Macromedia's Flash media player is raising concerns among privacy advocates for its little-known ability to store computer users' personal information and assign a unique identifier to their machines.

"A lot of media players come with identifiers embedded in them to track content usage and digital rights management," Chris Hoofnagle, director of the Electronic Privacy Information Center's West Coast office, said. "With respect to Windows Media Player and now the Macromedia player, we're realizing that the media players themselves are creating privacy risks."

Flash, popular for its ability to play animation and video clips, employs a technology known as local shared objects to save up to 100KB of information on users' hard drives. By assigning a unique identifier to a computer and preserving it in the space for the local shared object, a website can recognize that someone has already visited the site, and advertisers can use the information to determine that a visitor has previously viewed an ad. Websites that require users to fill out personal information can also associate that data with the identifier.

Macromedia does not view its software as a threat to user privacy.

"The Flash player by its nature doesn't by default gather any information," Jeff Whatcott, vice president of product management at Macromedia, said. " We designed that technology from the beginning to make sure that (computer) users are always in control of their key information."

Macromedia provides instructions on its website for how to disable local shared objects on an individual site or all sites, delete data that is already stored locally, and set the maximum space allowed for storage.

Unfortunately, most Flash users are unaware that the player is storing any information about them at all and are unlikely to see these instructions or understand how to follow them.

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