The popular online notes and web clippings saving service Evernote has suffered disruption after coming under cyberattack.
The firm tweeted that it was trying to neutralise a denial of service attack on Tuesday evening.
Some members have been unable to synchronise their filings from one device to another while it continues.
The California-based company announced last month that it had more than 100 million users.
Denial of service (DoS) attacks are caused by an attack directing huge amounts of data to a target's computer servers in an effort to overwhelm them.
This sometimes involves hijacked computers - whose owners may be unaware of their involvement - in what is known as a botnet.
This is not the first time the storage service has been compromised. In 2013 it said hackers had managed to access user names, email addresses and encrypted passwords.
However, it appears that the latest cyber-assault is more limited.
"We do not have an identity to tie to the attack," spokeswoman Ronda Scott told the BBC.
"It started at about 14:25 local time [22:25 BST] and continues. We're actively working to mitigate the attack and service is now intermittent."
James Potter