CVE-2025-39889: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-39889 is a vulnerability discovered in the Linux kernel's Bluetooth l2cap subsystem, specifically related to encryption key size validation during incoming connections. The vulnerability was disclosed on September 24, 2025, and affects various Linux distributions and their kernel versions (NVD).

The vulnerability stems from insufficient validation of encryption key sizes in the Bluetooth l2cap subsystem when handling incoming connections. This issue specifically affects the Security Mode 4 Level 4 implementation, where the system fails to properly verify encryption key sizes ranging from 1 to 15 bytes, while the security mode requires a 16-byte key size. The vulnerability has been assigned a CVSS v3 score of 7.0, indicating a moderate to high severity level (Red Hat Security).

The vulnerability could potentially allow an attacker to establish Bluetooth connections with weaker encryption than required by the security policy, potentially compromising the confidentiality and integrity of Bluetooth communications. This particularly affects systems implementing Security Mode 4 Level 4, which requires the highest level of Bluetooth security (NVD).

Multiple Linux distributions have released patches to address this vulnerability. Ubuntu has marked this as 'Some fixes available' with 39 of 83 affected packages being patched. Red Hat Enterprise Linux and other major distributions have also issued fixes. The primary mitigation is to update to the latest kernel version that includes the security patch (Ubuntu Security).