Facebook has blocked links between its social networking site and malware-infested Web sites to where malicious hackers have been trying to lure Facebook members.

"We've identified and blocked the ability to link to the malicious websites from anywhere on Facebook. Less than .002 percent of people on Facebook have been affected, all of whom we notified and suggested steps to remove the malware," wrote Max Kelly, Facebook's head of security, in a blog post early Friday.

[ Related reading: Malicious hackers use Facebook Wall for malware attack ]

Security company Sophos warned on Thursday about the attack, in which malicious hackers were targeting unsuspecting Facebook users via postings on the site's Wall feature.

The Wall, a core component of Facebook profile pages, is used by members to leave each other messages. Impersonating members' friends, malicious hackers posted messages urging users to click on a link to view a video on a Web site they falsely said was hosted by Google.

However, the link took users to a rogue Web page where they were told to download a new version of Adobe's Flash player in order to view the video. If users authorized the download, the site would install a Trojan horse, Troj/Dloadr-BPL, that funneled other malicious code detected as Troj/Agent-HJX into their PCs.

Then, an image of a court jester sticking his tongue out would appear, making it seem to Facebook members like an innocent practical joke by a friend. In fact, at that point, the PC had been seriously compromised and put in the control of malicious hackers for sending spam, distributing malware and performing other harmful actions, according to Sophos.

In its alert, the security company also addressed business and IT managers, saying that malware attacks via social networks are on the rise and that companies need to establish policies for employee use of these sites from the office.

If companies decide to allow employees to use Facebook, MySpace, LinkedIn and other sites, they should consider beefing up their security wares with, for example, devices that monitor Web traffic and scan software downloads authorized by end users, according to Sophos.

In his blog posting, Kelly asks Facebook members, of which there are about 80 million active ones, to report any spam messages they receive or postings they see, as well as any members who post threatening or inappropriate messages.

"The more reports we get, the easier it is for us to respond decisively," he wrote.

Another important tip: Never share a Facebook password, not even with someone purporting to be from the company.

"No Facebook employee will ever ask for it, and no one else should know it. If you are ever prompted to log in to Facebook, make sure it's from a legitimate Facebook web address. If something looks or feels off, go directly to www.facebook.com to log in," Kelly wrote.

The prompt to download an upgraded Flash player is apparently becoming popular with malicious hackers. This week, Adobe posted its own alert warning people not to fall for this trick. Apparently, the bogus Flash message is part of other malware attacks that use microblogging site Twitter and other social sites.

Last week, security company Kaspersky Lab warned of new worms targeting MySpace and Facebook users via automatically generated comments and messages to those on their lists of friends.

IDG News Service

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