Kr00k, KRACK, and the Seams in Wi-Fi, IoT Encryption

Earlier this year, two ESET researchers disclosed a flaw in processor chips powering over 1 billion Wi-Fi and Internet of Things (IoT) devices that would make it easy for attackers to snoop on encrypted traffic. Last week at Black Hat, the researchers explained that the attack surface area for these kinds of flaws is broader than they initially thought and that the weakness is present in a several other popular chipsets that could put even more IoT and Wi-Fi devices at risk.

Dubbed "Kr00k" by researchers Robert Lipovsky and Stefan Svorencik, the flaw in question occurs in how Wi-Fi chips handle the four-way handshake process that occurs between a device and an access point to facilitate WPA2 encryption. When devices associate and disassociate with a network, the handshake process governs authentication and how cryptographic keys are exchanged as connection is both established and broken between device and access point.

Kr00k is a flaw in how the chips handle the process of WLAN session disassociation, in which they overwrite the encryption keys with all zeros in the expectation that no further data will be transmitted after disassociation. The expectation is when the device reassociates with a new session, a new encryption key will be negotiated and encryption will remain seamless.

"This is expected behavior as no further data is supposed to be transmitted after disassociation. And it stays that way until a new session is generated after the new reassociation and the new four-way handshake," explained Lipovsky during the "Kr00k: Serious Vulnerability Affected Encryption of Billion+ Wi-Fi Devices" session. "But until that happens, the transmit buffer still may contain data. The transmit engine needs to send them away and continues to do so as usual. So all data which were left in that buffer [when] the association occurred are now sent encrypted with an all-zero encryption key." 

As a result, an attacker can easily decrypt the data frames in the buffer, and a savvy adversary that can artificially trigger a disassociation and prolong its state can slurp up sensitive information in the process. 

"So when you can grab these frames, you can easily decrypt them," Lipovsky says. "You know, the encryption algorithm, you know, the encryption key, you get the numbers from the header and you can easily decrypt them. The question is, how do you identify these particular frames in the air? The answer is simple. You don't. You just try to decrypt everything you see with an all-zero key."

Interestingly, Kr00k's discovery was somewhat of a by-product of the fallout from the key reinstallation attack (KRACK) techniques explored by Mathy Vanhoef at Black Hat Europe 2017. This attack and exploit was preying on the way Amazon Echo and other IoT devices handled the process of resetting encryption keys.

After the vendors made adjustments to harden themselves against attack, Lipovsky and Svorencik were trying other variations of the attack and soon stumbled on the Kr00k vulnerability. As they dug further, they found it wasn't a flaw in the device itself but the underlying chipset. When the pair first disclosed Kr00k at the RSA Conference in February, they could only confirm it impacted Broadcom and Cypress chips but suspected other manufacturers were also impacted.

Last week they confirmed that additional chip manufacturers were, indeed, impacted. During their talk they reported several additional chipsets were vulnerable to the flaw and dropped a new vuln on the audience — CVE-2020-3702 — a Kr00k-like flaw in Qualcomm chips that leads to disclosure of data through disassociation, with the main difference that instead of using an all-zero key, the data isn't encrypted at all during the state of disassociation.

The impacted chips are used in smart home hubs, wireless routers, and more. The researchers explained that Qualcomm has released a proprietary driver to fix the flaw but that not all devices with Qualcomm chips use that driver. 

This lack of uniformity of drivers highlights one of the challenges with Kr00k, which, because it is at the chip level, can be difficult to assess from an enterprise perspective in devices that may not necessarily be getting embedded software updates from the chip manufacturer. In order to help the audience better audit their environments for Kr00k, the duo released a script at Black Hat that they developed to test devices for their susceptibility to Kr00k.