Many technology companies consider Section 230 of the Communications Decency Act to be a foundation of the Internet economy. The 1996 law gives website owners broad immunity for content submitted by users. Advocates say that allows websites to host a wide variety of user-generated content without worrying about getting sued.

Now, Congress is considering the first significant change to the law in its 21-year history. Critics say certain websites have hidden behind the law while publishing ads for the sexual exploitation of children. Activists are pushing for legislation that would carve out a sex trafficking exception to Section 230, allowing state prosecutions and private lawsuits against websites that host ads for sex with children.

"We've crafted a law that protects a sector of business with complete immunity against civil actions by victims even where there's evidence that that company has knowingly facilitated the trafficking of that child," says Samantha Vardaman of the advocacy organization Shared Hope.

The legislation is popular on Capitol Hill. The Senate version of the legislation enjoys at least 27 co-sponsors from both parties. Similar legislation in the House has a bipartisan group of more than 100 co-sponsors. With that kind of momentum, the proposal has a real chance of becoming law.

But the technology industry and free-speech advocates say the law is a bad idea. They argue that even a narrow exception to Section 230 would open the floodgates for state officials and private plaintiffs to harass websites.

Critics point out that Congress passed legislation in 2015 designed to allow the feds to prosecute sites that host ads for sex with children. They argue that Congress should see how well that law works before legislating further.

Finally, critics say that shutting down high-profile sites would simply push advertisements underground where they will be harder for law enforcement to track.

This is really a fight over Backpage

I said earlier that "certain websites" have hidden behind Section 230, but the debate over the legislation has focused almost entirely on one website: Backpage.com. After Craigslist shut down its adult services section in 2010, Backpage quickly became one of the Internet's leading destinations for advertising sexual services.

Last year, a Senate investigative committee completed work on an in-depth investigation of Backpage and its business practices. According to internal Backpage documents obtained by Senate investigators, adult ads accounted for 93 percent of the company's ad revenue in 2011. Many of these ads were for prostitution—a service that's illegal everywhere in the United States, aside from a few counties in Nevada.

Senate investigators say Backpage executives knew that a large fraction of Backpage ads were for illegal services. In the site's early years, moderators were instructed to reject ads that appeared to be soliciting prostitution. But that was bad for business, so moderators began editing user-submitted ads instead, deleting words and phrases that indicated an explicit offer of sex for money.

In 2010, the company started automating the process so that words and phrases signaling illegal prostitution—like "full service," "no limits," and price lists for services completed in under an hour—were automatically stripped out of ads—but the rest of the ad was posted online. In October 2010, a Backpage official estimated that the company was editing 70 to 80 percent of ads to strip out problematic phrases.

Of course, changing the words in an ad doesn't change the kind of service the advertiser is offering. "Cleaning up" an ad for prostitution doesn't change the fact that it's an offer of sex for money. It simply helps to insulate Backpage from the backlash the site would otherwise get for facilitating an illegal transaction.

Disturbingly, Backpage allegedly took the same approach when it received ads for sexual exploitation of children. Rather than rejecting these ads outright, the Senate report claimed, Backpage's software would strip terms like “lolita,” “little girl,” “school girl," and “amber alert” out of ads before posting the rest of the ad online. (In a 2011 e-mail, a Backpage executive told a Texas law enforcement officer that "amber alert" was "either a horrible marketing ploy or some kind of bizarre new code word for an under-aged person.")

We asked Backpage for comment on the allegations in the Senate report, but it declined to comment on the record.

"Backpage horrifies people," says Daphne Keller, a legal expert at Stanford University. She pointed to a film about Backpage, called I Am Jane Doe, that was widely circulated on Capitol Hill and spurred legislators into action.

Hosting ads for underage sex services has made Backpage no shortage of enemies, and the company has relied heavily on Section 230 to dodge both criminal prosecution and civil lawsuits.

Last year, California filed pimping charges against the three men behind the site: Carl Ferrer, Michael Lacey, and James Larkin. But the case was dismissed because the trio enjoyed protection under Section 230. California quickly filed another case against Backpage on charges of money laundering.

Last year, courts also dismissed a lawsuit from three anonymous underage girls who say they were trafficked on Backpage. The court ruled that Section 230 doesn't allow victims to sue a website over ads submitted by third parties. The mother of another trafficking victim filed a lawsuit against Backpage this year.

With Backpage largely shielded from direct legal attacks, public officials have looked for ways to attack Backpage indirectly. In 2015, Thomas Dart, sheriff of Chicago's Cook County, used legal threats to convince Visa and MasterCard to drop Backpage as a customer. Backpage sued in federal court, which ruled that Dart's campaign violated Backpage's First Amendment rights.

Anti-trafficking advocates say that this illustrates a problem with Section 230. They argue that Backpage is facilitating and profiting from the sexual exploitation of children, and Section 230 leaves victims and law enforcement with no way to hold them accountable. So they've been lobbying Congress to pass legislation that allows state prosecutors and private plaintiffs to go after companies like Backpage.

Opponents say weakening Section 230 is a mistake

Free speech advocates and technology companies don't buy this argument, however. And they point to three big problems with the Senate's Stop Enabling Sex Traffickers Act and its House companion, the Allow States and Victims to Fight Online Sex Trafficking Act.

First, they say that Congress already took action to address this problem. Under Section 230, the courts have given websites broad immunity against state prosecutors and private plaintiffs, but the law leaves the door open for criminal prosecutions at the federal level. In 2015, Congress amended sex trafficking laws to make advertising underage sexual services a crime. This provision was widely viewed as a Backpage killer.

Two years later, federal prosecutors don't seem to have used this new law yet. But there are rumors that prosecutors in Arizona are working on a case against the Backpage founders. Critics of new sex trafficking legislation argue that Congress should wait and see if federal prosecutors can address the problem using the 2015 law before they consider re-writing Section 230.

Opponents also worry that creating a Section 230 exception for child exploitation could be a camel's nose under the tent that would ultimately expose a wide range of websites to frivolous litigation. The Senate bill would allow "any State criminal prosecution or civil enforcement action targeting conduct that violates a Federal criminal law prohibiting sex trafficking of children." But critics say it's not clear what "targeting" means here.

For example, Santa Clara University legal scholar Eric Goldman says we should imagine if a state passed a law saying, "If you don't authenticate all of your users, you are liable for any non-authenticated content promoting sex trafficking of children." That, Goldman says, could be considered a law "targeting" a federal crime. Yet it could also have far-reaching effects, since many websites—not just companies running adult personal ads—could be forced to add age-verification capabilities to their sites.

But that concern doesn't impress Mary Leary, a professor at the Catholic University Law School and a supporter of the legislation. "I think this language is drafted very narrowly to not go there," she told Ars in an August interview. "There's no federal criminal law that says you have to age-verify users."

We won't know for sure who is right unless Congress passes the legislation and states start taking advantage of the new exception to Section 230. But the technology industry and free speech advocates worry that even a small change to Section 230 could create headaches for website operators with no connection to the sex trade. Current law gives website owners blanket immunity, allowing them to dispose of lawsuits quickly and with minimal expense. A new exemption could open the door for frivolous litigation that claims to be related to sex trafficking. Even if these lawsuits are ultimately thrown out, technology companies could face significant legal expenses in the meantime.

Even if that's true, activists say it's a small price to pay to stop the sexual exploitation of children.

"The bottom line is the interests that are facing off here are economic versus human protections," Vardaman says. "It's protection of children. We can't fathom a world in which we place economics over the protection of our children."

Why shutting down Backpage might not reduce trafficking

Defenders of Section 230 also question whether shutting down a site like Backpage would actually help victims of trafficking.

"There is neither an empirical foundation for the assumption that the platforms cause trafficking nor any evidence that shuttering them would reduce trafficking," writes Notre Dame legal scholar Alexandra Levy. "To the contrary, allowing Internet platforms on which sexual services are brokered to thrive may be key to apprehending traffickers and recovering victims."

Backpage's critics have made much of how reports of online sex trafficking have skyrocketed in recent years. That might be because the problem has gotten worse. But another possibility, Levy points out, is that Backpage has made it easier for the authorities to find out about trafficking incidents and do something about them.

If activists succeed in shutting down Backpage, that won't spell the end of sex trafficking, Levy argues. Girls will continue to be trafficked, with their services advertised elsewhere. And, crucially, these underground venues are less likely to help law enforcement to identify and rescue the victims.

Vardaman isn't convinced by this argument. She notes that there appears to have been a temporary decrease in trafficking activity in the wake of the 2010 shutdown of Craigslist—but she acknowledges, "Backpage did eventually exceed Craigslist's level of profit and activity."

The problem, in Vardaman's view, is that the authorities didn't "fill the void with new or creative law enforcement technique" after Craigslist was shut down. Vardaman acknowledges that shutting down Backpage won't cause trafficking to go away—but she believes that it's a step in the right direction.