel warnings or memory corruption triggered by malformed or malicious USB devices. This could disrupt critical infrastructure or enterprise environments relying on Linux-based systems, especially those using USB ATM devices or similar hardware. Confidentiality and integrity impacts are limited since the vulnerability relates to endpoint validation rather than direct code execution or privilege escalation. However, availability could be affected if attackers exploit this flaw to cause kernel panics or system instability. Organizations with embedded Linux devices, network equipment, or industrial control systems using affected drivers are particularly at risk. Although no exploits are known, the ease of triggering the flaw via USB device interaction suggests a moderate risk of exploitation, especially in environments where physical access or USB device insertion is possible.

European organizations should promptly update their Linux kernels to versions where this vulnerability is patched, ensuring the use of the fixed usb_check_XXX_endpoints() validation method. For systems where immediate patching is not feasible, organizations should implement strict USB device control policies, including disabling unused USB ports, enforcing device whitelisting, and monitoring USB device connections for anomalies. Additionally, kernel hardening techniques such as enabling kernel lockdown modes and using security modules (e.g., SELinux, AppArmor) can reduce the attack surface. Organizations should audit their hardware inventory to identify devices using the cxacru driver and assess exposure. Network segmentation and endpoint protection can help contain potential impacts. Finally, monitoring kernel logs for unusual USB-related warnings or errors can provide early detection of attempted exploitation.