Mathy Vanhoef, a postdoctoral researcher at Belgian university KU Leuven, has discovered and disclosed major vulnerabilities in the WPA2 protocol that secures all modern protected Wi-Fi networks.
Vanhoef said an attacker within range of a victim can exploit these weaknesses using so-called KRACKs, or key reinstallation attacks, which can result in any data or information that the victim transmits being decrypted. Attackers can eavesdrop on network traffic on both private and public networks.
As explained by Ars Technica, the primary attack exploits a four-way handshake that is used to establish a key for encrypting traffic. During the third step, the key can be resent multiple times. When it's resent in certain ways, a cryptographic nonce can be reused in a way that completely undermines the encryption.
As a result, attackers can potentially intercept sensitive information, such as credit card numbers, passwords, emails, and photos. Depending on the network configuration, it is also possible to inject and manipulate data. For example, an attacker might be able to inject ransomware or other malware into websites.
Note that the attacks do not recover the password of any Wi-Fi network, according to Vanhoef. They also do not recover any parts of the fresh encryption key that is negotiated during the four-way handshake.
Websites properly configured with HTTPS have an additional layer of protection, but an improperly configured site can be exploited to drop this encryption, so Vanhoef warned that it is not reliable protection.
Since the vulnerabilities exist in the Wi-Fi standard itself, nearly any router and device that supports Wi-Fi is likely affected, including Macs and iOS devices. Android and Linux devices are particularly vulnerable since they can be tricked into installing an all-zero encryption key instead of reinstalling the real key.
This vulnerability appears to be caused by a remark in the Wi-Fi standard that suggests to clear the encryption key from memory once it has been installed for the first time. When the client now receives a retransmitted message 3 of the 4-way handshake, it will reinstall the now-cleared encryption key, effectively installing an all-zero key.
As a proof-of-concept, Vanhoef executed a key reinstallation attack against an Android smartphone. In the video demonstration below, the attacker is able to decrypt all data that the victim transmits.
iOS devices are vulnerable to attacks against the group key handshake, but they are not vulnerable to the key reinstallation attack.
Fortunately, the vulnerabilities can be patched, and in a backwards-compatible manner. In other words, a patched client like a smartphone can still communicate with an un-patched access point like a router.
Vanhoef said he began disclosing the vulnerabilities to vendors in July. US-CERT, short for the United States Computer Emergency Readiness Team, sent out a broad notification to vendors in late August. It is now up to device and router manufacturers to release any necessary security or firmware updates.
Despite the vulnerabilities, Vanhoef says the public should still use WPA2 while waiting for patches. In the meantime, steps users can take to mitigate their threat level in the meantime include using a VPN, using a wired Ethernet connection where possible, and avoiding public Wi-Fi networks.
Vanhoef is presenting his research behind the attack at both the Black Hat Europe and Computer and Communications Security conferences in early November. His detailed research paper (PDF) is available today.
