We often talk about malware here on the NetDefence blog and the importance in having effective anti-virus software on each computer you use, however sometimes even having your anti-virus software up to date will not protect you.
A Zero Day Exploit is any vulnerability in software that is unpatched at that point in time. Zero Day Exploits are constantly being found by anti-virus companies and patches are released, but the time in between is when your computer is at high risk.
Today (25th February 2009) a Zero Day Exploit has been found for Microsoft Excel and appears to be affecting all versions on all operating systems. This exploit allows someone to run code on your computer which can install a trojan called Trojan.Mdropper.AC. It is highly recommended you are very cautious in opening Excel files over the next 24 hours and ensure you run an update on your anti-virus software.
When a Zero Day Exploit like this occurs, your only defence may be using your own smarts to determine whether an email is legitimate or not. Ask yourself, do I know this sender? Am I expecting this email and attachment? If you can’t answer at lease one of these questions, it is best to delete the email and keep your computer virus free!