Campaigners claim that Elon Musk's X, previously Twitter, provided subscription benefits to designated terrorist groups and others forbidden from functioning in the United States.
The Tech Transparency Project (TTP) discovered that X had issued blue check marks to accounts linked to Hezbollah members, among others.
For $8 (£6.40) every month, a tick enables longer postings and better marketing.
Following the disclosure, X eliminated certain ticks, claiming its security was "robust."
Musk's decision to charge for check marks was one of the most contentious adjustments he made after purchasing Twitter in 2022, with opponents claiming it would exacerbate misinformation concerns while also exposing the network to impersonators.
The badge was originally free and indicated that the social media site had confirmed the identity of the account holder.
Many of the recipients were journalists, international leaders, and celebrities.
In other cases, these included persons facing sanctions in the United States, prompting concerns that the corporation was providing a platform to the wrong people and defying US sanctions laws.
Now that the technology has been paid for, "X may be raising new legal issues," according to the Tech Transparency Project.
It stated that X had deleted the ticks from the accounts it detected after the report was released.
The TTP said that an account operated by Ansar Allah, often known as the Houthis, appeared to have paid for the blue check mark. The checkmark has now been deleted. The account has more than 23,000 followers. The Houthis are sanctioned in both the United States and the United Kingdom.
According to the UK government's website, the Houthis have been sanctioned "to disrupt their ability to attack international shipping in the Red Sea and to promote Yemen's peace, stability, and security."
The US Treasury, which lists groups with which the US will not trade, did not immediately reply to a BBC request for comment.
"The United States imposes sanctions on individuals, groups, and countries that are deemed to pose a threat to national security." The TTP said that Elon Musk's X appeared to be offering premium service to some of them.
This article was originally published on the BBC.