The software, code-named Gamera, was obtained by Techpages.com. The Web site made the code available online yesterday, according to the site's administrator.
A source close to AOL confirmed the download is authentic. An AOL representative said the company is "aware of the posting and (is) looking into it."
The development of the preview software marks another concerted step in AOL's adoption of the Linux open-source operating system for its future services. As first reported by CNET News.com, AOL has been known to be working on a Linux-based device since May 1999.
The company plans to use its Linux-based service in future Net gadgets. It has already struck partnerships with chipmaker Transmeta and PC maker Gateway to produce Linux-powered appliances slated for release later this year.
For the giant Internet service provider to use Linux in its future products is a significant bonus for the OS. The free operating system is mostly used on heavy-duty computers, but it is becoming increasingly popular as a desktop operating system.
Furthermore, adopting Linux means companies do not need to pay costly licensing fees to operating system developers such as Microsoft.
In another vote of support for Linux, AOL last week released a beta Linux version of its popular AOL Instant Messenger client.
This week's posting of the Gamera preview is the latest in a series of security breaches that have dogged AOL development projects.
"The file was leaked by someone inside AOL," said Ryan Neeley, administrator of Techpages.com. The company "should have security in place to avoid this."
In May, Techpages.com obtained and posted a preview version of AOL's upcoming 6.0 software, slated for release this fall.
Also in May, a teenager identified as Kenton obtained confidential Gamera documents and posted them on his Web site.
According to those documents, AOL developers were planning to combine Web browsing, instant messaging, chat, email, graphics and file transfer capabilities into the service.
The latest download will only run on the Linux operating system.
Previous attempts to post test versions of AOL software have not been taken lightly by the online giant. The company is continuously testing versions of its software within its firewalls. AOL has issued warnings of legal action against people who have posted software versions without its consent.