CVE-2025-40020: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40020 is a shift-out-of-bounds vulnerability in the Linux kernel's can: peak_usb driver, affecting PC CAN FD interfaces. The flaw arises from improper bit-shifting operations where a 32-bit shift is performed without using a 64-bit constant, leading to undefined behavior. It was published on October 24, 2025, and affects Linux kernel versions from 3.4 up to the patched stable releases. The vulnerability carries a CVSS v3.1 base score of 7.1 (High), with local access required (Red Hat CVE, Feedly).

The root cause is an out-of-bounds shift operation (CWE-125/undefined behavior via shift-out-of-bounds) in the peak_usb CAN driver within the Linux kernel. Specifically, when handling PC CAN FD interfaces, the driver performs a 32-bit left shift without using a 64-bit integer constant, which can result in undefined behavior per the C standard when the shift amount equals or exceeds the width of the type. The attack vector is local (AV:L), requiring low privileges (PR:L) and no user interaction, meaning a local user with access to the CAN USB interface could trigger the flaw. The fix explicitly uses a 64-bit constant (ULL suffix) for the shift operation to prevent overflow (Red Hat CVE, ENISA EUVD).

Successful exploitation could result in memory corruption or unexpected kernel behavior, potentially leading to system instability, kernel memory access violations, denial of service, or in worst-case scenarios, local privilege escalation. The confidentiality and availability impacts are both rated High, while integrity impact is None, suggesting the primary risks are information disclosure from kernel memory and system crashes. The scope is limited to the local system and specifically to systems using PEAK USB CAN FD interfaces (Red Hat CVE, Feedly).

There is no public proof-of-concept exploit available, and no evidence of in-the-wild exploitation has been reported as of the time of disclosure (Feedly). The EPSS score is approximately 0.024% (0.000240), indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been identified.

The Linux kernel has been patched across multiple stable branches. Fixed versions include: 5.4.300, 5.10.245, 5.15.194, 6.1.155, 6.6.109, 6.12.50, 6.16.10, and 6.17. Downstream distributions including Debian (DSA-6053-1, DLA-4379-1, DLA-4404-1), Ubuntu (USN-8033 series, USN-8034 series, USN-8141-1, USN-8163 series, USN-8243-1), SUSE, and Oracle Linux have also released updated kernel packages. The recommended remediation is to update to the latest patched kernel version for your distribution. As a workaround, restricting access to CAN USB interface devices (e.g., via udev rules or removing the peak_usb module if not needed) can reduce exposure (Red Hat CVE, Debian LTS, Ubuntu USN-8033).