SEATTLE, April 13 (Reuters) - Computer security researchers said they have uncovered a new variation on an old weakness in Microsoft Corp's Windows operating system that could theoretically allow hackers to steal login credentials from hundreds of millions of PCs.

The vulnerability, named 'Redirect to SMB' by security firm Cylance, is similar to one found in the late 1990s that took advantage of a weakness in Windows and Microsoft's Internet Explorer browser which made it possible for attackers to trick Windows into signing on to a server controlled by hackers.

According to Cylance, if a hacker can get a Windows user to click on a bad link in an email or on a website, it can essentially hijack communications and steal sensitive information once the user's computer has logged on to the controlled sever.

The technique takes advantage of features in Windows Server Message Block, commonly known as SMB. The new variation, discovered by Cylance researcher Brian Wallace, has so far only been recreated in the laboratory and has not been seen on computers in the outside world.

Microsoft said the threat posed by the purported weakness was not as great as Cylance supposed.

"Several factors would need to converge for a 'man-in-the-middle' cyberattack to occur. Our guidance was updated in a Security Research and Defense blog in 2009, to help address potential threats of this nature," said Microsoft in an emailed statement. "There are also features in Windows, such as Extended Protection for Authentication, which enhances existing defenses for handling network connection credentials." (Reporting by Bill Rigby; Editing by Marguerita Choy)