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

Related readings:  Renren seeks to double Facebook's valuation  Facebook investors look for exits  Obama off to 'friend' Facebook in person  Baidu refuses to comment on Facebook deal

"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," posing a security threat, the blog post said.  

The third-parties may not have realized 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.

"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 unauthorized third parties," Facebook spokeswoman Malorie Lucich said in a statement.

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She also confirmed that the company removed the outdated API (Application Programming Interface) referred to in Symantec's report.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 Inc and Yahoo Inc for users' time online and for advertising dollars.