An exploit of unpatched Microsoft vulnerability has been added to a multi-strike attack toolkit, according to security company Symantec. The company said that this may mean attacks will increase soon.

Symantec said that an in-the-wild exploit of the DirectShow bug, which Microsoft acknowledged a month ago, has been added to at least one web-based attack kit. "This will likely lead to widespread use in a short time," said Liam Murchu, a researcher with Symantec's security response group, in blog entry.

Microsoft has not yet issued a fix for the DirectShow bug, which affects Windows 2000, XP and Server 2003, but not the newer Windows Vista or Server 2008. The flaw also doesn't affect the not-yet-released Windows 7.

However, attacks using the bug, which hits Windows XP and Server 2003, have been tracked since May, when Microsoft issued a security advisory and confirmed it had evidence of "limited, active attacks."

Unlike other recent exploits of Microsoft zero-days - vulnerabilities that haven't been patched by the time attack code surfaces - the DirectShow attacks are not targeting specific individuals or organisations. "This is not a targeted attack, but is one of limited distribution," Ben Greenbaum, a senior research manager with Symantec.

What caught researchers' attention, added Greenbaum, was that the DirectShow exploit piggybacked on a run-of-the-mill phishing attack. It's becoming more common, said Greenbaum, that a phishing site - in this case a bogus log-in page for Microsoft's Windows Live software - also hosts malware that tries to hijack PCs.

"They're thinking: 'Why not try to get them with everything we can?'" said Greenbaum, referring to the attackers.

The phishing site silently redirects visitors to another URL that hosts the DirectShow attack code, which is in the form of a malicious .avi file. Multiple malformed .dll files are also loaded onto the victimised system; those .dlls, in turn, load an encoded .exe payload that then downloads and installs a Trojan horse that adds the compromised PC to a growing botnet.

Although a patch is not yet available, Microsoft has suggested that users disable QuickTime parsing on Windows 2000, XP and Server 2003 machines. QuickTime, which is Apple's media player, is not flawed, but the QuickTime parser in DirectShow, a component of DirectX, is. Microsoft has posted a tool that automates the process of disabling QuickTime parsing in Windows; normally that requires editing the Windows registry, a chore many users avoid.

Microsoft's next regularly-scheduled security updates will be released 14 July. Most researches expect the company to patch the DirectShow bug then.