Bermuda-based law firm Appleby, which has offices in many tax havens, warned clients they may have their details exposed after the massive hack of its database.
The incident once again highlights why law firms are such as highly prized target for hackers, containing highly sensitive data on clients.
The law firm, founded in 1897, is in the process of writing to customers to warn them of the compromise, which occurred in 2016, and comes after the International Consortium of Investigative Journalists (ICIJ) threatened to open up the cache of files that it claims were leaked to it in September previous year.
"These requests are based on documents that the journalists claim to have seen and which involve the business of the firm and our clients".
The cyber security incident has emerged around a year after a trove of private financial information relating to hundreds of individuals, including celebrities and high-profile public figures, known as the Panama Papers was stolen from legal firm Mossack Fonseca. It instantly became the world's best-known law firm following the leak of 11.5 million documents, cherry-picked by global media outlets, that revealed confidential client information.
"Appleby has thoroughly and vigorously investigated the allegations and we are satisfied that there is no evidence of any wrongdoing, either on the part of ourselves or our clients", the law firm said in a statement on its website.
The firm said it does "not tolerate illegal behavior" but admitted "it is true that we are not infallible".
The company, which has officers in Jersey, Mauritius and Seychelles, said that it had reviewed its cyber-security and data-access arrangements following a "data-security incident" previous year. "These arrangements were reviewed and tested by a leading IT forensics team and we are confident that our data integrity is secure".
Appleby, which also has offices in the British Virgin Islands, the Cayman Islands, Guernsey, the Isle of Man, Jersey, Mauritius and the Seychelles, denied any wrongdoing. Appleby dealt with several worldwide organisations and the information breach could see a lot new revelations about irregularities in terms of tax affairs. "Where we find that mistakes have happened we act quickly to put things right and we make the necessary notifications to the relevant authorities", the firm said.
And Appleby also expressed its "disappointment" that the media is choosing to "use this information which could have emanated from material obtained illegally and that this may result in exposing innocent parties to data protection breaches".