Facebook users' personal information could have been accidentally leaked to third parties, in particular advertisers, over the past few years, Symantec said in its official blog.

Third-parties would have had access to personal information such as profiles, photographs and chat, and could have had the ability to post messages, the security software maker said.

"We estimate that as of April 2011, close to 100,000 applications were enabling this leakage," the blog post said. "Over the years, hundreds of thousands of applications may have inadvertently leaked millions of access tokens to third parties." This posed a security threat, the blog post said.

The third-parties may not have realised their ability to access the information, it said.

Facebook, the world's largest social networking website, was notified of this issue and confirmed the leakage, the blog post said. It said Facebook has taken steps to resolve the issue.

However, Facebook has disputed some of the report. "Unfortunately, their [Symantec's] resulting report has a few inaccuracies. Specifically, we have conducted a thorough investigation which revealed no evidence of this issue resulting in a user's private information being shared with unauthorised third parties," Facebook spokeswoman Malorie Lucich said in a statement.

Ms Lucich said the report also ignores the contractual obligations of advertisers and developers which prohibit them from obtaining or sharing user information in a way that "violates our policies."

She also confirmed that the company removed the outdated API (Application Programming Interface) referred to in Symantec's report.

Facebook has more than 500 million users and is challenging Google and Yahoo for users' time online and for advertising revenue.

Reuters