MessageLabs said that since 4.55 pm (GMT) on Monday, its spam filters have intercepted over 450 malicious emails containing attachments and purportedly coming from the U.S. Department of Justice and targeted specifically at CEOs, CIOs, CFOs and vice-presidents at financial institutions.

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By targeting C-level executives and downloading malware onto their computers, criminals might hope to obtain useful inside information about upcoming mergers and acquisitions, says Paul Wood, senior analyst at MessageLabs.

Then again, says Wood, they might be targeting these executives simply because they are perhaps the easiest to get information about, since news stories generally track the moves of top executives from one company to another.

"Criminals can then supplement their research from social networking sites," Wood adds, pointing to sites such as Linkedin and Plaxo which business executives routinely use to post professional information about themselves.

Of course, many busy C-level executives don't actually open emails themselves, but forward them to their assistant – who may or may not be more vigilant about malicious email.

Other C-level executives might simply forward emails to heads of departments or other employees in the company. But employees may still try to follow through and click on a malicious attachment, since they will recognize that the email comes from inside the company -- perhaps from their boss.

News of the attack comes just days after a new report revealed that 60% of phishing attacks against global banking brands are targeted against U.S. institutions.

MessageLabs points out that Monday's attack is unusual because the emails state the full name of the recipient and their company, in the subject line.

The attack is therefore more likely to succeed than when criminals randomly spam hundreds or thousands of employees.

The attack also marks a return to the use by criminals of malicious attachments, as opposed to malicious links, which had recently become more common, MessageLabs says.

Meanwhile, the number of cyber attacks is continuing to rise rapidly. "Tool kits have made it a lot easier for people to conduct these attacks. Previously criminals had to be quite technically sophisticated. Now they can just get a tool kit which makes it all very simple," Wood explains.