Security flaw found in Steam
Hackers could have a new means of accessing your computer through a browser command which utilizes Valve’s software distribution system Steam. When your browser accesses a URL that begins with the command “steam://”, it will prompt your copy of steam to launch and perform some operation. Usually, such an operation would be to launch a game, or install or uninstall software.
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Story added 18. October 2012, content source with full text you can find at link above.