A new vulnerability in Microsoft's Excel has been exposed and exploited in a targeted attack, according to reports. Microsoft's security blog says at least one customer has been affected.

The software giant says for the exploit to work a user must open a malicious Excel document, and reminds its customers to "be very careful opening unsolicited attachments from both known and unknown sources".

The timing of the attack - just days after Microsoft's monthly "patch Tuesday" is thought to be more than mere coincidence. Scott Carpenter, director of Security Labs at Secure Elements told CNet that in recent similar attacks, the software giant has not issued an "out of cycle patch", and suggests the attackers wanted to "take advantage of a full month before Microsoft is scheduled to patch it".

Meanwhile, Symantec warns that attackers are actively exploiting the vulnerability. It says malicious files (called "okN.xls") have been found containing the Trojans Mdropper.J and Booli.A, which can download other files to an infected PC. The company warns that an attacker can take full control of a PC using the vulnerability.

Microsoft says its Windows Live Safety Centre now has detection capabilities for "up-to-date removal of malicious software that attempts to exploit the vulnerability". However, it has said nothing about when it plans to issue a fix. ®