Deloitte, one of the world's "big four" accountancy firms, has fallen victim to a cyberattack that exposed sensitive emails to hackers.

The IT security breach dates back to November 2016 but was only discovered in March this year, according to The Guardian, which broke the news in an exclusive on Monday. Deloitte has reportedly informed six of its clients that their information was "impacted." The firm's internal review into the incident is ongoing.

Hackers gained access to Deloitte's email system through an administrative account that was not secured using two-factor authentication, The Guardian reports. Emails to and from Deloitte staff were hosted on Microsoft's Azure cloud service. As well as email, hackers may have had access to "usernames, passwords, IP addresses, architectural diagrams for businesses and health information."

The network breach is said to have been US-focused. A Deloitte spokeswoman confirmed over the phone that it had been the victim of a breach. The biz forwarded a lengthy statement (below) that said only a few customers had been affected but didn't get into numbers nor how much information was potentially exposed.

Deloitte's response to the cyber incident included the following: · Implementing its comprehensive security protocol and initiating an intensive and thorough review which included mobilizing a team of cyber-security and confidentiality experts inside and outside of Deloitte; · Contacting governmental authorities immediately after it became aware of the incident; and, · Contacting each of the very few clients impacted The attacker accessed data from an email platform. The review of that platform is complete. Importantly, the review enabled us to understand precisely what information was at risk and what the hacker actually did and to determine that: · Only very few clients were impacted · No disruption has occurred to client businesses, to Deloitte’s ability to continue to serve clients, or to consumers. Deloitte remains deeply committed to ensuring that its cyber-security defences are best in class, to investing heavily in protecting confidential information and to continually reviewing and enhancing cyber security.

Tony Pepper, co-founder and chief exec of encryption tech provider Egress, said: "Whilst it hasn't been confirmed exactly what was stolen, compromised mail servers can be a good source of sensitive information for an attacker, allowing them to siphon off message content and attachments. This is why multi-factor access control such as two-factor authentication is important, especially for admins. It makes it much harder to gain illicit access in the first place, and provides a warning if someone is trying to log in without your knowledge."

In the meantime, maybe Deloitte ought to get some help shoring up its network security...

Deloittes’ US offices have everything from Netbios to RDP to Exchange Admin (single factor) etc etc etc. They should get an auditor. pic.twitter.com/C8aoN5YQMn — Kevin Beaumont 🙃 (@GossiTheDog) September 25, 2017

Deloitte provides auditing, tax consultancy and a range of cybersecurity services to banks, multinationals and government agencies. ®