Deloitte is registered in London and its global headquarters is in NY.
The firm provides auditing, consulting and cybersecurity advice to government agencies and to the world's largest companies.
Several Fortune 500 companies make up the bulk of Deloitte's client catalog, including big banks, government agencies in the US and the United Kingdom, and more staple firms from North America and Europe.
Deloitte is said to be still investigating what areas of its networks hackers managed to access.
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The attack that appeared to target the firm's USA operations was discovered in March and could have begun as early as October a year ago, the Guardian said.
Richard Stiennon, chief strategy officer at security firm Blancco Technology Group, commented on the importance of protecting emails: 'Deloitte's experience with a simplistic breach of their Microsoft 365 infrastructure through an easy to access administrator account highlights how easy it is to overlook critical information stores.
The exploited admin account had just a single password and lacked two-step authentication.. The hackers had access to up to 5 million sensitive emails and documents of its clients. Because of the sensitive nature of the breach, only senior partners and legal professionals were initially informed.
Following on from an attack which exposed millions of Equifax customer details earlier this month, The Guardian is reporting Deloitte has been similarly targeted by an especially sophisticated hack. It has involved specialists trying to map out exactly where the hackers went by analysing the electronic trail of the searches that were made.
The internal review is still ongoing and so far it is not known who is responsible: whether it was a lone wolf, a business rival or the result of a state-sponsored hacker. This kind of reverse-engineering is not foolproof, however.
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The publication has confirmed that only six of Deloitte's clients have been notified of the data breach.
With that said, the data that did get impacted by the breach could still pose a major problem.
The "intensive" review included mobilising a team of cyber-security and confidentiality experts inside and outside of Deloitte.
The accounting firm stated they have reviewed the hack after going through a "comprehensive security protocol", and had also contacted the authorities and the clients affected immediately. "No disruption has occurred to client business, to Deloitte's ability to continue to serve, or to consumers".
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