Remote login software TeamViewer beefs up its security controls following a spate of users reporting their accounts have been hacked.
Installed on millions of PCs, the TeamViewer software lets people access a machine via the web.
Reports of attacks on TeamViewer users grew sharply last week, as many shared their experiences via social media.
TeamViewer said the attacks used credentials found in giant caches of data stolen from other companies.
Many people have taken to Reddit and Twitter to report their machines have been remotely penetrated by hackers abusing TeamViewer accounts.
In some cases, victims said they had lost cash from bank accounts accessible via the compromised computer.
TeamViewer blamed "unprecedented large scale data thefts on popular social media platforms and other web service providers" for the series of attacks.
During the past two weeks, hundreds of millions of hacked account details from MySpace, Tumblr and other companies have been offered for sale online.
TeamViewer said it was likely passwords stolen in those "mega-breaches" that had also been used for TeamViewer accounts were helping cyber-thieves win access to users' computers.
"They have taken advantage of common use of the same account information across multiple services to cause damage," it said.
In an interview with Ars Technica, a spokesman for the company said it had seen a "significant" number of accounts taken over but declined to give exact figures.
The spokesman denied claims the hacks were taking place because attackers had managed to penetrate its network and steal login names and passwords.
In response to the continuing attacks, TeamViewer has set up a system that will log which devices people use for the service and require them to confirm access from a new machine or device.
It said it was also introducing monitoring systems that would seek to spot when a TeamViewer account showed "unusual behaviour".
TeamViewer said users might experience "minor inconveniences" as it set up the security systems.
It added users should chose hard-to-guess passwords and set up two-factor authentication for their accounts to thwart attackers.
Paul Ducklin, writing on the Sophos security blog, recommended users set up TeamViewer to ask for approval before allowing access to a remote machine.
"That's a simple and effective way to prevent crooks from wandering in while you aren't there," he said.