WPA2 security hole found
Vulnerability allows hackers to spoof WiFi packets, compromise WLAN
By Joanie Wexler | Network World US | Published: 11:55, 26 July 2010
Perhaps it was only a matter of time. But wireless security researchers say they have uncovered a vulnerability in the WPA2 security protocol, which is the strongest form of WiFi encryption and authentication currently standardised and available.
Malicious insiders can exploit the vulnerability, named "Hole 196" by the researcher who discovered it at wireless security company AirTight Networks. The moniker refers to the page of the IEEE 802.11 Standard (Revision, 2007) on which the vulnerability is buried.
Hole 196 lends itself to man-in-the-middle-style exploits, whereby an internal, authorized Wi-Fi user can decrypt, over the air, the private data of others, inject malicious traffic into the network and compromise other authorised devices using open source software, according to AirTight.
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The researcher who discovered Hole 196, Md Sohail Ahmad, AirTight technology manager, intends to demonstrate it at two conferences taking place in Las Vegas next week: Black Hat Arsenal and DEF CON 18.
The Advanced Encryption Standard (AES) derivative on which WPA2 is based has not been cracked and no brute force is required to exploit the vulnerability, Ahmad says. Rather, a stipulation in the standard that allows all clients to receive broadcast traffic from an access point (AP) using a common shared key creates the vulnerability when an authorized user uses the common key in reverse and sends spoofed packets encrypted using the shared group key.
Ahmad explains it this way: WPA2 uses two types of keys: 1) Pairwise Transient Key (PTK), which is unique to each client, for protecting unicast traffic; and 2) Group Temporal Key (GTK) to protect broadcast data sent to multiple clients in a network. PTKs can detect address spoofing and data forgery. "GTKs do not have this property," according to page 196 of the IEEE 802.11 standard.
These six words comprise the loophole, Ahmad says.
Because a client has the GTK protocol for receiving broadcast traffic, the user of that client device could exploit GTK to create its own broadcast packet. From there, clients will respond to the sending MAC address with their own private key information.
Ahmad says it took about 10 lines of code in open source MadWiFi driver software, freely available on the Internet, and an off-the-shelf client card for him to spoof the MAC address of the AP, pretending to be the gateway for sending out traffic. Clients who receive the message see the client as the gateway and "respond with PTKs", which are private and which the insider can decrypt, Ahmad explains.
From there, "the malicious insider could drop traffic, drop a [denial-of-service] attack, or snoop," Ahmad says.
The ability to exploit the vulnerability is limited to authorized users, AirTight says. Still, year-after-year security studies show that insider security breaches continue to be the biggest source of loss to businesses, whether from disgruntled employees or spies who steal and sell confidential data.
What can we do about Hole 196? "There's nothing in the standard to upgrade to in order to patch or fix the hole," says Kaustubh Phanse, AirTight's wireless architect who describes Hole 196 as a "zero-day vulnerability that creates a window of opportunity" for exploitation.