"We're actively working to neutralise a denial of service attack," Evernote tweeted. "You may experience problems accessing your Evernote while we resolve this."
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It is not clear how many of Evernote’s 100 million registered users were affected by the DDoS attack, which security firms report are once again gaining in popularity with cyber attackers.
While DDoS attacks do not involve a breach of systems, various techniques are used to make websites or services inaccessible to users.
Evernote reported that its services were disrupted for almost four hours in its first-ever DDoS attack.
Such attacks can be used by rivals for competitive advantage or by hacktivists to highlight a particular cause or issue.
More on DDoS attacks
- More than one-fifth of UK firms hit by DDoS attacks in 2012
- Activists unleash biggest DDoS cyber attack to date
- DDoS attack trends highlight increasing sophistication, larger size
- Business struggling with DDoS and other cyber threats, poll reveals
- A quarter of 2013 DDoS attacks will be app-based, says Gartner
- Police arrest man for DDoS attacks on Theresa May sites
Security researchers are also beginning to see that DDoS attacks are seldom one-off events, but rather pre-cursors to further attacks and even smokescreens for other malicious activity.
The identity of the attackers and the motive for the DDoS attack against Evernote is not known, but the cloud-based service emphasised that no accounts were compromised and that no data was lost.
In March 2013, Evernote was forced to carry out a service-wide password reset after its computer systems were breached by hackers.
The company said its security team had discovered and blocked suspicious activity on the Evernote network that appeared to be a co-ordinated attempt to access secure areas of the Evernote service.
Evernote said its investigation revealed that hackers had gained access to user information, including user names, email addresses and encrypted passwords.
However, it said there was "no evidence" that payment information or stored content had been accessed, changed or lost.
Despite an increase of 36% in the number of UK companies hit by DDoS attacks in 2013, most firms are still ill-equipped to deal with such attacks, according to a report published in March 2014.
The cost of DDoS attacks is significant, with 32% of companies estimating losses of more than £240,000 a day, according to the second annual UK DDoS report by communications and analysis firm Neustar.