President Donald Trump has signed the Allow States and Victims to Fight Online Sex Trafficking Act (FOSTA), a bill that aims to fight sex trafficking by reducing legal protections for online platforms. FOSTA passed the Senate in March by an overwhelming majority, and it’s been endorsed by the Internet Association, which represents major companies like Facebook and Google. But privacy and civil liberties advocates say it’s a fatally flawed bill that would hurt small online communities, and sex workers say it will make them less safe by driving them offline.

FOSTA carves out a new exception to Section 230 of the Communication Decency Act, which shields website operators from liability for user-generated content. It states that Section 230 doesn’t apply to civil and criminal charges of sex trafficking, or to conduct that “promotes or facilitates prostitution.” The rule applies retroactively to sites that violate it. (It’s sometimes known as the Stop Enabling Sex Traffickers Act or SESTA, after an earlier version of the bill.)

Great news! @POTUS has signed #SESTA into law! A momentous day for survivors of online trafficking and a big victory in our fight to help end online #SexTrafficking in this country. — Rob Portman (@senrobportman) April 11, 2018

FOSTA is clearly aimed at sites like Backpage.com, which are hubs for illegal sex work. But it could make site operators think twice about letting users post any sexual material, especially if they don’t have the legal or technical resources of a huge web platform. Even before the bill had passed, Craigslist removed its “personals” section to avoid legal risk. The Electronic Frontier Foundation has called FOSTA “the most significant rollback to date of the protections for online speech in Section 230.”

We also don’t know how greatly FOSTA will supplement existing anti-prostitution and anti-trafficking law. The Justice Department shut down Backpage and filed criminal charges against its founders last week, before FOSTA was signed. The owner of Rentboy.com, another sex work site, was sentenced to prison for promoting prostitution last year. And FOSTA conflates stopping sex trafficking with stopping consensual sex work, making it difficult for sex workers to screen clients or build communities through online services. Now that the bill is signed into law, we’ll see exactly how web platforms — and sex workers who operate online — respond to this new legal liability.