A massive cyberattack that hit Iran last month threatened the stability of its banking system and forced the country’s regime to agree to a ransom deal of millions of dollars, people familiar with the case say.

An Iranian firm paid at least $3 million in ransom last month to stop an anonymous group of hackers from releasing individual account data from as many as 20 domestic banks in what appears to be the worst cyberattack the country has seen, according to industry analysts and western officials briefed on the matter.