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A new Bluetooth vulnerability named "KNOB" has been disclosed that allow attackers to more easily brute force the encryption key used during pairing to monitor or manipulate the data transferred between two paired devices.

In a coordinated disclosure between Center for IT-Security, Privacy and Accountability (CISPA), ICASI, and ICASI members such as Microsoft, Apple, Intel, Cisco, and Amazon, a new vulnerability called "KNOB" has been disclosed that affects Bluetooth BR/EDR devices, otherwise known as Bluetooth Classic, using specification versions 1.0 - 5.1.

This flaw has been assigned CVE ID CVE-2019-9506 and allows an attacker to reduce the length of the encryption key used for establishing a connection. In some cases, an attacker could reduce the length of an encryption key to a single octet.

"The researchers identified that it is possible for an attacking device to interfere with the procedure used to set up encryption on a BR/EDR connection between two devices in such a way as to reduce the length of the encryption key used," stated an advisory on Bluetooth.com. "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."
Microsoft has released an update today titled "CVE-2019-9506 | Encryption Key Negotiation of Bluetooth Vulnerability" that will mitigate this vulnerability by enforcing "a default 7-octet minimum key length to ensure that the key negotiation does not trivialize the encryption."