Instead , they are required to co-license their contributions under the copyleft licenses Creative Commons Attribution-ShareAlike 3.0 Unported License ( CC-BY-SA ) and GNU Free Documentation License ( GFDL ) .
However , they must license their changes under the copyleft licenses Wikipedia uses , which are the Creative Commons Attribution-ShareAlike 3.0 Unported License ( CC-BY-SA ) and GNU Free Documentation License ( GFDL ) .