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Recently, a group of researchers exposed a severe vulnerability called Key Negotiation Of Bluetooth (KNOB) that allows an attacker to break the Bluetooth Basic Rate/Extended Data Rate (BR/EDR) security. The vulnerability allows the attacker to intercept, monitor, or manipulate encrypted Bluetooth traffic between two paired devices, without being detected.

The vulnerability was identified by researchers at the Center for IT-Security, Privacy and Accountability (CISPA) who shared their findings in a paper “The KNOB is Broken: Exploiting Low Entropy in the Encryption Key Negotiation Of Bluetooth BR/EDR”. The paper was included in the proceedings of the 28th USENIX Security Symposium (August 14–16), USA.

In November 2018, the researchers of the paper shared the details of the attack with the Bluetooth SIG, the CERT Coordination Center, and the International Consortium for the Advancement of Cybersecurity on the Internet (ICASI), which is an industry-led coordination body founded by Intel, Microsoft, Cisco, Juniper and IBM. The vulnerability has been assigned CVE ID CVE-2019-9506.

[box type=”shadow” align=”” class=”” width=””]The Bluetooth BR/EDR is a popular wireless technology which is used for low-power short-range communications, and is maintained by the Bluetooth Special Interest Group (SIG).[/box]

How does the KNOB attack the victim’s devices

The researchers specify that such an attack would “allow a third party, without knowledge of any secret material (such as link and encryption keys), to make two (or more) victims agree on an encryption key—enabling the attacker to easily brute force the negotiated encryption keys, decrypt the eavesdropped ciphertext, and inject valid encrypted messages (in real-time).”

Researchers add that the attack is “standard-compliant because all Bluetooth BR/EDR versions require to support encryption keys with entropy between 1 and 16 bytes and do not secure the key negotiation protocol. As a result, the attacker completely breaks Bluetooth BR/EDR security without being detected.”

In some cases, it can also allow an attacker to reduce the length of an encryption key to a single octet. “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,” according to an advisory released by Bluetooth. This in turn would make it much easier for an attacker to brute force the encryption key used by the paired devices to communicate with each other.

The KNOB attack is effective, stealthy and cheap

The KNOB attack is a serious threat to the security and privacy of all Bluetooth device users.

It exploits the vulnerable encryption key negotiation protocol, hence risking all standard compliant Bluetooth devices irrespective of their Bluetooth version number and implementation details. This attack is highly ‘effective’ and severe as it can even attack secure Bluetooth connections.

The KNOB attack is considered ‘stealthy’ (secretive), as the users and the Bluetooth application developers do not come to know about the attack, since it generally uses a Bluetooth link-layer encryption as a trusted service. Also, the protocol is transparent to the Bluetooth host (OS) and the Bluetooth application used by the victims. The KNOB attack is also cheap because the attacker does not need an expensive resource or an attacker model to conduct the attack.

The researchers say, “We were surprised to discover such fundamental issues in a widely used and 20 years old standard. We urge the Bluetooth SIG to update the specification of Bluetooth according to our findings. Until the specification is not fixed, we do not recommend to trust any link-layer encrypted Bluetooth BR/EDR link.”

Proposed countermeasures to the KNOB attack

The researchers have proposed two classes of countermeasures to the KNOB attack. The first class is called the Legacy compliant countermeasure which requires a standard amount of negotiable entropy that cannot be easily brute-forced, e.g.,16 bytes of entropy. It also includes automated checks by the Bluetooth host to confirm the amount of negotiated entropy each time the link layer encryption is activated. This will enable the hosts to abort the connection if the entropy does not meet the minimum requirement.

Another class of countermeasure is called the Non-legacy compliant which modifies the encryption key negotiation protocol by securing it using the link key. The link key should be a shared and an authenticated secret should always be made available before starting the entropy negotiation protocol. It should also have message integrity and confidentiality.

Devices vulnerable to the KNOB attack

The researchers have conducted the attack on more than 17 unique Bluetooth chips including Broadcom, Qualcomm, Apple, Intel, and Chicony manufacturers and all the devices were found to be vulnerable to the KNOB attack.

On August 13th, Bluetooth released a Security Notice stating that the Bluetooth SIG has updated the Bluetooth Core Specification to recommend a minimum encryption key length of 7 octets for further BR/EDR connections. However, the Bluetooth SIG says, “There is no evidence that the vulnerability has been exploited maliciously and the Bluetooth SIG is not aware of any devices implementing the attack having been developed, including by the researchers who identified the vulnerability. ”

The researchers of this paper also disclosed KNOB attack to the Bluetooth Chip vendors in late 2018, following which some vendors have implemented workarounds for the vulnerability on their devices. These vendors include Apple macOS, iOS, and watchOS, Google, Cisco IP phones and Webex and Blackberry powered by Android phones who have added fixes to this vulnerability in their latest updates. Last week, the CERT Coordination Center also released an advisory to this attack.

Last week, Microsoft released an update titled “CVE-2019-9506 | Encryption Key Negotiation of Bluetooth Vulnerability” They have proposed “a default 7-octet minimum key length to ensure that the key negotiation does not trivialize the encryption.”

The researchers of this paper have also notified users that if their device has not been updated since late 2018, then it is likely to be vulnerable.

Many people  are surprised to learn about the KNOB attack and are advising others to update their devices.

To know more details about the KNOB attack, check out the “The KNOB is Broken” paper.

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