SAN FRANCISCO — Billions of devices—many of them already patched—are affected by a Wi-Fi vulnerability that allows nearby attackers to decrypt sensitive data sent over the air, researchers said on Wednesday at the RSA security conference.

The vulnerability exists in Wi-Fi chips made by Cypress Semiconductor and Broadcom, the latter whose Wi-Fi business was acquired by Cypress in 2016. The affected devices include iPhones, iPads, Macs, Amazon Echos and Kindles, Android devices, Raspberry Pi 3’s, and Wi-Fi routers from Asus and Huawei. Eset, the security company that discovered the vulnerability, said the flaw primarily affects Cyperess’ and Broadcom’s FullMAC WLAN chips, which are used in billions of devices. Eset has named the vulnerability Kr00k, and it is tracked as CVE-2019-15126.

Manufacturers have made patches available for most or all of the affected devices, but it’s not clear how many devices have installed the patches. Of greatest concern are vulnerable wireless routers, which often go unpatched indefinitely.

“This results in scenarios where client devices that are unaffected (either patched or using different Wi-Fi chips not vulnerable to Kr00k) can be connected to an access point (often times beyond an individual’s control) that is vulnerable,” Eset researchers wrote in a research paper published on Wednesday. “The attack surface is greatly increased, since an adversary can decrypt data that was transmitted by a vulnerable access point to a specific client (which may or may not be vulnerable itself).”

A key consisting of all zeros

Kr00k exploits a weakness that occurs when wireless devices disassociate from a wireless access point. If either the end-user device or the access point is vulnerable, it will put any unsent data frames into a transmit buffer and then send them over the air. Rather than encrypt this data with the session key negotiated earlier and used during the normal connection, vulnerable devices use a key consisting of all zeros, a move that makes decryption trivial.

Disassociation typically happens when a client device roams from one Wi-Fi access point to another, encounters signal interference, or has its Wi-Fi turned off. Hackers within range of a vulnerable client device or access point can easily send disassociation frames to trigger the vulnerability because these frames aren't authenticated.

With the forced disassociation, vulnerable devices will typically transmit several kilobytes of data that’s encrypted with the all-zero session key. The hacker can then capture and decrypt the data. Eset researcher Robert Lipovsky told me hackers can trigger multiple disassociations to further the chances of obtaining useful data.

The following two diagrams help illustrate how the attack works.

Eset researchers determined that a variety of devices are vulnerable, including:

Amazon Echo 2nd gen

Amazon Kindle 8th gen

Apple iPad mini 2

Apple iPhone 6, 6S, 8, XR

Apple MacBook Air Retina 13-inch 2018

Google Nexus 5

Google Nexus 6

Google Nexus 6S

Raspberry Pi 3

Samsung Galaxy S4 GT-I9505

Samsung Galaxy S8

Xiaomi Redmi 3S

The researchers also found that the following wireless routers are vulnerable:

Asus RT-N12

Huawei B612S-25d

Huawei EchoLife HG8245H

Huawei E5577Cs-321

An Apple spokesman said the vulnerabilities were patched last October with details for macOS here and for iOS and iPadOS here.

Amazon officials, meanwhile, wrote in a statement: “Customer trust is important to us and we take the security of our devices seriously. The Echo and Kindle devices detailed in this research have received automatic security updates over the Internet addressing this issue.”

Manufacturers of other vulnerable devices that still receive patch support couldn't immediately be reached for comment.

The researchers tested Wi-Fi chips from other manufacturers, including Qualcomm, Realtek, Ralink, and Mediatek and found no evidence any of them were vulnerable. Since it was impossible for the researchers to test all devices, it’s possible that other devices using Cypress and Broadcom chips are also affected.

While the vulnerability is interesting and users should make sure their devices are patched quickly—if they aren’t already—there are a few things that minimize the real-world threat posed. For one thing, most sensitive communications in 2020 are already encrypted, usually with the transport layer security protocol or by other methods. A glaring exception to this is domain name lookups, which, unless a computer is using DNS over HTTPS or DNS over TLS, are sent entirely over plaintext. Hackers who viewed these requests would be able to learn what domain names users were accessing.

Even if a vulnerable device is communicating over HTTP or another unencrypted channel, hackers could recover only several kilobytes of the data flowing over it at any one time. It’s doubtful attackers could time the disassociations in a way that would ensure passwords or other sensitive information would be captured. That means useful attacks would have to involve a large amount of luck or disassociations that occurred over and over in rapid succession.

It also seems likely that repeated attacks would be easy to detect since Wi-Fi connections would start and stop repeatedly with no clear reason why.

Despite the limited threat posed, readers should ensure their devices have received updates issued by the manufacturers. This advice is most important for users of vulnerable Wi-Fi routers, since routers are often hard to patch and because vulnerable routers leave communications open to interception even when client devices are unaffected or are already patched.

Post updated to add Amazon statement.