Anthony Luke Fulton, 22, targeted the personalised greetings card business over four days.
It is said about 18,000 records were accessed in July last year.
Carlisle Crown Court was told that the company had estimated their losses during the shutdown period, over several days, to be in the region of £150,000.
Fulton, of Mill Hill, Cleator Moor, appeared before Judge Barbara Forrester having earlier pleaded guilty to three hacking offences during a magistrates’ court hearing.
The charges state that he caused a computer to perform a function with intent to enable or secure unauthorised access.
Jacob Dyer, prosecuting, confirmed that Moonpig.com had estimated the “significant” loss to its business to be in the region of £150,000.
“It is a sustained, brute force attack. Just over 18,000 records have been accessed,” said Mr Dyer.
The court heard the website was shut down in the UK, the USA and Australia while an internal probe into the cyber attack was carried out.
It is said that during the course of the Moonpig.com investigation, Fulton was traced through an online IP address.
He was found to have launched the online attack from a computer while at his girlfriend’s home.
Members of a cyber crime team were informed and Fulton was arrested.
The court heard that this was not the first time the hacker had fallen foul of the law.
He appeared in court late last year and was given a 112-day custodial sentence, suspended for a year, for fraud by false representation.
But Brendan Burke, defending, said that crime had not involved the “kind of brute force hacking” shown by the attack on Moonpig.com.
For the fraud, Fulton had been ordered to complete unpaid work and given a rehabilitation requirement.
A progress report had been prepared by the probation service, and Mr Burke added: “I am told the progress has been good.”
Judge Forrester had been set to sentence Fulton for the Moonpig.com attack.
However, she adjourned the case after announcing that clarification was needed on a number of factors.
Judge Forrester also asked whether software could be fitted to Fulton’s computer equipment to keep a check on his use of technology.
The case is due to come back to court later this month.
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