Evernote's security team discovered suspicious activity in its network which attempted to access secure areas of its service.
Evernote says there's no evidence that shows payment information or content stored in Evernote was accessed or lots.
However, hackers were able to gain access to Evernote user information, which includes usernames, email addresses and encrypted passwords.
The company has decided to implement a password reset for 50 million users. Evernote assures that their password encryption measures are "robust" and protected by one-way encryption but the company is taking precautions by implementing a password reset for its 50 million users.
"While our password encryption measures are robust, we are taking additional steps to ensure that your personal data remains secure," the company said in a statement.
Security experts say that companies are relying too much on encryption and anti-virus solutions, which have limitations.
"Not only does this news follow hot on the heels of recent cyber attacks on Apple, Facebook, and Twitter, but it harks back to 2011's stream of notorious data breaches, which saw the IT security defences of other large companies such as Lockheed Martin, RSA, Sony and Google also compromised," said Ross Brewer, vice president and managing director of international markets at LogRhythm. "It's disappointing to think that as an industry, it seems very few lessons have been learned since then - organisations of all sizes are still relying all too heavily on traditional point security tools such as encryption and anti-virus solutions, which have repeatedly proven their limitations.
"With cyber attacks becoming increasingly frequent and sophisticated, today's organisations must be constantly aware of the evolving cyber threat - ditching the common and outdated reactive approach to security - if they are to have any hope at protecting themselves. As such, companies need to start introducing mechanisms that give context to data and facilitate a deeper understanding of all network activity in real time. "
Other experts say that theft of customer data isn't going away any time soon.
"Evernote has done exactly the right thing by requiring all users to reset their passwords," said Terry Greer-King, Check Point UK managing director. "Users should do this by directly accessing the website and should be cautious about clicking on emails they receive, no matter how authentic the emails appear to be. There's a risk that external parties could use the email addresses exposed in the attack to send phishing emails to users, to try and harvest sensitive data.
"In 2012, we surveyed over 2,600 C-level and IT staff at firms globally and found they reported an average of 68 new security attack attempts per week, with theft of customer data as the primary target. This issue isn't going away."