The move was a rare one for Microsoft. Although the company is often the target of patent litigation, it's rarely the instigator. In Microsoft's , it's accused others of infringing its patents only three times before today.

The last time was in 2009 when it over the company's in-car navigational devices.

The other patent infringement lawsuits Microsoft initiated were against Taiwanese mouse maker Primax Electronics in 2008, and peripherals manufacturer Belkin in 2006. Microsoft settled with both companies before the lawsuits reached trial.

"They don't sue often," said Rob Enderle, an analyst at Enderle Group. "It's very rare that they aggressively go after someone [on patents]. But these are core Microsoft patents, the company's crown jewels. They'll heavily defend these against any software developer."

According to Microsoft's complaint, which was filed on Tuesday in a Seattle federal court, Salesforce.com is using Microsoft-patented technologies in its Web-based CRM software and service service, and in supporting hardware and software.