File-sharing site BitTorrent has warned users that any downloads taken from its site during a two-hour period overnight may contain malware.
It revealed that hackers had injected scareware into its download software.
When installed, the program pops up fake warnings that a virus has been detected and prompts users for payment to remove it.
BitTorrent has become synonymous with illegal downloads but it also has legitimate content.
"This morning at approximately 4.20 am Pacific Time, the uTorrent.com and BitTorrent web services were compromised. Our standard software download was replaced with a type of fake anti-virus 'scareware' program," the company said in its blog.
The scareware is identified as belonging to the Security Shield malware brand.
"Just after 6am Pacific Time we took the affected servers offline to neutralise the threat. Our servers are now back online and functioning normally...We sincerely apologise to any users who were affected," it added.
Later it issued an update, claiming just uTorrent was affected.
Despite clarifications, Sophos security expert Paul Ducklin suggested users needed to be extra careful.
"Since the two sites share the same network infrastructure you might want to ignore that blog update and assume that any downloads from BitTorrent Inc were dodgy and give yourself a thorough anti-malware checkover," he said.
In separate news, the ongoing battle between file-sharing site and the creative industries has taken an unusual turn as digital locker Hotfile counter-sues Warner Bros.
The movie studio had originally taken Hotfile to court in the US, claiming it was hosting pirated films.
But, in a twist, Hotfile claims that the automated software Warner uses to take down content has removed files over which the movie house has no rights.
In one instance, it said, Warner's attempts to remove copies of its movie 'The Box', also took down the audio book 'Cancer: Out of the Box' by Ty M.Bollinger as well as the BBC show 'The Box that saved Britain'.