From the Guardian:

The Russian government is suspected of being behind a cyber-attack on parliament that breached dozens of email accounts belonging to MPs and peers. Although the investigation is at an early stage and the identity of those responsible may prove impossible to establish with absolute certainty, Moscow is deemed the most likely culprit.

Why? Just because. They even add in a few misleading lies to help convince you it must be Russia, despite the acknowledgment it could have been anyone:

In May, Russia was linked to the hacking of France’s computer systems during the presidential campaign, taking data from Emmanuel Macron’s campaign and leaking it to the public. US officials have previously said they were seeking to share their experience of the 2016 presidential election, where US intelligence agencies concluded that Russia hacked and leaked Democratic party communications and disseminated fake news with the aim of getting Donald Trump elected.

Except the French Intelligence Services shot down this particular lie a long time ago:

The head of the French government’s cyber security agency, which investigated leaks from President Emmanuel Macron’s election campaign, says they found no trace of a notorious Russian hacking group behind the attack. In an interview in his office Thursday with The Associated Press, Guillaume Poupard said the Macron campaign hack “was so generic and simple that it could have been practically anyone.” He said they found no trace that the Russian hacking group known as APT28, blamed for other attacks including on the U.S. presidential campaign , was responsible. Poupard is director general of the government cyber-defense agency known in France by its acronym, ANSSI. Its experts were immediately dispatched when documents stolen from the Macron campaign leaked online on May 5 in the closing hours of the presidential race. Poupard says the attack’s simplicity “means that we can imagine that it was a person who did this alone. They could be in any country.”

So what was hacked from the MPs in the British Parliament? Not much:

Fewer than 90 email accounts belonging to parliamentarians are believed to have been hacked, a parliamentary spokesman said. [...] Amid fears that the breach could lead to blackmail attempts, officials were forced to lock MPs out of their own email accounts as they scrambled to minimise the damage from the incident. {...] The number of states who might mount such an attack on the UK is limited, and, in addition to Russia, includes North Korea, China and Iran. A security source said: “It was a brute force attack. It appears to have been state-sponsored.” [...] A 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.

This doesn't sound like a very sophisticated cyber attack to to me, and at least one source thinks so, too:

Henry Smith, the Tory MP, said: “Sorry no parliamentary email access today – we’re under cyber-attack from Kim Jong Un, Putin or a kid in his mom’s basement or something.”

My money is on the kid in the basement. Or something.