Elon Musk's X, formerly Twitter, granted subscription perks to designated terrorist groups and others barred from operating in the US, according to campaigners.
The Tech Transparency Project (TTP) found X had granted blue check marks to accounts tied to Hezbollah members, among others.
For $8 (£6.40) a month, a tick allows longer posts and better promotion.
X removed some ticks after the report, saying its security was "robust".
Mr Musk's decision to charge for check marks was one of the most controversial changes he made after he bought Twitter in 2022, with critics saying the move would make issues of disinformation worse, opening the platform to impersonators.
The badge was previously free, meant to indicate that the social media platform had verified the identity behind the account.
Many of the recipients were journalists, as well as world leaders and celebrities.
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