One day after patching a widely exploited flaw in its IE (Internet Explorer) browser, Microsoft has another bug to worry about, this time in PowerPoint.

Attackers have been exploiting a newly discovered bug in Microsoft's Office presentation software in extremely targeted attacks, McAfee reported yesterday.

Researchers were made aware of the attacks when a customer submitted two different malicious PowerPoint files, both of which exploited the same vulnerability, said Craig Schmugar, a virus researcher at McAfee. Both files installed malicious remote access Trojan software that then attempted to connect to an outside web server, he said.

Though McAfee is not releasing technical details of the exploit, the security vendor says it has confirmed that the attack works on three versions of Office running on the Windows 2000 OS (operating system): Office 2000, Office XP and Office 2003. Other platforms and other Office applications may also be affected, but McAfee has not yet had time to complete its testing, Schmugar said.

Schmugar has blogged about the issue here.

Microsoft issued a security advisory on the matter yesterday, saying the issue affects users of Office 2000, Office 2003 and Office XP, as well as Microsoft PowerPoint 2004 for Mac. Microsoft's advisory can be found here.

As a workaround, Microsoft suggests that users open and view files using PowerPoint Viewer 2003. This software "does not contain the vulnerable code and is not susceptible to this attack", the advisory states. The PowerPoint viewer can be downloaded here.

Microsoft and other security vendors, including Symantec and McAfee, have added signatures to their security products so that they can detect this malicious code.

Over the past few months, attackers have focused on Office, exploiting a number of undisclosed Office bugs in extremely targeted attacks, often on government agencies or contractors. These attacks usually take the form of an email that has a malicious Office document attached and is sent to a small number of target victims.

This latest PowerPoint attack fits that pattern and was sent to a defence contractor, Schmugar said. He declined to provide further details on the intended victim.

Because the attack has been extremely limited in scope it is considered to be a low risk for most users, Schmugar said.

News of the attack comes the day after Microsoft issued an emergency patch for a widely exploited bug that affected the VML (vector markup language) rendering engine used by IE and Outlook. Hackers are exploiting this critical flaw in the browser via mass email and on thousands of websites, security experts said.

Microsoft's next set of security patches will be released 10 October. The software vendor did not say whether it plans to patch this latest PowerPoint flaw.