Security researchers have given Microsoft more work to do as it prepares an upcoming IE (Internet Explorer) security fix. Yesterday, they disclosed a critical vulnerability in Microsoft's browser - the third flaw to be disclosed in the past week.

This bug is rated critical because it could allow an attacker to seize control of a victim's system, according to UK-based Computer Terrorism, which posted an advisory on the flaw.

The problem relates to the way that 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.

Computer Terrorism did not release sample code showing how the bug works, but the security researchers did post some details of the problem to the Full Disclosure mailing list yesterday.

The Computer Terrorism researchers said they had produced a "reliable proof of concept" for this vulnerability that had been tested on IE 6.0 and the new IE 7.0 beta 2.0 release running on the latest version of Windows XP.

Microsoft is working on a patch for the bug, according to Secunia, which has published an advisory on the vulnerability. Secunia rates the issue as "highly critical".

Microsoft has two other IE flaws to consider as it readies its next round of security patches, scheduled for 11 April.

One of these bugs, which was disclosed yesterday, is considered critical because it could also be used to seize control of a system. The second bug, published last Thursday, is considered less critical, but it can cause IE to crash.

This latest vulnerability appears to be the most significant of the three, because it can apparently be exploited without difficulty, according to one security researcher. "This seems very likely to be exploited soon," said Cesar Cerrudo, chief executive officer of security research firm Argeniss.

With the information that has already been made public about the createTextRange() flaw, "you can reproduce the IE crash and by playing with IE then you can build an exploit," Cerrudo said.

Microsoft confirmed on Tuesday that it is working on an IE security update. "That update is currently in our testing process and could come out as early as April," said Stephen Toulouse, a security program manager with Microsoft's security response centre. "However, there's no firm date".

Representatives from the company were not immediately available yesterday to comment on the createTextRange() vulnerability.

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