Extremely critical Windows flaw code appears

A UK security researcher has released exploit code for a bug in Internet Explorer that could allow a remote attacker to take over Windows systems - even Windows XP with Service Pack 2 installed, according to security experts.

The proof-of-concept code was developed by ComputerTerrorism, and takes advantage of an unpatched Explorer bug previously thought to be relatively harmless. "Contrary to popular belief, the aforementioned security issue is susceptible to remote, arbitrary code execution, yielding full system access with the privileges of the underlying user," ComputerTerrorism said in an advisory.

Until now the bug was thought to allow attackers only to crash a target system.

The bug could be exploited by luring a user to a malicious website, or by embedding malicious code in a trusted site. Because Microsoft hasn't yet patched the flaw, the only reliable protection for Explorer users is to disable Active Scripting, said security researchers.

"Currently, the only way to protect against exploitation of this vulnerability is by disabling active scripting or by using another browser," said Thomas Kristensen, chief technical officer of security firm Secunia, which maintains a vulnerabilities database. "Secunia has raised the criticality of this vulnerability to Extremely Critical because working exploit code is publicly available." Secunia's advisory is here.

Explorer versions 5.5 and 6.x are affected, according to ComputerTerrorism. The firm noted that Microsoft has not yet released a fix for the bug despite the fact that the original bug was brought to the public's attention at the end of May. "To date, the vendor has failed to publicly acknowledge the presence of the flaw, or provide any timescales for an appropriate fix," ComputerTerrorism said.

Microsoft said it is investigating the issue, and may issue a patch. It also criticised ComputerTerrorism's disclosure of the bug, saying it may have made things worse. "Microsoft is concerned that this new report of a vulnerability in Internet Explorer was not disclosed responsibly, potentially putting computer users at risk," the company said in an advisory.

Some Windows systems aren't vulnerable to the attack, Microsoft said. "By default, Internet Explorer on Windows Server 2003, on Windows Server 2003 Service Pack 1, on Windows Server 2003 with Service Pack 1 for Itanium-based Systems, and on Windows Server 2003 x64 Edition runs in a restricted mode that is known as Enhanced Security Configuration. This mode mitigates this vulnerability," the company said.

ComputerTerrorism said it has confirmed the bug on a fully patched system running Windows XP SP2 and Windows 2000 SP4.

Last week, Microsoft issued an advisory warning users about a newly disclosed denial-of-service vulnerability in Windows 2000 Service Pack 4 and Windows XP Service Pack 1.

The company was prompted to issue the advisory because of reports about proof-of-concept code that seeks to exploit the flaw, the company said in its advisory. "Microsoft is concerned that this new report of a vulnerability in Windows 2000 Service Pack 4 and Windows XP Service Pack 1 was not disclosed responsibly, potentially putting computer users at risk," the company said.