Twitter accounts belonging to Joe Biden, Bill Gates, Elon Musk and Apple, among other prominent handles, were compromised on Wednesday and posted tweets that appeared to promote a cryptocurrency scam.
The accounts, along with those of former President Barack Obama, Kanye West, Kim Kardashian West, Warren Buffett, Jeff Bezos and Mike Bloomberg, posted similar tweets soliciting donations via Bitcoin to their verified profiles on Wednesday.
"Everyone is asking me to give back, and now is the time," Gates' tweet said, promising to double all payments to a Bitcoin address for the next 30 minutes.
In a tweet on Wednesday, Twitter's support account said: "We are aware of a security incident impacting accounts on Twitter. We are investigating and taking steps to fix it. We will update everyone shortly."
A little more than an hour after the attack began, Twitter moved to prevent holders of verified accounts from tweeting. Non-verified accounts could still tweet, however.
"You may be unable to Tweet or reset your password while we review and address this incident," Twitter's support account said, subsequently adding that "some other account functionalities" would also be limited.
Around 8:30 pm ET, roughly three hours after Twitter first said that it was investigating the apparent hack and a little more than two hours after it shut down tweeting for some accounts, Twitter said the majority of accounts had been restored to full functionality.
"Most accounts should be able to Tweet again. As we continue working on a fix, this functionality may come and go." the company said. "We're working to get things back to normal as quickly as possible."
The sheer number of prominent accounts impacted made it arguably the most significant security incident in Twitter's history.
A hack like this mainly concerns not just because of any financial scam that can be run, but because so many world leaders use Twitter -- and some, like President Donald Trump, use it to announce major policy decisions.
A hack that took over an account belonging to one of those leaders could have devastating consequences.
Last year, Twitter CEO Jack Dorsey's account was hacked, raising concerns about whether any account on the platform can genuinely avoid being compromised. Twitter fixed the mechanisms by which that hack occurred after Dorsey's hack, and there's no reason to believe it is to blame here.
A campaign aide for Biden said Twitter "locked down" his account immediately. "We remain in touch with Twitter on the matter," the aide added.
Barack Obama's Twitter account also appeared to be compromised as part of a broader security incident on the platform Wednesday.
"We can confirm that Bill Gates did not send this tweet," a spokesperson for Gates told CNN Business. "This appears to be part of a larger issue that Twitter is facing. Twitter is aware and working to restore the account."
Asked whether it was concerned about the President's account potentially being affected, or whether it was in touch with Twitter about the issue, the White House declined to comment.
The first Bitcoin wallet featured in some of the tweets only became active on Wednesday, Tim Cotten, a Bitcoin researcher, told CNN Business. In the hours immediately after the wallet's identification number was posted to Twitter, it received more than $100,000 worth of Bitcoins through hundreds of transactions, Cotten said.
Some of that Bitcoin was then transferred to other wallets, he added.
The apparent scam has also caught the attention of the FBI.
"We are aware of today's security incident involving several Twitter accounts belonging to high profile individuals," the FBI's San Francisco field office said in a statement. "The accounts appear to have been compromised to perpetuate cryptocurrency fraud. We advise the public not to fall victim to this scam by sending cryptocurrency or money concerning this incident."