Microsoft will patch just one bug in its first security release of 2009.

The release, which will be made available next Tuesday, is a critical patch for server and desktop versions of Windows and fixes at least one bug that could allow attackers to install unauthorised software on a victim's computer.

Microsoft did not say which bugs it would be fixing with next week's updates, but the company has several to choose from. In the past month, Microsoft has warned of flaws in its WordPad Text Converter and SQL Server database software.

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The researcher who disclosed the SQL Server flaw said recently that Microsoft has known about the issue since April, and had written a patch for it back in September.

One security researcher has also claimed that there is a bug in Microsoft's Windows Media Player, but Microsoft has disputed his findings.

But none of these flaws is in all versions of the underlying Windows operating system, which are being patched next week. According to nCircle's director of security operations, Andrew Storms, Microsoft could be fixing a known Windows flaw that would allow hackers to gain unauthorised privileges on a computer.

"An exploit has already been published for some time along with a whitepaper by the author," Storms said.

Microsoft has offered workarounds for this flaw already, but it has now had enough time to prepare a full-blown security patch, Storms said.

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However, the security researcher who found the flaw said he doesn't expect to see it patched next week. "I don't think they will patch my bug because it's rated critical (remote code execution) and my bug is about local elevation of privileges," said Cesar Cerrudo, chief executive of security research firm Argeniss.

Local elevation of privilege flaws are not typically considered critical, although Storms said Microsoft may have discovered while fixing the problem that it was more severe than previously thought.

The update comes nearly a month after Microsoft was forced to issue an emergency patch for Internet Explorer, after criminals began exploiting the flaw to install password-stealing software on victims' machines.

See also: 'Why we failed' - Microsoft admits IE bug mistakes