"This bill is an important first step towards addressing significant problems in cyber security," the company said at the time.
The goal of CISPA is to help companies beef up their defenses against hackers who steal business secrets, rob customers' financial information and wreak havoc on computer systems. The bill would remove legal barriers that discourage companies from sharing information about cyber threats.
But in recent weeks, privacy groups and Internet activists have launched a campaign against the bill.
Groups including the American Civil Liberties Union, the Center for Democracy and Technology and the Electronic Frontier Foundation warn CISPA would encourage Internet companies to hand over their customers' private information to military spy agencies.
The campaign aims to recreate the public backlash that forced Congress to drop anti-piracy legislation earlier this year.
Amid the heightened scrutiny over the issue, news site CNET reported last week that Microsoft had backed away from the bill, "citing privacy."
In a statement to CNET, Microsoft said it "believes that any proposed legislation should facilitate the voluntary sharing of cyber threat information in a manner that allows us to honor the privacy and security promises we make to our customers."
"We look forward to continuing to work with members of Congress, consumer groups, the civil liberties community and industry colleagues as the debate moves to the Senate to ensure the final legislation helps to tackle the real threat of cybercrime while protecting consumer privacy," the company said.
CNET interpreted that statement to mean that Microsoft was "no longer as enthusiastic" about the bill.
"That's a noticeable change — albeit not a complete reversal — from Microsoft's position when CISPA was introduced in November 2011," the news site said.
CNET's story was quickly picked up by other technology blogs and discussion forums.
Microsoft is not the only technology company lobbying for CISPA. Facebook, IBM, Oracle, Symantec, AT&T and Verizon have also declared their support.
The White House has threatened to veto the bill, saying it would undermine Internet privacy and fail to protect critical infrastructure systems.
The administration has endorsed an alternative measure in the Senate authored by Sens. Joe Lieberman (I-Conn.) and Susan Collins (R-Maine) that includes tougher protections for people's personal information and would empower the Homeland Security Department to set mandatory security standards for critical infrastructure.
But House GOP leaders have indicated they will not allow a vote on any bill that would create new regulations for cybersecurity.
The House approved CISPA last week on a vote of 248-168, with 42 Democrats supporting the bill and 28 Republicans opposed.