Researchers from security firm Zscaler have published free software that detects when users' web connections are being monitored by a controversial tool that steals log-in credentials from Facebook, Google and dozens of other websites.
Dubbed BlackSheep, the Firefox extension alerts users when computers on a local area network are using FireSheep to steal unencrypted cookies the websites use to grant users access to their account pages. When BlackSheep detects the snoop software in a hotspot or other open Wi-Fi network, it displays a message that reads “Somebody is using FireSheep on this network.” It then displays the LAN IP address of the offending party.
BlackSheep's release comes two weeks after security researcher Eric Butler published FireSheep in an attempt to expose the bovid practices of Facebook and other websites that don't bother to encrypt session cookies used to authenticate their users. With the exception of Twitter and a handful of Google services, few popular websites offer the end-to-end encryption needed to foil such attacks, a shortcoming that seemed to be lost on the masses until FireSheep made it easy to exploit the weakness.
FireSheep listens to web traffic for instances when users on an unsecured network log in to known websites such as Facebook, Google, and Yahoo. Although such websites encrypt the login name and password entered by the user, most send the corresponding session cookie in the clear, making it easy for people performing man-in-the-middle attacks to appropriate them and use them to access the user's account.
FireSheep had been downloaded 659,620 times at time of writing, according to figures supplied by Butler.
BlackSheep, which is based on the FireSheep source code, works by broadcasting fake session IDs over the network and then monitoring to see if other machines use them to sign in. By default, it checks a network every five minutes, but can be configured to send out faux cookies more or less frequently as preferred by the user. Zscaler has more information here.
Since FireSheep's release, Microsoft appears to have added detection for it to its antivirus software. This seems like a reactionary and futile move on the part of Microsoft, since detecting the snoop software will in no way protect users from the underlying vulnerability. ®