"Even in the free software community most users are not aware of this issue; the browsers' silence tends to conceal it," Stallman has written.
The warning echoes a similar call by Stallman in 2004 to beware of inadvertently downloading Sun Microsystems’ Java, before Java was open-sourced.
Stallman criticized Silverlight for distributing non-free codecs and - by implication - the Novell-backed Moonlight, which is also licensed to use those codecs. The media codecs are closed, and have been licensed by Microsoft over the years from the media industry to play audio and video on Windows. Microsoft has extended the codec patent licensing to Moonlight.
"A free replacement for Silverlight would hardly be of use in the free world without free replacement codecs," Stallman said. ®