Facebook users' personal data could have been accidentally leaked to third parties over the past few years, Symantec said.
Third-parties, particularly advertisers, would have had access to personal information such as profiles, photographs and chat, the security software maker said in a blog post.
They could even have had the ability to post messages, Symantec added.
"We estimate that as of April 2011, close to 100,000 applications were enabling this leakage," the Symantec 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 Symantec's resulting report has a few inaccuracies," Facebook spokeswoman Malorie Lucich said.
"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."
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", Lucich added.
She also confirmed that the company removed the outdated API (Application Programming Interface) referred to in Symantec's report.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 dollars.