The hack into the accountancy giant Deloitte compromised a server that contained the emails of an estimated 350 clients, including four US government departments, the United Nations and some of the world's biggest multinationals, the Guardian has been told.
Sources with knowledge of the hack say the incident was potentially more widespread than Deloitte has been prepared to acknowledge and that the company cannot be 100% sure what was taken.
Deloitte said it believed the hack had only "impacted" six clients, and that it was confident it knew where the hackers had been. It said it believed the attack on its systems, which began a year ago, was now over.
However, sources who have spoken to the Guardian, on condition of anonymity, say the company red-flagged, and has been reviewing, a cache of emails and attachments that may have been compromised from a host of other entities.
The Guardian has established that a host of clients had material that was made vulnerable by the hack, including:
• The US departments of state, energy, homeland security and defense.
• The US Postal Service.
• The National Institutes of Health.
• "Fannie Mae" and "Freddie Mac," the housing giants that fund and guarantee mortgages in the US.
Football's world governing body, Fifa, had emails in the server that was breached, along with four global banks, three airlines, two multinational car manufacturers, energy giants and big pharmaceutical companies.
The Guardian has been given the names of more than 30 blue-chip businesses whose data was vulnerable to attack, with sources saying the list "is far from exhaustive."
Deloitte did not deny any of these clients had information in the system that was the target of the hack, but it said none of the companies or government departments had been "impacted." It said "the number of email messages targeted by the attacker was a small fraction of those stored on the platform."
This assurance has been contested by sources that spoke to the Guardian. They said Deloitte's public position belied concern within the company about exactly what had happened and why.
The Guardian first revealed the existence of the hack on 25 September.
Since then, the Guardian has been provided with further details of the attack, which seems to have started in autumn last year at a time Deloitte was migrating and updating its email from an in-house system to Microsoft's cloud-based Office 365 service.
The work was being undertaken at Deloitte's Hermitage office in Nashville, Tennessee.
The hackers got into the system using an administrator's account that, theoretically, gave them access to the entire email database, which included Deloitte's US staff and their correspondence with clients.
Deloitte realized it had a substantial problem in spring this year, when it retained the Washington-based law firm, Hogan Lovells, on "special assignment" to review and advise about what it called "a possible cybersecurity incident."
In addition to emails, the Guardian understands the hackers had potential access to usernames, passwords, IP addresses, architectural diagrams for businesses and health information.
It is also thought that some emails had attachments with sensitive security and design details.
Deloitte has insisted its internal inquiry, codenamed Windham, found that only six clients had information that had been compromised. The review had also been able to establish "precisely what information was at risk," the company said.
However, that analysis has been contested by informed sources that have spoken to the Guardian. They say the investigation has not been able to establish definitively when the hackers got in and where they went; nor can they be completely sure that the electronic trail they left is complete.
"The hackers had free rein in the network for a long time and nobody knows the amount of the data taken," said one source.
"A large amount of data was extracted, not the small amount reported. The hacker accessed the entire email database."
Another source added: "There is an ongoing effort to determine the damage. There is a team looking at records that have been tagged for further analysis. It is all deeply embarrassing."
The Guardian has been told Deloitte did not at the time have multi-factor authentication as standard on the server that was breached. A cybersecurity specialist told the Guardian this was "astonishing."
The expert said the migration to the new email system would have "utterly complicated the kind of forensic investigation required to see what had happened."
"A hacker has got into Deloitte's email system and been undetected for months, and only six clients have been compromised? That does not sound right. If the hackers had been in there that long, they would have covered their tracks."
When the Guardian put all these points to Deloitte, it declined to answer specific questions, but a spokesman said: "We dispute in the strongest terms that Deloitte is 'downplaying' the breach. We take any attack on our systems very seriously.
"We are confident that we know what information was targeted and what the hacker actually did. Very few clients were impacted, although we want to stress that even when one client is impacted, that is one client too many.
"We have concluded that the attacker is no longer in Deloitte's systems and haven't seen any signs of any subsequent activities.
"Our review determined what the hacker actually did. The attacker accessed data from an email platform. The review of that platform is complete."
In recent months, Deloitte has introduced multi-factor authentication and encryption software to try to stop further hacks.
Dmitri Sirota, co-founder and CEO of the cybersecurity firm BigID, warned that many companies had failed to use such methods because they were inconvenient and complex.
"Privileged accounts are like keys that unlock everything, from the castle to the treasury. They provide unfettered access to all systems, which is why they are so valuable.
"Organizations are monitoring databases, not the data in it. It's hard to detect changes, prevent incidents or compare your data to notice breached information unless you have an inventory of what you have."