Microsoft has released its security software patches for April, addressing an unpatched bug in the Internet Explorer browser that hackers had been exploiting for several weeks.

As expected, the company released five patches, called "updates" in Microsoft parlance, addressing a number of critical vulnerabilities in Internet Explorer and the Windows operating system. Microsoft also released an update for Outlook Express, rated "important", and a fix for Windows FrontPage Server Extensions and SharePoint Team Services 2002, rated "moderate".

In Microsoft's rating system, the most serious vulnerabilities are rated "critical", meaning they could allow unauthorised software to be installed without user action. The "critical" designation is followed by "important", and then "moderate" ratings.

The most anticipated of this month's updates is the MS06-013 patch, which fixes several IE bugs, including the "create TextRange ()" vulnerability reported last month. Hackers had been exploiting this problem by installing unauthorised software on PCs by tricking users into visiting sites that took advantage of the bug.

The problem was serious enough that security vendors eEye Digital Security and Determina created patches to address it. Yesterday, eEye said that it had seen more than 156,000 downloads of its software, which Microsoft does not recommend.

Because of this vulnerability, Isabel Maldonado ended up following Microsoft's advice and disabling Active Scripting on the 1,100 workstations she administers at the county attorney's office in Maricopa, Arizona. Her office's support desk ended up fielding about 100 support calls as a result of these changes over the two-week period that the flaw remained unpatched, said Maldonado, a LAN (local area network) administrator with the county.

Microsoft has said that it tends to avoid releasing early patches - even when they relate to "0day" bugs that hackers are already exploiting - because customers find the regular monthly patch releases far less disruptive.

But Maldonado said she would have been happy to have the TextRange () problem patched earlier. "I would have much rather they'd rushed out a patch," she said. "I can't think of a customer that would say, 'Oh no. Don't send me the patch right now,' if there's a 0day alert."

Security researchers had reported four separate IE vulnerabilities prior to the release of the MS06-13 update, but Microsoft said it has addressed a total of 10 issues - including address spoofing and cross-site scripting vulnerabilities - with the patch.

The IE update will include changes to the way that IE handles ActiveX controls, meaning that on some websites, users may now have to click through some extra steps when using dynamic content such as Flash animation. These changes come as the result of a $521 million judgement against Microsoft in a patent lawsuit brought by Eolas Technologies Inc and the University of California.

Though he does not expect a major malware outbreak following the release of Tuesday's patches, Qualys Product Manager Jonathan Bitle said that hackers are likely to take advantage of some of the vulnerabilities.

"With so many issues addressed by these patches... we expect that we might see some aftershocks," he said. "These issues could easily be exploited leveraging the naivete of inexperience users."

Yesterday Microsoft also patched a similarly critical vulnerability in the way Windows Explorer handles Component Object Model objects. Attackers could take over a system by tricking users into visiting a website that would then connect them to a remote file server. "This remote file server could then cause Windows Explorer to fail in a way that could allow code execution," Microsoft said.

This vulnerability affects all supported versions of Windows, Microsoft said.

The third critical fix in April's updates addresses a vulnerability in an ActiveX control, called RDS.Dataspace, which is distributed with the Microsoft Data Access Components. This software is included with the Windows operating system and is typically used by database software.

The RDS.Dataspace component problem is rated critical for Windows 98, Windows 2000 and Windows XP. It is considered a moderate risk for Windows Server 2003 users.

Information on all the patches released can be found here.

Click here for the best Microsoft voucher codes.