Online tracking service comScore Inc siphons confidential data including passwords, credit card numbers and Social Security numbers from unsuspecting users, according to a lawsuit filed on Tuesday.

The proposed class action lawsuit, filed on behalf of two plaintiffs who downloaded comScore software, also says comScore scans all files on users' personal computers and modifies security settings, among other allegations.

The lawsuit against comScore, one of the leading companies that measures and analyzes Internet traffic, seeks an injunction against several alleged practices, as well as damages under U.S. electronic communications privacy laws.

ComScore collects data from people who get free software and chances to enter sweepstakes in exchange for their participation. It sells that information to more than 1,800 businesses around the world, including Best Buy Co, Facebook, Microsoft Corp and Yahoo Inc, according to comScore's website.

Concerns have surfaced about comScore's data collection practices in the past, though the complaint filed on Tuesday by Chicago-based law firm Edelson McGuire appears to be the first such legal action taken against the company. It comes amid heightened concerns about privacy as consumers share more personal information online.

The lawsuit says comScore's software scans all accessible files on a user's computer, as well as all files from other users on the same network, and transmits information about those files back to the company.

We have reviewed the lawsuit and find it to be without merit and full of factual inaccuracies, said comScore spokesman Andrew Lipsman. ComScore intends to aggressively defend itself against these claims.

The case in U.S. District Court, Northern District of Illinois, is Mike Harris and Jeff Dunstan, individually, and on behalf of a class of similarly situated individuals v. comScore Inc, case no. 11-cv-5807.

(Editing by Ted Kerr and Robert MacMillan)

Lawsuit