Viral license

Viral license is a pejorative name for copyleft licenses, especially the GPL, that allows derivative works only when permissions are preserved in modified versions of the work. Copyleft licenses include several common open source and free content licenses, such as the GNU General Public License (GPL) and the Creative Commons Attribution-Sharealike license (CC-BY-SA).

The term is most often used to describe the GPL, which requires that any derivative work also be licensed under compatible licenses with the GPL. The viral component is described as such because the licenses spreads a continuing use of the licenses in its derivatives. This can lead to problems when software is derived from two or more sources having incompatible viral licenses in which the derivative work could not be re-licensed at all.

Although the concept is generally associated with licenses that promote free content, proprietary licenses also have viral characteristics. For example, original equipment manufacturer source code software distribution agreements generally grant licensees the right to redistribute copies of the software, but restrict what terms can be in the end user license agreement. However, derivative work is much less common with proprietary licensed work and so the issue of licenses becomes moot.

The term 'General Public Virus' or 'GNU Public Virus' (GPV) as a pejorative name dates back to a year after the GPLv1 was released. In 2001 Microsoft vice-president Craig Mundie remarked "This viral aspect of the GPL poses a threat to the intellectual property of any organization making use of it." In another context, Steve Ballmer declared that code released under GPL is useless to the commercial sector (since it can only be used if the resulting surrounding code becomes GPL), describing it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches". In response to Microsoft's attacks on the GPL, several prominent Free Software developers and advocates released a joint statement supporting the license.

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