CVE-2025-40301: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40301 is an uninitialized memory access vulnerability in the Linux kernel's Bluetooth HCI event handling subsystem. Specifically, the flaw exists in hci_cmd_complete_evt() within hci_event.c: when a command complete event carries an unknown opcode, the handler assumes skb->data[0] contains the return status, but if the socket buffer (skb) has already been emptied by prior parameter extraction in hci_event_func(), this access reads uninitialized memory. The vulnerability was published on December 8, 2025, and affects Linux kernel versions from commit afcb3369f46e up to the respective stable branch fixes. It carries a CVSS v3.1 base score of 7.1 (High) (Red Hat CVE, Microsoft MSRC).

The root cause is classified as use of uninitialized memory (CWE-908), triggered in the Bluetooth HCI layer's hci_cmd_complete_evt() function. When hci_event_func() pulls parameter data from the skb prior to dispatching to the command-complete handler, the skb may be left with zero remaining bytes. If the opcode is unknown, the code unconditionally reads skb->data[0] to obtain the return status without first validating skb->len, resulting in a read of uninitialized or out-of-bounds kernel memory. The attack vector is local, requiring low privileges and no user interaction, and is reachable by any process with access to Bluetooth HCI functionality. The fix adds a bounds check (if (!skb->len)) before accessing skb->data[0] (Red Hat CVE, EUVD).

Successful exploitation could allow a low-privileged local attacker to read sensitive kernel memory, potentially exposing cryptographic material, credentials, or other confidential data processed by the Bluetooth subsystem. The vulnerability also carries a risk of system instability or crash (denial of service) due to the use of uninitialized memory in kernel context. In more severe scenarios, depending on memory layout, it may contribute to privilege escalation or arbitrary code execution, though no such exploitation has been demonstrated publicly (Red Hat CVE, Microsoft MSRC).

No public proof-of-concept exploit code or active in-the-wild exploitation has been observed for CVE-2025-40301 as of the time of this report (Red Hat CVE). The EPSS score is approximately 0.018% (0.000180), indicating a very low probability of exploitation in the near term. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires local access with at least low privileges and the presence of Bluetooth hardware or a virtual HCI interface.

Patches have been backported to multiple stable Linux kernel branches. Fixed versions include: 6.1.159, 6.6.117, 6.12.58, 6.17.8, and 6.18 (mainline). Administrators should apply the relevant kernel update for their distribution — Red Hat has issued RHSA-2026:1690 and RHSA-2026:1727; Debian has issued DLA-4436-1 for the 6.1 LTS branch; Rocky Linux and AlmaLinux have issued corresponding advisories. As a workaround where patching is not immediately possible, disabling Bluetooth functionality (rfkill block bluetooth or blacklisting the bluetooth kernel module) eliminates the attack surface (Red Hat Advisory, Debian LTS, Microsoft MSRC).

The vulnerability was noted in Microsoft's December 2025 Patch Tuesday coverage in the context of Azure Linux kernel updates, and was covered by Rapid7 and Lansweeper in their Patch Tuesday summaries (Rapid7 Blog, Lansweeper Blog). No significant independent researcher commentary or social media discussion has been identified beyond standard vulnerability tracking and distribution advisory channels.