CVE-2019-9506 in Bluetooth
Summary
by MITRE
The Bluetooth BR/EDR specification up to and including version 5.1 permits sufficiently low encryption key length and does not prevent an attacker from influencing the key length negotiation. This allows practical brute-force attacks (aka "KNOB") that can decrypt traffic and inject arbitrary ciphertext without the victim noticing.
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Analysis
by VulDB Data Team • 07/28/2020
The vulnerability identified as CVE-2019-9506 represents a critical flaw in the Bluetooth Basic Rate/Enhanced Data Rate specification that affects versions up to and including 5.1. This weakness stems from the protocol's permissive approach to encryption key length negotiation, creating an exploitable condition where attackers can manipulate the cryptographic parameters during the connection establishment process. The vulnerability specifically targets the key exchange mechanism that occurs during Bluetooth pairing and connection setup, allowing adversaries to downgrade encryption to weak key lengths that are susceptible to brute-force attacks.
The technical implementation of this vulnerability enables what researchers have termed the "KNOB" attack, where malicious actors can influence the key length negotiation process to force the use of insufficiently long encryption keys. This occurs because the Bluetooth specification does not adequately enforce minimum key length requirements during the cryptographic handshake, allowing attackers to manipulate the key size negotiation to select weaker encryption parameters. The flaw operates at the protocol level, specifically within the Bluetooth security management protocol and key generation mechanisms, making it particularly dangerous as it affects the fundamental security assumptions of the Bluetooth connection process.
From an operational perspective, this vulnerability creates a severe risk for any device that relies on Bluetooth BR/EDR for communication, as it enables attackers to decrypt sensitive traffic flowing between paired devices without detection. The attack can be executed remotely and does not require physical proximity to the target device, though it does require successful pairing or connection establishment. Once an attacker successfully exploits this vulnerability, they can inject arbitrary ciphertext into the communication stream, potentially leading to data manipulation, unauthorized access to sensitive information, and complete compromise of the Bluetooth communication channel. The stealthy nature of this attack means that victims typically remain unaware of the compromise, as the malicious activity occurs within the normal Bluetooth protocol behavior.
The security implications of CVE-2019-9506 align with CWE-327, which addresses the use of weak cryptographic algorithms, and can be mapped to ATT&CK technique T1041, which covers data obfuscation through encryption. Organizations should implement immediate mitigations including firmware updates from device manufacturers, disabling Bluetooth when not in use, and implementing network segmentation to limit potential attack vectors. Additionally, security teams should conduct thorough vulnerability assessments to identify affected devices and consider implementing Bluetooth traffic monitoring to detect anomalous behavior patterns that might indicate exploitation attempts. The vulnerability highlights the importance of cryptographic protocol design and the necessity of enforcing minimum security requirements during key exchange processes to prevent downgrade attacks and maintain the integrity of wireless communication channels.