Security researchers have released attack code that will crash Windows machines susceptible to a recently patched bug in the operating system.
The code, which is not available to the general public, has been released to security professionals who use Immunity's Canvas computer security testing software. It causes the Windows system to crash but does not let the attacker run malicious software on the victim's system.
"It reliably crashes Windows machines," said Dave Aitel, Immunity's chief technology officer. "In fact, it blue-screened our print server by accident - this is a broadcast attack, after all."
That's the biggest concern for security experts who worry that a more dangerous attack may soon follow as researchers dig further into the vulnerability. The bug is particularly troublesome for two reasons. First, it affects a widely used Windows component that is turned on by default. Worse, no user interaction is required to trigger the flaw, meaning that it could be exploited in a self-copying worm attack.
Microsoft patched the flaw in its MS08-001 update, released last week, but it takes time for enterprise users to test and install Microsoft's patches.
The flaw lies in the way Windows processes networking traffic that uses IGMP (Internet Group Management Protocol) and the MLD (Multicast Listener Discovery) protocol, which are used to send data to many systems at the same time. The protocols are used by a range of applications including messaging, web conferencing and software distribution products.
For a worm attack to work, the attacker would have to send specially crafted packets to a victim's machine, which could then allow the attacker to run unauthorised code on the PC. The worm could then spread from computer to computer within a LAN, but would generally be stopped from travelling to another network by a firewall.
A reliable exploit could be combined with malicious botnet software, giving attackers a way to widen the size of their networks of infected computers. The flaw is rated critical for Windows XP and Vista systems, according to Microsoft.
After patching the flaw, Microsoft published some technical research indicating that it would be hard for an attacker to exploit this vulnerability.
But Aitel believes that Microsoft may have overestimated how difficult it would be to create reliable attack code. Because it could spread so quickly through a network, a reliable exploit "is going to be worth the effort," Aitel said. "You can be assured lots of smart people are working on it."
Part of the problem is that IT staff may not be aware of how widely these multicasting protocols are used within their companies, said Russ Cooper, a senior network consultant with Verizon Business. "I am extremely worried that this becomes a problem simply because people are unaware of what they're already allowing," he said.
If one machine were infected within a network subnet, its attempt to attack other machines might not even be noticeable, he added. "It may look like a large file transfer."
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