Belkin ships many wireless routers with an encrypted wireless network configured by default. The network name (ESSID) and the (seemingly random) password is printed on a label at the bottom of the device.
II. Description of vulnerability
Having a preconfigured randomly generated WPA2-PSK passphrase for wireless routers is basically a good idea since a vendor-generated passphrase can be much more secure than most user-generated passwords. However, in the case of Belkin the default password is calculated solely based on the mac address of the device. Since the mac address is broadcasted with the beacon frames sent out by the device, a wireless attacker can calculate the default passphrase and then connect to the wireless network.
Each of the eight characters of the default passphrase are created by substituting a corresponding hex-digit of the wan mac address using a static substitution table. Since the wan mac address is the wlan mac address + one or two (depending on the model), a wireless attacker can easily guess the wan mac address of the device and thus calculate the default WPA2 passphrase.
Moreover, the default WPA2-PSK passphrase solely consists of 8 hexadecimal digits, which means that the entropy is limited to only 32 bits (or 33 bits since some models use uppercase hex digits). After sniffing one successful association of a client to the wireless network, an attacker can carry out an offline brute-force attack to crack the password. The program oclhashcat-plus can try 131,000 passwords per second on one high end GPU (AMD Radeon hd7970) . Doing a full search of the 32-bit key space takes about 9 hours at this rate.
An attacker can exploit this vulnerability to calculate the WPA2-PSK passphrase of a wireless network. This allows sniffing and decrypting all wireless traffic in a purely passive attack given that the attacker has also sniffed the association.
The attacker may also connect to the wireless network, which may allow further exploitation of unprotected systems in the local network. An attacker may furthermore use the wireless network to access the internet from the owner's network. The network owner may then be held responsible for any illegal activities perpetrated by the unauthorized users.
IV. Affected devices
Belkin Surf N150 Model F7D1301v1
The official Belkin support page  contains pictures of the label of several other WiFi devices, which show that the following devices are vulnerable as well:
Belkin N900 Model F9K1104v1
Belkin N450 Model F9K1105V2
The following device uses a variation of the algorithm and the password consists of uppercase hex digits. When using our algorithm with the wlan mac of the device, the first 5 digits of the password are calculated correctly. It is likely that the algorithm differs only in the tables used.
Belkin N300 Model F7D2301v1
It is likely that other Belkin devices are affected as well. Unfortunately, Belkin has not yet cooperated with us to fix the vulnerability and/or confirm a list of other affected devices. If you own a Belkin wireless router and want to know whether it is vulnerable as well, you should change the passphrase and then send me the relevant data (model number, wan/wlan mac address and original, default WPA2 passphrase).
Users of potentially affected wireless routers should change the wireless passphrase to something more secure.
6.1.2012: Vendor contacted
29.10.2012: Another contact attempt, still no response
19.11.2012: Public disclosure
Advisory location: http://www.jakoblell.com/blog/?p=15