Facebook worm “Koobface,” a web application virus that deceives users into compromising their online security, has struck again. Despite earlier efforts last winter to combat the worm, Huliq reported by way of The Washington Post that the worm has again infiltrated both Facebook and MySpace.

The Post advised users of the popular social networking sites to “to be on guard” against the new strain, which “spreads by tricking users into responding to a message apparently sent from one of their friends.”

The Facebook worm works like this: After users log into their Facebook or MySpace accounts, they will see a new message from one of their friends encouraging them to click on a new video at a fake, YouTube-esque site.

From there, the fake YouTube site tells the user that they need to download Adobe Flash plug-in in order to properly watch the video. Except that, rather than install the new Adobe program, a trojan horse program which allows the worm and hacker to completely control the user’s Facebook account is downloaded.

After that, security firm Trend Micro, which broke the news of the resurgent Facebook worm, reports that the virus “searches for cookies created by [social networking sites]” like Facebook, MySpace, Friendster, and Livejournal, amongst others, in order to obtain log-in credentials stored therein.

Trend Micro elaborated upon the threat that the Facebook worm poses by saying that the virus “then searches for an infected user’s friends, who are then sent messages containing a link where a copy of the worm is downloaded.” The worm can also send and receive information from the infected machine by connecting to multiple servers, Trend Micro reported.

Huliq, for its part, compiled twol practical tips for avoiding the Facebook worm, which include the following:

1. Don’t open any new videos from strangers
2. Don’t click on video message asking to update Flash.

Facebook has yet to comment on its official blog about the Koobface worm.