Creative Commons BY-SA 4.0 declared one-way compatible with GNU GPL version 3
On Thursday, October 8, Creative Commons (CC) announced the addition of the GNU General Public License version 3.0 (GPLv3) to the list of licenses compatible with the Creative Commons Attribution ShareAlike 4.0 (CC BY-SA 4.0) license. Compatibility means that a person can now take a work they received under the terms of CC BY-SA 4.0 and then distribute adaptations of that work under the terms of GPLv3. However, this compatibility is one-way only, meaning you can not release adaptations GPLv3-covered works under the terms of CC BY-SA 4.0.
The FSF provided extensive feedback throughout the drafting process of the 4.0 suite of CC licenses, and began discussing the possibility of one-way compatibility of CC BY-SA 4.0 and GPLv3 in 2011. As a steward of public licenses, CC has displayed tremendous leadership throughout the entire drafting process of the 4.0 license suite, and this leadership has continued through the license compatibility process. In January 2015, CC officially opened a public consultation on CC BY-SA 4.0 and GPLv3 compatibility. They facilitated this discussion via a public mailing list, and supported it by creating strong educational resources such as their analysis of GPLv3 and their analysis of BY-SA compatibility. One of the most important ways in which the community contributed to the compatibility discussion was by identifying various use cases in which a person may wish to combine a CC BY-SA licensed work with a piece of software or similar work. One increasingly common use case is with free hardware design projects, in which contributors to the project choose to put the entire repository (for example, an aggregate of art, design documents, and software) under the terms of the GNU GPL. Similar use cases are documented with games and the artwork distributed alongside the games.
While we do not anticipate that many people will choose GPLv3 for their adapted versions of CC BY-SA licensed works of art, we do expect compatibility will be especially useful for individuals working in niche areas where a creative work that is licensed under CC BY-SA needs to be adapted and melded into source code form and combined with some GPLv3-covered software. As Mike Linksvayer stated in CC's announcement, this important interoperability between two of the world's most popular copyleft licenses "not only removes a barrier, but helps inspire new and creative combinations of software and culture, design, education, and science, and the adoption of software best practices such as source control (e.g., through 'git') in these fields."
When combining works licensed under CC BY-SA with ones under GPLv3, individuals should use caution and make sure that they can actually comply with the source requirements of GPLv3, specifically that they are able to show that they can provide the source code, which is defined as the preferred form of the work for making modifications. Further, when licensing a CC BY-SA work under the terms of GPLv3, the FSF urges individuals to take advantage of Section 14 of the GNU GPLv3, which allows a licensor to specify a proxy to determine whether future versions of the GPL can be used, i.e., "GNU GPL version 3 or (at your option) any later version." Creative Commons has kindly provided instructions stating that "individuals can specify Creative Commons as their proxy (via http://creativecommons.org/compatiblelicenses) so that if and when Creative Commons determines that a future version of the GPL is a compatible license, the adapted and combined work could be used under that later version of GPL."
Lastly, the FSF is grateful to all of the individuals who helped contribute to the compatibility discussion and process, including the many emails and questions we received from members of our community, but most especially to those individuals directly involved over the past five years, including: Sarah Hinchliff Pearson, Diane Peters, Lawrence Lessig, Mike Linksvayer, Kat Walsh, and Christopher Allan Webber from Creative Commons; Joshua Gay, Donald Robertson, III, Brett Smith, Richard Stallman, and John Sullivan of the Free Software Foundation, along with Eben Moglen of the Software Freedom Law Center and Aaron Williamson (who is now an attorney at Tor Ekeland).