CVE-2021-34145 in WICED BT Stack (BrakTooth)
Summary
by MITRE • 09/07/2021
The Bluetooth Classic implementation in the Cypress WICED BT stack through 2.9.0 for CYW20735B1 devices does not properly handle the reception of LMP_max_slot with an invalid Baseband packet type (and LT_ADDRESS and LT_ADDR) after completion of the LMP setup procedure, allowing attackers in radio range to trigger a denial of service (firmware crash) via a crafted LMP packet.
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Analysis
by VulDB Data Team • 09/10/2021
The vulnerability CVE-2021-34145 represents a critical denial of service flaw within the Bluetooth Classic implementation of Cypress WICED BT stack version 2.9.0 and earlier, specifically affecting CYW20735B1 devices. This issue stems from improper handling of LMP_max_slot commands when received with invalid Baseband packet types, creating a condition where malicious actors within radio range can exploit the firmware by transmitting crafted LMP packets that trigger system crashes. The vulnerability occurs during or after the LMP setup procedure completion, making it particularly dangerous as it can be initiated at any point during Bluetooth communication sessions.
The technical flaw resides in the stack's insufficient input validation and error handling mechanisms when processing LMP_max_slot commands. When the system receives a malformed LMP packet containing invalid Baseband packet type information alongside LT_ADDRESS and LT_ADDR fields, the firmware fails to properly sanitize or reject the invalid data before processing. This leads to memory corruption or execution flow disruption that ultimately results in a complete system crash. The vulnerability is classified under CWE-129 as an insufficient input validation issue, where the system does not properly validate the range or type of incoming data before processing it. The attack vector requires only proximity to the target device since Bluetooth communication operates within a defined radio range, making it accessible to nearby adversaries.
The operational impact of this vulnerability extends beyond simple service disruption as it can cause complete system unavailability for the affected CYW20735B1 devices. Once exploited, the firmware crash renders the Bluetooth functionality inoperable until manual intervention or device reboot occurs, potentially disrupting critical applications that depend on wireless connectivity. This is particularly concerning for IoT devices, automotive systems, and industrial equipment where Bluetooth Classic connectivity is integral to operation. The vulnerability can be exploited repeatedly, making it a persistent threat that can be used to maintain denial of service conditions without requiring physical access to the device.
Mitigation strategies should focus on immediate firmware updates from Cypress to address the specific handling of LMP_max_slot commands with invalid packet types. Organizations should implement network monitoring to detect anomalous Bluetooth traffic patterns that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1499.004 for network denial of service attacks, where adversaries leverage protocol-level weaknesses to disrupt services. Additionally, implementing proper input validation at the application layer and network segmentation can help reduce the attack surface. Device manufacturers should also consider implementing robust error handling mechanisms that gracefully reject malformed packets rather than allowing them to cause system crashes. The vulnerability demonstrates the importance of proper protocol implementation and input validation in embedded systems where resource constraints may lead to inadequate security measures.