A hacker group called OurMine reportedly took over the billionaire's Twitter, Instagram and Pinterest account on Sunday according to screengrabs of Tweets since removed from Zuckerberg's accounts, which are rarely used by the Facebook mogul.
Tech news magazine Engadget took a screengrab of the Tweet in which OurMine said: "Hey @finkd (Mark Zuckerberg's Twitter handle) we got access to your Twitter & Instagram & Pinterest, we are just testing your security, please dm (direct message) us."
Those accounts have since been "re-secured using best practices," a Facebook spokesman told Reuters, adding, "no Facebook systems or accounts were accessed."
The group, whose principal Twitter account has since been suspended, then messaged him to say that it had found his password on a LinkedIn database of user details that was leaked online last month.
"We've invalidated all accounts believed to be at risk here," a LinkedIn spokesperson told Reuters.
Since OurMine's main Twitter account has been suspended, the original tweet is not publicly available, but other Twitter users, including a U.S.-based social media reporter for CNBC, picked up on the hack.
The group said in a secondary Twitter account, @OurMineTeam43, that it was just trying to alert Zuckerberg to the security flaw.
"A number of other online services have seen millions of passwords stolen in the past several weeks," a Twitter spokesperson told Reuters. "We recommend people use a unique, strong password for Twitter."
Pinterest did not immediately respond to Reuters' request for comment.
— Reuters contributed to this report.