Microsoft is warning users of its IE (Internet Explorer) browser to use caution on the web, after the disclosure of an unpatched bug that could allow attackers to seize control of a PC running the browser software.

The bug, disclosed on Wednesday, relates to the way IE processes information using the 'createTextRange()' method. By presenting the browser with specially crafted code, attackers could corrupt the system's memory and trick it into running unauthorised software.

"We're still investigating, but we have confirmed this vulnerability and I am writing an advisory on this," wrote Lennart Wistrand, security program manager at the Microsoft Security Response Center, in a blog posting. "We will address it in a security update."

Microsoft's next scheduled set of security updates are due on 11 April, but Wistrand did not say whether the TextRange() bug would be patched then. Microsoft executives were not immediately available to comment on this story.

Wistrand offered IE users a workaround to avoid the problem. "Our initial investigation has revealed that if you turn off Active Scripting, that will prevent the attack," he wrote.

IE users with the latest refresh of the IE 7.0 Beta 2.0 preview software, announced this week, are "not affected" by the problem, Wistrand said. Outlook and Outlook Express users are not at risk either, he added.

This is the third IE bug to be reported in recent days. A bug disclosed on Tuesday is also considered critical because it could also be used to seize control of a system. A third flaw, made public last Thursday, is considered less severe, but it can cause IE to crash.

The TextRange() method bug is considered the most serious of the three because it appears to be relatively easy to exploit.

Wistrand's blog posting can be found here.