LinkedIn's online learning unit has been notifying its 9.5 million users of an unauthorised database breach that contained the contact information and courses viewed of around 55,000 users. The company has confirmed the hack and said it has reset the passwords for all 55,000 accounts as a precautionary measure.

It has notified all of its users about the breach "out of an abundance of caution" even though their data was not compromised in the breach.

"You may have received an email notification from explaining that we recently became aware that an unauthorised third party accessed a database that included some learning data, such as contact information and courses viewed," said in a statement. "We are informing users out of an abundance of caution."

The company also noted it has found "no evidence that any data has been made publicly available".

In response to a user query on Twitter, the company said it was "taking this issue very seriously and working with law enforcement".

LinkedIn also suffered a massive data breach in 2012 that compromised more than 100 million users' emails and passwords. In May, the company revealed that the much-publicised breach actually affected many more accounts than previously thought, and urged its millions of users to reset their passwords and enable two-step verification. The stolen user credentials were put up for sale on the Dark Web by a hacker named "Peace" earlier this year.

In April 2015, LinkedIn acquired for $1.5bn (£1.2bn). In June this year, Microsoft bought professional network LinkedIn in an all-cash transaction worth $26.2bn - the largest acquisition in its history. Microsoft officially sealed the acquisition earlier this month.

The latest breach comes just days after Yahoo announced yet another massive breach that occurred in 2013 and compromised the personal data of more than one billion users.

In September, the internet giant disclosed a separate massive breach that affected more than half a billion user accounts. Following the second disclosure, telecom company Verizon, which agreed to a deal to acquire Yahoo's core business for $4.85bn, said it would review the impact of this new development before reaching any final conclusions.

An unidentified source familiar with the matter told Bloomberg that Verizon might be looking for a discount or a possible exit from the deal. Yahoo, however, said in a statement that it was "confident in Yahoo's value and we continue to work towards integration with Verizon".