Deloitte, which is registered in London and has its global headquarters in New York, was the victim of a cybersecurity attack that went unnoticed for months.
One of the largest private firms in the US, which reported a record $37bn (£27.3bn) revenue last year, Deloitte provides auditing, tax consultancy and high-end cybersecurity advice to some of the world’s biggest banks, multinational companies, media enterprises, pharmaceutical firms and government agencies.
The Guardian understands Deloitte clients across all of these sectors had material in the company email system that was breached. The companies include household names as well as US government departments.
So far, six of Deloitte’s clients have been told their information was “impacted” by the hack. Deloitte’s internal review into the incident is ongoing.
The Guardian understands Deloitte discovered the hack in March this year, but it is believed the attackers may have had access to its systems since October or November 2016.
The hacker compromised the firm’s global email server through an “administrator’s account” that, in theory, gave them privileged, unrestricted “access to all areas”.
The account required only a single password and did not have “two-step“ verification, sources said.
Emails to and from Deloitte’s 244,000 staff were stored in the Azure cloud service, which was provided by Microsoft. This is Microsoft’s equivalent to Amazon Web Service and Google’s Cloud Platform.