A vulnerability in Adobe Systems Inc.'s Acrobat Reader software is more dangerous than originally thought, allowing cyber-intruders to access information to a user's hard drive, according to security experts.

Security officials warned Wednesday that a flaw in earlier versions of Acrobat Reader could allow online thieves and hackers to steal web-related information from users who posted portable document format — or PDF — files on their website.

But security experts on Thursday said the vulnerability could also affect a personal computer by linking directly to PDF files found on a victim's PC.

Acrobat's inclusion of sample PDF's in predictable places on the hard drive makes such files easier to find, warns Websense Security Labs.

"Because known PDF files are stored on the local computer, this vulnerability can be used to execute JavaScript in the context of the local user, granting access to the local file system," Websense Security Labs said in a release on Thursday.

Adobe issued it own advisory, encouraging customers using older versions of the software to upgrade to Acrobat Reader 8. The company will be offering patches early next week to users with systems unable to upgrade to the latest version.

The flaw is exploitable in all versions of Mozilla Firefox running versions of Acrobat Reader earlier than 8. It also appears to affect Microsoft's Internet Explorer running some but not all earlier versions of Reader.

The vulnerability, first discovered at a hacker conference in Germany over the holidays, takes advantage of a plug-in allowing JavaScript code appended to links to PDF files to run when the link is clicked.

Attackers could create a hostile website linking to another site's PDF. When the link is clicked, a malicious JavaScript program is also activated and runs on the system hosting the PDF.

If the host of the PDF is a website, it would allow the hacker to steal cookies and other web-related information from the site. Should the host of the PDF file be a personal computer, the malicious program would allow much greater access to personal information.

"Given that it is easy to exploit, I would expect that we will see this method used considerably in the coming days and weeks, until it is resolved," a researcher from Symantec Corp. said in a posting on the California-based company's web log.

With files from the Associated Press