Virus authors have reportedly created a Trojan that exploits an unpatched vulnerability in a range of versions of Excel.

The malware comes in the form of a maliciously constructed spreadsheet file with a malicious payload identified by McAfee, for example, as the BackDoor-DUE trojan. Many versions of Excel are vulnerable, including 2000, 2002, 2003, 2007, 2004/2008 for Mac, Excel Viewer/Excel Viewer 2003.

Opening up an infected file using vulnerable software packages creates a backdoor. Attacks thus far are "very targeted and limited" and similar in this respect to malware targeting the also unpatched Adobe PDF flaw, McAfee reports.

Microsoft said it was investigating reports of the vulnerability and its exploitation, in a holding statement. It's a toss up at the time as to whether an out-of-sequence patch will follow or whether Microsoft elects to wait two weeks until the next scheduled Patch Tuesday update or (if fixing the flaw turns out to be (particularly tricky) even longer before releasing a patch.

In the meantime, users are urged to practice caution, especially about Excel files from untrusted sources.

(The Ragister)