Microsoft confirmed last week that it has been working on a critical vulnerability in SQL Server for more than eight months.

However the software giant declined to say whether it has had a patch ready since September, as an Austrian security researcher has alleged.

Last week, Redmond warned customers of a bug that could be used to compromise servers running older versions of the database software, which is widely used to power websites and applications.

"Microsoft opened an investigation for this vulnerability in April upon the initial report by the security researcher," said a company spokesman in an email. "We immediately started an investigation and have been working on this issue since that time," he added.

The researcher, Bernhard Mueller of SEC Consult Security, a Vienna-based security consulting company, went public with details of the vulnerability as well as exploit code on 9 December, apparently after tiring of Microsoft's lack of communication.

According to Mueller, who posted findings in an advisory on the SEC Consult site, as well as to prominent security mailing lists, the bug was reported to Microsoft on 17 April, 2008, and Microsoft's last message to him was on 29 September. After four requests for an update on a patch's status during October and November, Mueller disclosed the vulnerability.

Mueller also said that Microsoft had informed him in September that it had completed a fix.

The Microsoft spokesman didn't directly respond to a question about whether the company had a patch in hand, as Mueller claimed, but instead said, "At this time, security updates are not available for the affected versions listed in Microsoft Security Advisory 961040."

Although it is true that Microsoft has not yet issued an update to the affected software - which includes SQL Server 2000 , SQL Server 2005, SQL Server 2005 Express Edition, SQL Server 2000 Desktop Engine, Microsoft SQL Server 2000 Desktop Engine and Windows Internal Database - one security expert said he's betting that the company will release one soon.

"We expect that Microsoft is currently working on patch and will release it out of band," said Wolfgang Kandek , chief technology officer at security company Qualys.

So-called "out-0f-band" or "out-of-cycle" updates are those that Microsoft issues on days other than its regularly scheduled monthly Patch Tuesday. Microsoft's next scheduled update is set for 13 January, 2009.

Microsoft has released two out-of-cycle emergency updates in the last two months, the most recent a fix to plug a hole in all versions of Internet Explorer. The IE vulnerability, however, was already being exploited by hackers prior to the patch's release; Microsoft has said it has no reports of in-the-wild exploitation of the SQL Server bug.

Kandek believes that a SQL Server patch will present more problems for companies than they faced with the IE fix. "Patch deployment will be slow," he said. "SQL [Server] is part of the core server infrastructure of many enterprise companies and is subject to lengthy patch and testing cycles before any such fix can be deployed."

In lieu of a patch, Microsoft has urged users to deny permissions to the SQL procedure that can be used to trigger the bug. It then updated that recommendation by posting a Visual Basic script which, when run, automates the workaround. "Essentially, the script iterates through the running instances of SQL Server and denies execute permissions on 'sp_replwritetovarbin' to 'public' on all the affected versions," said Microsoft spokesman Bill Sisk in an entry to the Microsoft Security Response Center blog.

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