Researchers at the Center for IT-Security, Privacy and Accountability (CISPA) found a new Bluetooth vulnerability, referred as Key Negotiation of Bluetooth (KNOB) attack, that could allow attackers to spy on encrypted connections.
The vulnerability, tracked as CVE-2019-9506, resides in the way ‘encryption key negotiation protocol’ lets
An attacker in close proximity to the victim’s device could trigger the vulnerability to intercept or manipulate encrypted Bluetooth traffic between two paired devices.
“The encryption key length negotiation process in Bluetooth BR/EDR Core v5
When two Bluetooth devices attempt to establish an encrypted connection, they must pair with each other and establish a link key that is used to generate the encryption key. After authenticating the link key, the first device proposes that the use of 16 bytes of entropy. The second device can either accept this value, reject it, or propose a smaller value (for example, because it is not able to manage a large number of bytes proposed by the first device). After proposing a smaller amount, the
An attacker could force the two devices to use a smaller number of bytes of entropy. To do it, the attacker could intercept the proposal request sent by the first device to the second one, and change the number with a Number equal to 1 byte, then, the second device would accept this value.
The attacker could then intercept the acceptance message sent by the second device and change the entropy proposal to 1 byte, which the first one would likely accept. Thus, both devices would accept N and inform the Bluetooth hosts that encryption is active, but at this point, it could be easier for the attackers to brute-force the negotiated encryption keys.
Once the attacker has obtained the key, it can monitor and manipulate the Bluetooth traffic in real-time, even if it is encrypted.
“For an attack to be successful, an attacking device would need to be within wireless range of two vulnerable Bluetooth devices that
“In addition, since not all Bluetooth specifications mandate a minimum encryption key length, it is possible that some vendors may have developed Bluetooth products where the length of the encryption key used on a BR/EDR connection could be set by an attacking device down to a single octet.” added the advisory.
The experts tested the KNOB attack against more than 14 Bluetooth chips from different vendors such as Intel, Broadcom, Apple, and Qualcomm. The result is that all the chips accept 1 byte of entropy except the Apple W1 chip that accepts (at least) 7 bytes of entropy.
Further technical details are reported in the research paper published by the experts
To mitigate KNOB attack, the maintainers of the Bluetooth specifications recommended device manufacturers and vendors to enforce a minimum encryption key length of 7 octets for BR/EDR connections.
Many vendors have already released security updates to address the flaw, including:
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(SecurityAffairs – KNOB attack, hacking)