KRACKs Against Wi-Fi Serious But Not End of the World

The attack can potentially allow attackers to send attacker controlled data to your Wi-Fi connected device. In some situations, the attacker can break the built in Wi-Fi protocol’s encryption to reveal users’ Wi-Fi messages to the attacker.

However key reinstallation depends on either working with the inherent timing of a Wi-Fi during a discreet, somewhat rare (in computer terms) exchange or the technique depends upon the attacker forcing the vulnerable exchange through some method (see below for examples). Both of these scenarios take a bit of attacker effort, perhaps more effort than using any one of the existing methods for attacking users over Wi-Fi?

For this reason, while KRACKs is a serious issue that should be fixed as soon as possible (see below), our collective digital lives probably won’t experience a tectonic shift due to Wi-Fi key reinstallation attacks.

Please read on for a technical analysis of the issue.

“… because messages may be lost or dropped, the Access Point (AP) will retransmit message 3 if it did not receive an appropriate response as acknowledgment. As a result, the client may receive message 3 multiple times. Each time it receives this message, it will reinstall the same encryption key, and thereby reset the incremental transmit packet number (nonce) and receive replay counter used by the encryption protocol. We show that an attacker can force these nonce resets by collecting and replaying retransmissions of message 3 of the 4-way handshake. By forcing nonce reuse in this manner, the encryption protocol can be attacked, e.g., packets can be replayed, decrypted, and/or forged.”

–Mathy Vanhoef, “Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2”

The highlighted text above is Vanhoef’s, not mine.

Depending upon which key establishment exchange is attacked, as Vanhoef notes, injection of messages might be the result. But, for some exchanges, decryption might also be possible.

For typical Wi-Fi traffic exchange, AES-CCMP is used between the client (user) and the access point (AP, the Wi-Fi router). Decryption using KRACKs is not thought to be possible. But decryption is not the only thing to worry about.

An attacker might craft a message that contains malware, or at least, a “dropper,” a small program that when run will attempt to install malware. Having received the dropper, even though the message may make no sense to the receiving program, the dropper is still retained in memory or temporary, perhaps even permanent, storage by the receiving computer. The attacker then has to figure out some way to run the dropper—in attack parlance, to detonate the exploit that has been set up through the Wi-Fi message forgery.

“Simplified, against AES-CCMP an adversary can replay and decrypt (but not forge) packets. This makes it possible to hijack TCP streams and inject malicious data into them. Against WPA-TKIP and GCMP the impact is catastrophic: packets can be replayed, decrypted, and forged.“

–Mathy Vanhoef, “Key Reinstallation Attacks: Forcing Nonce Reuse in WPA2”

Packet forgery is worse, as the receiver may consider that the data is legitimate, making it easier for an attacker to have the receiver accept unexpected data items, influencing the course of an exchange and establish a basis upon which to conduct follow-on, next-step exploits.

In my opinion KRACKs are a serious problem to which the wise will wish to respond.

The essential problem with Wi-Fi is that it is free to intercept. Radio waves travel over the air, which, as we all know, is a shared medium. One need not “plug in” to capture Wi-Fi (or any radio-carried) packets. One simply must craft a receiver strong enough to “hear,” that is, receive the transmissions of interest.

But although certainly serious, are KRACKs the end of the known digital universe? I think not.

First, the attacker has to be present and alert to the four-way, WPA2 handshake. If this has already passed successfully, the attacker’s key reinstallation is no longer possible. The handshake must be interjected at exchange #3, which will require some precision. Or, the attacker must force the key exchange.

One could, I imagine, write an attack that would identify all WPA2 handshake exchanges within range, and inject the attacker’s message #3 to each handshake to deliver some sort of follow-on exploit. This would be a shotgun approach, and might on large, perhaps relatively open networks have potential attacker benefits.[1]

Supplicants (as Wi-Fi clients are called) do not constantly handshake. The handshake interchange takes place upon authentication and reauthentication, and in some networks upon shifting from one AP to another when the client changes location (a “hand-off”). These are discreet triggers, not a constant flow of handshake message #3. Johnny or Janie attacker has to be ready and set up in anticipation of message #3. That is, right after message #3 goes by, attacker’s #3 must be sent. Once the completed handshake message #4 goes by, I believe that the attack opportunity closes.

I fear that the timing part might be tricky for the average criminal attacker. It would be much easier to employ readily available Wi-Fi sniffing tools to capture sufficient traffic to allow the attacker to crack weak Wi-Fi passwords.

There are well-understood methods for forcing Wi-Fi authentication (see DEAUTH, below). Since these methods can be used to reveal the password for the network or the user, using one of these may be a more productive attack?

There are other methods for obtaining a Wi-Fi password, as well: The password must be stored somewhere on each connecting device or the user would need to reenter the password upon every connection/reconnection. Any attack that gives attacker access to privileged information kept by any device’s operating system can be used to gain locally stored secrets like Wi-Fi passwords.

Wi-Fi password theft (whether WPA-Enterprise or WPA-Personal) through Wi-Fi deauthentication, “DEAUTH,” has been around for some time; numerous tools make the attack relatively trivial and straightforward.

Nation-state attackers may have access to banks of supercomputers or massive parallel processing systems that they can apply for rapid password cracking of even complex passwords. A targeted nation-state–sponsored Wi-Fi password-cracking attack is difficult to entirely prevent with today’s technologies. There are likely other adversaries with access to sufficient resources to crack complex passwords in a reasonable amount of time, as well.[2]

Obviously, as Vanhoef suggests, as soon as your operating system or Wi-Fi client vendor offers an update to this issue, patch it quickly. Problem, hopefully, solved. Please see https://www.krackattacks.com for updates from security vendors that are working with the discoverer. Question your vendor. If your vendor does not respond, pressure them; that’s my suggestion.

Other actions that organizations can couple with good Wi-Fi hygiene are to use good traffic and event analysis tools, such as modern Security Information Event Management (SIEM) software, network ingress and egress capture for anomaly analysis, and perhaps ingress/egress gateways that prevent many types of attack and forms of traffic.

“One can use VPN, SSH2 tunnels, etc., to ensure some safety from ARP poisoning attacks. Mathy Vanhoef also uses an SSL stripper that depends on badly configured web servers. Make sure your TLS implementation is correct!”

– Carric Dooley, Global Lead, Foundstone Consulting Services

Organization Wi-Fi hygiene includes rapidly removing rogue access points, Wi-Fi authentication based upon the organization’s central authentication mechanism (thus do not employ a WPA password), strong password construction rules, and certificates issued to each Wi-Fi client without which access to Wi-Fi will be denied. Add Wi-Fi event logs to SIEM collection and analysis. Find out whether organization access points generate an event on key reinstallation. This is a fairly rare event. If above-normal events are being generated, your Wi-Fi may be suffering from a KRACK.

None of the foregoing measures will directly prevent key reinstallation attacks. Reasonable security practices do make gaining access to Wi-Fi more difficult, which will prevent or slow other Wi-Fi attacks. Plus, physical security ought to pay attention to anyone sitting in the parking lot with a Wi-Fi antenna pointing at a campus building. But that was just as true before KRACKs were discovered.

For home users, use a complex password on your home Wi-Fi. Make cracking your password difficult. Remember, you usually must enter the password only once for each client device.

Maintain multiple Wi-Fi networks (probably, multiple access points), each with different passwords: one for work or other sensitive use; and another for smart TVs and the like and other potentially vulnerable devices such as Internet of Things devices—isolate those from your network shares, printers, and other sensitive data and trusted devices. Install a third Wi-Fi for your guests. That network should be set up so that each session is isolated from the others; all your guests need is to get to the internet. Each SSID network must have a separate, highly varied password: numbers, lowercase, uppercase, symbols. Make it a long passphrase that resists dictionary attacks.

Wi-Fi routers are a commodity; I do not suggest spending a great deal of money. How much speed do your occasional guests really need? Few Internet connections are as fast as even bottom-tier home Wi-Fi. Most of your visitors probably do not need access to your sensitive data, yes? Should your kids have their own Wi-Fi so that their indiscretions do not become yours?

Multiple Wi-Fi networks will help to slow and perhaps confound KRACKs cybercriminals. “Which network should I focus on?” You increase the reconnaissance the attacker must perform to promulgate a successful attack. Maybe they will move on to simpler home network.

If you happen to notice someone sitting in a car in your neighborhood with an open laptop, be suspicious. If they have what looks like an antenna, maybe let law enforcement know.

Basic digital security practices can perhaps help. For instance, use a VPN even over Wi-Fi. Treat Wi-Fi, particularly, publicly available Wi-Fi as an untrustable network. Be cognizant of where the Wi-Fi exists. WPA2 is vulnerable to decryption, so don’t trust airports, hotels, cafes, anywhere where the implementers and maintainers of the network are unknown.

Users of some Android and Linux clients should be aware that an additional implementation error allows an attacker immediate access to the decryption of client and access point Wi-Fi traffic. Remember, all those smart TVs, smart scales, home automation centers, and thermostats are usually nothing more than specialized versions of Linux. Hence, each may be vulnerable. On the other hand, if these are segregated onto a separate, insecure Wi-Fi, at least attackers will not have readily gained your more sensitive network and devices. While waiting for a patch for your vulnerable Android device, perhaps it makes sense to put it on the untrusted home Wi-Fi as a precaution.

You may have noticed that I did not suggest changing the home Wi-Fi passwords. Doing so will not prevent this attack. Still, the harder your WPA2 password is to crack, the more likely common cybercriminals will move on to easier pickings.

As is often the case in these serious issues, reasonable security practices may help. At the same time, failure to patch quickly leaves one vulnerable for as long as it takes to patch. I try to remember that attackers will become ever more facile with these methods and what they can do with them. They will build tools, which, if these have value, will then become ever more widespread. It is a race between attacker capability and patching. To delay deploying patches is typically a dance with increasing risk of exploitation. In the meantime, the traffic from this attack will be anomalous. Watch for it, if you can.

Many thanks to Carric Dooley of Foundstone Professional Services for his help with this analysis.

[1] “When a client joins a network, it […] will install this key after receiving message 3 of the 4-way handshake. Once the key is installed, it will be used to encrypt normal data frames using an encryption protocol. However, because messages may be lost or dropped, the Access Point (AP) will retransmit message 3 if it did not receive an appropriate response as acknowledgment. […] Each time it receives this message, it will reinstall the same encryption key, and thereby reset the incremental transmit packet number (nonce) and receive replay counter used by the encryption protocol. We show that an attacker can force these nonce resets by collecting and replaying retransmissions of message 3 of the 4-way handshake. By forcing nonce reuse in this manner, the encryption protocol can be attacked, e.g., packets can be replayed, decrypted, and/or forged.”

[2] My LinkedIn password was among those stolen during the 2011 LinkedIn breach. A research team attempted to crack passwords. After three months, they cracked something like 75% of the passwords. However, my highly varied, but merely six-character password was never cracked. Although today’s cracking is factors more sophisticated and rapid, a varied nondictionary password still slows the process  considerably.