Fewer than 90 email accounts belonging to peers and MPs are believed to have been hacked by an orchestrated cyber-attack, a parliamentary spokesman said on Sunday.
The Houses of Parliament were targeted by hackers on Friday in an attack that sought to gain access to accounts protected by weak passwords.
MPs said they were unable to access their emails after the attack began and cited blackmail as a possible motive.
The parliamentary spokesman said the authorities had found that less than 1% of parliament’s 9,000 email addresses had been compromised. “The figure is less than many feared, but is still a breach. It is being taken very seriously.”
The estate’s digital services team said they had made changes to accounts to block out the hackers, and that the changes could mean staff were unable to access their emails.
The parliamentary spokesman said those whose emails were compromised had used weak passwords despite advice to the contrary. “Investigations are ongoing, but it has become clear that significantly fewer than 1% of the 9,000 accounts on the parliamentary network have been compromised as a result of the use of weak passwords that did not conform to guidance issued by the Parliamentary Digital Service.
“As they are identified, the individuals whose accounts have been compromised have been contacted and investigations to determine whether any data has been lost are under way,” he said.
Tory MP Andrew Bridgen said such an attack “absolutely” could leave some people open to blackmail. “Constituents want to know the information they send to us is completely secure,” he said.
An email sent to all those affected, seen by the Guardian, said: “Earlier this morning, we discovered unusual activity and evidence of an attempted cyber-attack on our computer network. Closer investigation by our team confirmed that hackers were carrying out a sustained and determined attack on all parliamentary user accounts in an attempt to identify weak passwords.
“These attempts specifically were trying to gain access to our emails. We have been working closely with the National Cyber Security Centre to identify the method of the attack and have made changes to prevent the attackers gaining access, however our investigation continues.”
The changes are believed to have stopped MPs and their offices from accessing emails on mobile phones and tablets outside Westminster. “Access to systems from the Westminster estate has not been affected,” the email said, before adding that further disruption was likely.
