A US military computer science professor on Friday warned that a trend to push software into the "clouds" exacerbates privacy risks as people trust information to the Internet.

Websites routinely capture data that can reveal pictures of users' lives, US military academy professor Greg Conti told an audience at the annual DefCon hackers gathering in Las Vegas.

The danger is being heightened by a growing Internet trend toward "cloud computing," software being offered online with applications hosted on outside computers instead of programs being installed on people's machines.

A common example of the practice is Web-based email services such as those offered by Google and Yahoo.

The world of cloud computing is expanding to include software for documents, accounting, spread sheets, photo editing and more.

"With cloud computer looming on the horizon it is important for us all to think of the privacy threats there as well," Conti said.

"The tool resides with someone else and the data is stored somewhere else. Generally, that is a bad idea."

Internet users are already giving away copious amounts of information using online search and mapping software.

Prime examples are social networking websites where people post personal videos, pictures, and thoughts that supposedly can only be viewed by selected friends.

The potentially revealing data in people's profiles is stored on computers maintained by the social networking firms.

If someone does an Internet search of their own name and then maps a route from their home, who they are and where they live is on record indefinitely in data bases of the firms that provided the services.

With cloud computing, copies of documents, spreadsheets or other files created using outside applications could be stored by companies providing the services.

"When information is in the public domain, it is game over," Conti said.

"Information on your computer may get protection under the law, but on someone else's it gets less protection."

The US Department of Justice has tried to pry search data from Google, and China pressured Yahoo to reveal the identities of pro-democracy advocates voicing opinions online.

"The information we are all giving to online companies is massive and dangerous and it's going to get worse before it gets better," Conti said. "Giving them our data is a clear and present risk."

Records of email and text messaging are routinely saved and it is common for websites to use software that tracks where online visitors came from and where they go next, according to security specialists.

Software used to post online ads collects information on people's online activities to more effectively target messages.

"How hard would it be to target someone as a political activist or a person with AIDS?" Conti asked rhetorically.

And even if Internet firms champion privacy for users, there is no guarantee they won't be forced to yield to courts or get new owners with different ideas.

"Companies consolidate but also companies die," Conti said.

"This is heresy, but one day Google will die. What happens to data when a company dies is a big question."