Late last month, the former White House social media account for President Barack Obama suddenly began posting odd things on its Instagram page.

The account had been dormant since 2017, when Mr. Obama left office. The new posts — which included messages deriding President Trump and saying the White House was “under Shiite control,” referring to the branch of Islam — were out of character for Mr. Obama’s social media activities.

It turned out the posts were not made by Mr. Obama’s office at all. In May, a group of hackers discovered a bug in a Meta customer service tool that allowed anyone to use an artificial-intelligence-powered chatbot to reset the passwords for Instagram accounts. All the hacker had to do was ask the chatbot to change someone’s password — and it would be done.

Roughly 34,000 Instagram accounts were affected, including the accounts of the home security monitoring company SimpliSafe and a senior official in Mr. Trump’s Space Force department, according to internal Meta documents viewed by The New York Times. In the Space Force official’s case, hackers began posting pro-Iran messages comparing the war in Iran to U.S. involvement in Vietnam in the 1960s.

Of the 34,000 accounts, 20,000 were breached, giving hackers access to the related email addresses, phone numbers, birth dates and other personal data. More than 3,500 of the accounts had their user names taken over and changed from the hack, according to the internal documents. Meta has said it could not determine what information was viewed or stolen by the attackers.

In a statement, Meta said it had fixed the flaw, which was reported by 404 Media this month, and secured the affected accounts.