One of the largest advertising holdings in the world, Dentsu, has confirmed a leak of data of its British employees due to a hack of the systems of its subsidiary Merkle. The company warns: attackers could have gained access to sensitive personal and financial information of employees.
Dentsu announced “unusual activity” on the network of Merkle, a data processing consulting company that is part of the holding. The investigation revealed that the attackers had stolen files containing information about current and former Dentsu employees in the UK. The documents could have included salary details, national insurance numbers and personal contacts.
The holding company did not specify the extent of the leak, but its UK workforce numbers more than 2,500, and the number of former employees could reach tens of thousands. Dentsu said it had “taken steps to prevent the data from being publicly disclosed”, which could indicate negotiations with the attackers or a ransom payment – but there has been no official confirmation.
Despite the lack of claims from known ransomware groups, experts do not rule out the possibility of extortion. The company has already offered victims annual credit monitoring and darknet surveillance, and has also notified the ICO, NCSC and law enforcement.
Dentsu is a Japanese advertising and PR giant with over 60,000 employees and an annual turnover of around $8 billion. Merkle specializes in collecting and analyzing marketing data, working with major brands around the world. 2025 was a record year for cybercriminals: companies working with large amounts of data are becoming key targets for extortion attacks.
The incident at Dentsu proves once again that even technology leaders with enormous resources remain vulnerable to sophisticated attacks and the human factor. Disclosure of employee data is not only a matter of reputation, but also a direct threat of phishing attacks, social engineering and fraud.
