CVE-2021-3564 in Linux
Summary
by MITRE • 06/08/2021
A flaw double-free memory corruption in the Linux kernel HCI device initialization subsystem was found in the way user attach malicious HCI TTY Bluetooth device. A local user could use this flaw to crash the system. This flaw affects all the Linux kernel versions starting from 3.13.
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Analysis
by VulDB Data Team • 06/11/2021
The vulnerability identified as CVE-2021-3564 represents a critical double-free memory corruption flaw within the Linux kernel's Bluetooth HCI (Host Controller Interface) device initialization subsystem. This security defect manifests when the kernel processes user-controlled HCI TTY Bluetooth device attachments, creating a scenario where maliciously crafted device initialization sequences can trigger improper memory management operations. The vulnerability resides in the kernel's handling of Bluetooth device registration and resource allocation processes, specifically affecting the HCI subsystem's ability to properly manage memory allocations during device attachment operations. The flaw was introduced in kernel versions starting from 3.13 and has remained unpatched in many distributions, making it a persistent threat to systems running affected kernel versions. The root cause of this vulnerability can be categorized under CWE-415 as double free conditions, where the same memory block is deallocated twice, leading to memory corruption and potential system instability.
The technical exploitation of this vulnerability occurs when a local attacker creates or attaches a malicious HCI TTY Bluetooth device that triggers improper memory deallocation sequences within the kernel's HCI subsystem. During the device initialization process, the kernel allocates memory structures to manage the Bluetooth device state and configuration. When the malicious device attachment sequence is processed, the kernel's resource management code fails to properly track memory allocations, resulting in the same memory block being freed twice. This double-free condition corrupts the kernel's memory management structures, particularly affecting the slab allocator or page allocator mechanisms. The memory corruption typically manifests as heap corruption, which can lead to kernel crashes, system hangs, or potentially more severe consequences if the corruption affects critical kernel data structures. The vulnerability is particularly dangerous because it requires only local user privileges to exploit, making it accessible to any user with access to the system's Bluetooth subsystem.
The operational impact of CVE-2021-3564 extends beyond simple system crashes, as the memory corruption can potentially lead to more serious consequences within the kernel environment. When the double-free condition occurs, it can cause the kernel to become unstable and crash, resulting in denial of service for Bluetooth services and potentially affecting other system functionalities that depend on proper memory management. In some scenarios, the memory corruption could be leveraged to corrupt kernel data structures, potentially leading to privilege escalation or other advanced exploitation techniques. The vulnerability affects all Linux kernel versions starting from 3.13, which encompasses a vast range of systems including desktop environments, servers, and embedded devices that rely on Bluetooth connectivity. The widespread adoption of these kernel versions means that many systems remain vulnerable to exploitation, particularly in enterprise environments where kernel updates may be delayed or restricted. The vulnerability's impact is further compounded by the fact that Bluetooth device initialization occurs frequently during system boot processes and device attachment events, increasing the attack surface and exploitation opportunities.
Mitigation strategies for CVE-2021-3564 primarily focus on kernel updates and system hardening measures. The most effective approach involves applying the latest kernel patches that address the double-free condition in the HCI subsystem's device initialization code. System administrators should prioritize updating to kernel versions that contain the specific fixes for this vulnerability, typically found in kernel versions 5.13 and later, though some distributions may have backported fixes for older versions. Additional mitigations include disabling unnecessary Bluetooth functionality when not required, implementing proper access controls for Bluetooth device management, and monitoring for suspicious Bluetooth device attachment events. The vulnerability aligns with ATT&CK technique T1059.007 for command and scripting interpreter and T1068 for exploit for privilege escalation, as local attackers can leverage this flaw to gain system-level control. Organizations should also consider implementing kernel lockdown mechanisms and restricting local user access to Bluetooth device management interfaces. Network segmentation and monitoring solutions should be deployed to detect anomalous Bluetooth device attachment patterns that could indicate exploitation attempts, as the vulnerability can be exploited through legitimate Bluetooth device initialization sequences that appear normal to system monitoring tools.