AllFacebook, a blog devoted to Facebook-related news, first reported that a second search box had begun to appear on Facebook interfaces, right next to the legitimate site search bar.
The mysterious Web search box appeared perfectly integrated into the Facebook page layout, as if it were a native Facebook feature. However, Facebook is now saying that it didn't put that second search box there and that it could be a sign of malware infection.
"We are not testing the placement of a separate web search field and have no plans to do so. We believe the second search field or 'Search the Web" box appeared on peoples' accounts as the result of unknown actions by a third party targeting the browser -- potentially a browser plugin or malware -- unrelated to Facebook," a Facebook official told technology news blog Search Engine Land.
As Facebook members, users who think they might be affected by this situation have access to a free, browser-based virus scanning tool from McAfee, according to the company.
As the most popular social network and one of the world's largest sites, Facebook is in a constant battle against malicious hackers and online scammers who want to take advantage of its massive user base to commit fraud and spread malware.
At this point, it's not clear whether the sinister search box is the result of an external malware exploit or the work of a rogue Facebook application.