The NSA-contractor-turned-whistleblower joined digital rights groups including Fight for the Future and the Electronic Frontier Foundation, along with and outspoken CISA opponent Senator Ron Wyden, for an Ask Me Anything session on Reddit, which has also come out against the bill.
"CISA isn't a cybersecurity bill," Snowden wrote during the Q&A. "It's not going to stop any attacks. It's not going to make us any safer. It's a surveillance bill."
The bill's supporters say it would make it easier for tech companies to share data and thwart cyber attacks. Critics, however, argue that the legislation fails to protect user privacy and would only serve to help intelligence agencies track users.
"What it allows is for the companies you interact with everyday – visibly, like Facebook, or invisibly, like AT&T – to indiscriminately share private records about your interactions and activities with the government," Snowden wrote.
"CISA allows private companies to immediately share a perfect record of your private activities the instant you click a link, log in, make a purchase, and so on – and the government with reward for doing it by granting them a special form of legal immunity for their cooperation."
— Edward Snowden (@Snowden) October 27, 2015
Fight for the Future campaign director Evan Greer said the Senate's vote "will go down in history as the moment that lawmakers decided not only what sort of Internet our children and our children's children will have, but what sort of world they will live in."
Added Greer: "Every Senator who votes for CISA will be voting for a world without freedom of expression, a world without true democracy, a world without basic human rights. And they will be voting for their own removal from office, because the Internet will not forget which side of history they stood on."
The House of Representatives passed its version of CISA in April with strong support from Republicans and Democrats.
Any version of CISA passed by the Senate would have to be reconciled with the House bill before it could be sent to the White House for President Barack Obama to sign into law.