Microsoft has warned that Windows Small Business Server has a critical vulnerability, first patched two weeks ago.
In an update to its MS08-001 security bulletin, Microsoft said that the latest release of Windows Small Business Server was also critically at risk from a bug in Windows' networking software. The flaw is also considered critical for Windows XP and Vista users.
Microsoft did not say why it had initially omitted Small Business Server from its list of critically affected operating systems, but it said that the product's users were being offered patches via Microsoft's various automatic update services. "Customers with Windows Small Business Server 2003 Service Pack 2 should apply the update to remain secure," Microsoft said in its updated bulletin.
The bug lies in the way Windows processes networking traffic that uses Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) protocols, which are used to send data to many systems at the same time. Microsoft said that an attacker could send specially crafted packets to a victim's machine, which could then allow the attacker to run unauthorised code on a system.
Microsoft rates the flaw as 'important' for Windows Server 2003, meaning that it would be more difficult for attackers to exploit the flaw on this operating system.
Security experts are paying particular attention to this vulnerability because it could be exploited by attackers to create a self-replicating worm attack.
The flaw is not being exploited in online attacks, but last week researchers at penetration-testing-software vendor Immunity made a sample exploit available to their customers. That software causes an unpatched system to crash, but the company is close to developing code that could be used to install unauthorised software on a victim's computer, according to Immunity Chief Technology Officer Dave Aitel.
Aitel said it's no surprise that the small business version of Windows Server 2003 is at risk.
"I assumed most 2003 servers in the real world were vulnerable," he said. "Windows Server 2003 by default does not have any multicast addresses active and would not be affected by this vulnerability. However, installing applications that use multicasting could cause the operating system to become vulnerable."
He said that Microsoft could help its customers by giving them more details on how to avoid being at risk to this problem. "What features can I enable on Windows Server 2003 to become vulnerable?" he asked.