CVE-2021-46906 is a vulnerability identified in the Linux kernel's USB Human Interface Device (HID) subsystem, specifically within the function hid_submit_ctrl(). The issue arises from improper handling of the report length calculation when the HID report size is zero. The function hid_submit_ctrl() calculates the transfer_buffer_length based on the report size, but it does not account for the case where report->size is zero. This leads to an incorrect calculation where the transfer_buffer_length is set to 16384 bytes. When this oversized buffer length is passed to the USB core layer, Kernel Memory Sanitizer (KMSAN) detects an information leak of 16384 bytes. The root cause is that the function hid_report_len() does not properly handle zero-sized reports, resulting in an overestimation of the buffer length. The fix involves modifying hid_report_len() to correctly handle zero report sizes by using the DIV_ROUND_UP macro for division, ensuring accurate length calculation, and then calling this corrected function from hid_submit_ctrl(). This vulnerability could potentially allow an attacker to leak kernel memory contents, which may include sensitive information, through crafted USB HID reports. Although no known exploits are reported in the wild, the vulnerability represents an information leak risk in the Linux kernel's USB HID handling code. The affected versions are identified by a specific commit hash, indicating that this issue is present in certain Linux kernel builds prior to the patch date of February 26, 2024.

For European organizations, the impact of CVE-2021-46906 primarily concerns confidentiality risks due to potential kernel memory information leaks. Attackers with local access or the ability to interact with USB HID devices could exploit this flaw to extract sensitive kernel memory data, which might include cryptographic keys, passwords, or other confidential information. This could facilitate further privilege escalation or lateral movement within affected systems. Given the widespread use of Linux in European enterprise servers, cloud infrastructure, and embedded systems, especially in sectors like finance, telecommunications, and critical infrastructure, the vulnerability could expose sensitive operational data. However, exploitation requires interaction with USB HID devices and possibly local access, limiting remote exploitation scenarios. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially in environments with untrusted USB devices or where USB device usage is common. Organizations relying on Linux-based systems for critical operations should consider this vulnerability as a potential vector for information disclosure that could undermine system integrity and confidentiality.

European organizations should implement the following specific mitigation measures: 1) Apply the official Linux kernel patches that address CVE-2021-46906 as soon as they become available from trusted Linux distribution vendors or the Linux kernel mainline. 2) Restrict physical and logical access to USB ports, especially on critical systems, to prevent unauthorized USB device connections that could trigger the vulnerability. This can be achieved through USB port control policies, endpoint security solutions, or hardware port blockers. 3) Employ USB device whitelisting to allow only trusted HID devices, reducing the risk of malicious device interaction. 4) Monitor kernel logs and USB subsystem activity for unusual or unexpected HID report sizes or errors that could indicate exploitation attempts. 5) In environments where patching is delayed, consider disabling USB HID support if feasible, or isolate vulnerable systems from untrusted networks and users. 6) Educate system administrators and security teams about the vulnerability to ensure timely detection and response. These measures go beyond generic advice by focusing on controlling USB device interactions and monitoring kernel-level behavior specific to this vulnerability.