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

Salesforce.com bills its Web-based CRM (customer relationship management) products as easier to use and maintain than traditional CRM software, but this latest development underlines the security risks that come with this more open model.

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, e-mail addresses and telephone numbers of Salesforce.com customers.

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

Some of those customers then fell victim to the scam and gave up their passwords to the criminals, too. When Salesforce.com started seeing malicious software being attached to these e-mails, 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 victimized by this scam.

Jan Sabelstrom noticed that something was amiss when an e-mail purporting to be from the U.S. Federal Trade Commission landed in his inbox. This phishing attempt 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 Oct. 30—the same day that Salesforce.com notified its customers of the problem.

"I basically shot them an e-mail 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."