Mr Callow said he believed the post, which he said was published in the past 24 hours, was a “warning shot” to pressure the firm to pay up.
REvil’s is understood to have been responsible for the hack on Travelex, which caused services to be disrupted for several days after it was attacked and held to a £4.6m ransom on New Year’s Eve.
The attack took down Travelex’s online systems and has also hit banks including HSBC, Barclays and Natwest owner Royal Bank of Scotland which used its services for their own clients. It has refused to say if it paid the ransom.
Other victims are believed to include biotech 10x Genomics, consultancy Brooks International and the National Eating Disorders Association.
Grubman, Shire, Meiselas and Sacks has been contacted for comment.
Thousands of Hollywood heavyweight’s emails were leaked after Sony entertainment was hacked in 2014, revealing correspondence where a producer made a joke about Barack Obama’s race.
In another a producer described Angelina Jolie as a “minimally talented spoiled brat who thought nothing of shoving this off her plate for eighteen months so she could go direct a movie” while discussing casting for a Cleopatra film that was in development.
The hack was later attributed to North Korea because the attackers reportedly requested that an upcoming film, The Interview, which made fun of Kim Jong-un be pulled from cinemas.