Hackers accessed Epik, a web platform that hosted sites run by groups like QAnon and the Proud Boys.
Some 15 million names, email addresses, and phone numbers were leaked in the September 13 hack.
The hacking group Anonymous claimed credit.
About 15 million names, phone numbers, email addresses, and physical addresses held by the platform, Epik, were revealed.
The hacking group Anonymous claimed credit for the breach.
Epik's clients have included the Texas Republican Party, Parler, Gab, 8chan, Infowars, BitChute, the Proud Boys, members of QAnon, the pro-gun forum AR15.com, the pro-Trump site The Donald, and the neo-Nazi blog the Daily Stormer, according to Ars Technica and NPR.
Anonymous said on September 13 that it had accessed about 180 gigabytes of archived information, describing it as a "decade's worth of data from the company." News of the breach was first reported by the journalist Steven Monacelli.
Last Friday, Epik CEO Rob Monster held a four-hour online Q&A to address the hack. He said there had been a "hijack of data that should not have been hijacked."
Many far-right and conservative groups have struggled to maintain web-hosting services as a result of their incendiary content.
ProLifeWhistleblower, a site created by the Texas Right to Life movement that allowed people to submit tips about people helping facilitate abortions in Texas, was kicked off GoDaddy on September 2. It moved to Epik.
Parler, a social-media site popular among far-right groups, was dropped by Amazon Web Services following the attack on the US Capitol on January 6.
Epik has also kicked off right-wing clients before.
It banned 8chan after the manifesto of the shooter in Christchurch, New Zealand, was posted on the forum in 2019.
Experts believe hacks like the one claimed by Anonymous will drive the groups to companies outside the US.
"A breach like this will force some of these actors to find security providers outside of North America, in Europe, to possibly step up their security game," Gabriella Coleman, a professor of anthropology at Harvard University, told CNN.
While Anonymous said the hack was aimed at identifying people behind right-wing sites, the personal details of some people who are not Epik clients were also released, the data-privacy monitor Have I Been Pwned said.
"The breach exposed a huge volume of data not just of Epik customers, but also scraped WHOIS records belonging to individuals and organisations who were not Epik customers," the site said.
Correction: This story was updated to clarify Epik's relationship with Gab.
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