After being locked out of their accounts, users report having their profile picture, username and bio changed. The accounts then remain dormant, awaiting whatever fate the hackers have planned for them.
It is unclear why the hackers are breaking into the accounts. And the nature of the attack means people can't get their accounts back – even if they plead with Instagram.
One user whose account was taken over earlier this month told The Independent that she has repeatedly reached out to Instagram's support but the account has still not been returned to her.
"The problem is that my hacker has changed the email address associated with the account, locking me out. At the same time, I never received an email telling me that my email address or password had been changed, so I'm unable to recover my account," said Emma (who preferred her surname was not published), 26, from London said.
"The 'support' service' I've received from Instagram has been shockingly poor. Attempts to block and report the account were ignored – instead I repeatedly received the same automated message telling me to change my password."
People first began reporting the issue in August. But the widespread campaign is showing no sign of stopping, and there is still no clue about why hackers are gaining control of such a huge number accounts.
An Instagram spokesperson told The Independent: "We are investigating claims of some hacked accounts and will take the necessary steps to help those impacted."
For Emma, the reason behind her account being taken over remains a complete mystery.
"The hackers have changed my account name to Zulugius Lydia," she said. "None of the photos have changed but they have weirdly changed the bio bit."
The Facebook-owned firm claims that users will be notified of any change to the email address associated with an account, however Emma claimed this was not the case – an issue reported by dozens of other Instagram users across social media.
Instagram first acknowledged the issue in August, issuing guidance to people who have difficulty accessing their accounts.
Advice included strengthening passwords to improve security and revoking access to any "suspicious third-party apps". No specific definition was given for what these apps might look like.
Since then, Instagram users have continued to report mysterious account takeovers. However, the advice does not appear to have stemmed the flow of attacks.
Security experts have speculated that the hacked accounts could be being gathered to eventually spread spam and other online ads to the followers of the accounts.
The lack of apparent action from Instagram means frustrated users may give up trying to take back control of their account and either quit Instagram or open a brand new account, warned privacy advocate Paul Bischoff.
"Even if some victims regain control of their accounts, many of those affected have likely quit the platform or won't go through the trouble, adding soldiers to the spambot army," he said.
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