Microsoft has offered users a few tips on how to avoid falling victim to a critical bug in its Excel spreadsheet software. The company also offered a handful of workarounds to mitigate the risk.

Microsoft stopped short of issuing a fix for the vulnerability, which has to do with the way that Excel uses the computer's memory, but the company said that such an update is in the works.

Reports of the vulnerability first began circulating late last week, when Microsoft said that hackers had launched a targeted attack against one of its customers using the vulnerability. The flaw could be exploited to run unauthorised software on a Windows PC, but for this to happen, attackers would first need to either trick an Excel user into visiting a malicious website or to open a malicious Excel attachment.

The bug exists in many versions of the spreadsheet software, including Excel 2000, Excel 2002 and Excel 2003, the advisory states.

Advanced Windows users can block the vulnerability by editing their registry settings or by setting up their e-mail gateway to block Excel attachments, Microsoft said. Users can also cut down on the risk by simply avoiding Excel documents that are sent from untrusted sources.

Microsoft is testing a security patch that fixes the problem, but a spokeswoman for the company's public relations agency could not say whether it would be released as part of the company's next round of security updates, expected 11 July.

Microsoft's security researchers have been busy over the past week. The Excel bug is being patched just days after the June security updates, which included 12 patches. Microsoft researchers also spent Monday investigating a hack that shut down part of the company's French webite.

Both Microsoft and security vendor Symantec say that the vulnerability is being used in small-scale, targeted attacks and has not yet been seen in any widespread malware.

An unofficial Frequently Asked Questions list on the vulnerability, with more details on the malware that exploits it, can be found here.