SAN FRANCISCO (Reuters) - Computer security experts on Tuesday said they had found the first virus designed to attack Macromedia Inc. (Nasdaq:MACR - news) Flash animation files, which are predominating used in Internet advertising and on glitzy Web sites.

The virus, dubbed SWF/LFM.926, is low risk because it must be downloaded manually and cannot spread itself to other computers over e-mail or through Web browsers like many other viruses can, experts said.

However, people who browse an affected Web site or use e-mail could still become infected if they download and open an infected Flash file on their computer outside of the browser, Macromedia said.

A spokeswoman at San Francisco-based Macromedia said the company would release a patch for the virus within the next few days.

A sample of the virus was sent to antivirus vendor Sophos Inc. from a Hotmail e-mail account and is not spreading ``in the wild,'' said Craig Schmugar, virus research engineer at Network Associates Inc. (Nasdaq:NETA - news).

The virus affects Windows NT- and XP-based computers, but only those storing Flash files, which can be movies, games and other animations, according to Schmugar.

``Most people aren't going to have any Shockwave Flash files on their computer,'' he said, using an old name for the Flash file format. ``It's very unlikely the virus will spread very far.''

When executed, the virus displays a message saying ``Loading Flash Movie'' and shows a number puzzle that people might try to solve, Schmugar said.

The virus also creates a program that infects other Flash files on the same system, he said.