Searching for Flash Player on Bing and Yahoo can lead to rogue pages distributing a hard-to-remove rootkit, according to security researchers from antivirus vendor GFI Software.

The problem resides with the so-called sponsored results, the advertisements displayed at the top of search results for particular keywords. These look slightly different from the organic results normally returned by Bing's algorithm, but close enough for users to frequently click on them.

In the new attack observed by GFI Software, a sponsored result shown when searching for "Adobe Flash" linked to a page called "Download Flash Player" under the GetAdobeFlash.com domain.

However, according to Alex Eckelberry, vice president and general manager of the security software division at GFI, clicking on the link redirected users to a rogue page that was advertising Flash Player 10 but distributed a dangerous rootkit instead.

"In this case, we're talking Sirefef (ZeroAccess aka Max++), probably the nastiest piece of malware circulating on the 'net right now," said Eckelberry. "Sirefef kills any attempt to remove it, and is nearly impossible to clean (short of booting onto a rescue disk and performing cleanup actions, or reformatting)," he added.

However, the problem is not limited to this particular threat, because this isn't the first time that Bing's sponsored results have been poisoned in this manner. In September, GFI's researchers reported a similar attack , which targeted keywords for several popular programs, including Firefox, Skype and uTorrent.

"Microsoft needs to get a handle on ad placements on Bing," Eckelberry stressed, pointing out that this also affects Yahoo since it uses the same engine. According to September statistics from comScore, the two Web search services have a combined market share of nearly 30%.

Google used to have similar problems, with cybercriminals regularly tricking its sales team into accepting rogue ads. However, the company is now much more vigilant and such attacks are extremely rare.

It's obvious that attackers have moved to Bing in search of new victims after Google became a difficult target. Fortunately, the attacks detected so far have mainly relied on social engineering to infect users. However, that might not be the case for long.

"If the user is asked to download a file from a potentially suspicious website then they can likely escape unharmed, but if the rogue sites are serving up exploits and drive-by installs then things could become a lot more problematic," warned GFI senior threat researcher Christopher Boyd. His advice for users is to download programs from their respective developer's website by typing the URL directly rather than searching for it online.