Online storage service Dropbox accidentally turned off password authentication for its 25 million users for four hours on Monday -- although "much less than 1 percent" of those accounts were accessed during the period, the company said. It is still investigating whether any of those accounts were improperly accessed.

Dropbox CTO Arash Ferdowsi wrote that the company introduced a code change at 1:54 p.m. PST that caused a problem in the authentication mechanism. About four hours later, the problem was discovered, and Dropbox killed all of the sessions of those who were logged in at the time -- "much less" than 1 percent of its users, Ferdowsi wrote.

A fix was introduced at 5:46 p.m. PST, he said.

"We're conducting a thorough investigation of related activity to understand whether any accounts were improperly accessed," Ferdowsi wrote. "This should never have happened. We are scrutinizing our controls, and we will be implementing additional safeguards to prevent this from happening again."

The company later said it had notified all those who were logged in at the time of the error and asked them to review details of activity on their account. Those concerned can also query Dropbox at "support@dropbox.com."

The issue was noticed by some users. Christopher Soghoian , a University of Indiana doctoral candidate and security researcher, posted a tip-off from an unnamed source to the website Pastebin.

In May, Soghoian wrote a complaint letter to the FTC, alleging that the company has deceived consumers about the level of encryption security it offers. Dropbox said the complaint was without merit.

Several Dropbox users were upset by Monday's authentication problems, while others brushed it off.

"Every single Dropbox customer should be getting an e-mail right now about this -- not hearing about it from other sources or from a seemingly calm-toned blog post," wrote a user going by the name of Tony Webster. "Dropbox hasn't even tweeted about this a full 24 hours after it happened. I know I would like disclosure of every single action happening on my Dropbox account during the four hours anybody could access it, and I need that information immediately."

But an anonymous poster wrote, "What kind of amazing cancer/AIDS curing research have you stored on your Dropbox account? Mistakes happen and they fixed it. At least they told you about it. How many other companies do that?"

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