BOSTON (Reuters) - Microsoft Corp said on Tuesday that hackers are seeking to attack users of its PowerPoint presentation software for Windows PCs and released patches to protect them against the threat.

The world's No. 1 software maker said that a version of PowerPoint for Apple Inc's Mac computers is also vulnerable, though it has yet to find any evidence that hackers are actively seeking to exploit it.

Microsoft defined the threat as "critical" -- the most severe on the scale by which it ranks vulnerabilities to its software.

Hackers are seeking to exploit the vulnerability in PowerPoint by persuading the intended victim to open a tainted PowerPoint file -- that they either download from a Website or receive in an email, according to Symantec Corp, the world's top maker of security software.

"At that point, the attacker would then have complete control over everything the user's account has permission to do on the system," said Alfred Huger, a senior researcher with Symantec.

Huger said that Symantec has so far only observed a limited number of hacker attempts to exploit the vulnerability in PowerPoint.

Microsoft did not release a patch for Mac computers, though company spokesman Christopher Budd said that one is in development.

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