Salesforce.com's customers are targets of malicious hackers and scammers, after an employee was tricked into handing a corporate password to a phisher.

In a note to customers, Salesforce said that criminals have been sending customers fake invoices and, starting just a few days ago, viruses and key logging software. The emails were sent using information that was illegally obtained from Salesforce.com.

The problems began a few months ago, when a Salesforce.com employee fell for a phishing scam and divulged a company password that gave attackers access to a customer contact list. With this password, the criminals were able to obtain first and last names, company names, email addresses and telephone numbers of Salesforce.com customers.

"As a result of this, a small number of our customers began receiving bogus emails that looked like Salesforce.com invoices," Salesforce.com said.

Some of those customers also fell for the scam and gave up their passwords too. When Salesforce.com started seeing malicious software being attached to these emails, the company decided to issue a general alert to its nearly 1 million subscribers.

According to the Washington Post, Suntrust Banks was one of the customers victimised by this scam.

Jan Sabelstrom noticed that something was amiss when an email purporting to be from the US Federal Trade Commission landed in his inbox. This attack contained information about one of his company's customers that would have been available to Salesforce.com, but not the public at large, he said.

Sabelstrom, managing director of CaSa Customer Solutions, a Chicago-based CRM consultancy, said he emailed Salesforce employees, including CEO Marc Benioff, about the message on the same day last week that Salesforce.com notified its customers of the problem.

"I basically shot them an email saying... I would like to understand how this came to be," he said. "It seems a little bit dubious to me that there's this connection between me and my customers."

Salesforce.com's response showed him that the company was taking the issue seriously, Sabelstrom said. Within two hours he heard back from Benioff, and soon the company's security team was walking him through what had happened, and assuring him that his customer's data had not been breached. "I was impressed," he said. "You can call it damage control but it was attentiveness."

Salesforce.com is working with law enforcement to resolve the problem, but in the meantime it is recommending that customers implement a number of security measures in order to cut down on the phisher's chance of succeeding.

Suggested actions include restricting Salesforce.com account access to users who are within the corporate network, phishing education or the use of stronger user authentication techniques to log on to the Salesforce.com servers.

Salesforce.com has declined to comment further on the matter. "Everything that they have to say about it is in this note," a spokesman said.