Microsoft and Salesforce both declined to comment.
San Francisco-based Salesforce is No. 1 in the $23 billion-a-year customer relationship management (CRM) market, according to tech research firm Gartner. It helps corporations organize and track sales calls and leads.
Salesforce's services are entirely provided over the cloud, with no software directly installed on PCs. Oracle and Microsoft, which were relatively late to the cloud model, have much smaller online CRM revenues.
As a result, Microsoft and several other big software companies are seeking to beef up their presence in cloud computing.
Read MoreMicrosoft buying spree continues with acquisition
Yet any deal for Salesforce, which has a market capitalization of $49 billion, would be pricey. Salesforce's shares, which have risen 48 percent in the last 12 months, trade at 106.8 times the company's forward earnings, well above Microsoft's multiple of 19.1, according to Thomson Reuters data.
Last month, Oracle Chief Executive Safra Catz said her company could benefit if Microsoft or another rival bought Salesforce.com.
"It would cause a lot of disruption in that market and so I would view that as something that would be helpful to us, especially in the short or medium term, dependent on who it was," Catz said. She declined to comment on whether Oracle had made an approach to buy Salesforce.
Another potential buyer for Salesforce has ruled itself out. SAP Chief Executive Bill McDermott said earlier this week his company has "zero interest" in its software rival.
"We have never bought something that was impaired and in decline," he said, saying that Salesforce's cloud computing software was becoming commoditized.