As of January 2012 Roundcube is licensed under the GNU General Public License (GPL) Version 3 or later (with exceptions for skins & plugins) while it was licensed under GPL (v2) before that change. Actually the license situation wasn't absolutely clear before because some parts of the Roundcube code didn't specify a version number of the GPL while others did. Those parts not specifying a version number could, according to the GPL license text, be considered as either version 2 or any later version.
Clarification and Preparation for the Future
Because of the incorporated numerous other components, the only options for licenses are in the GNU GPL family, namely GNU GPLv2, GNU GPLv3, GNU AGPLv3.
In order to become license compatible with some of the newer technologies we would like to be able to use and to be well prepared for future development and integration we decided to officially change to GNU GPL "version 3 or any later version".
Together with the change to GPLv3+ we also added the following exception to the Roundcube license agreement which explicitly excludes skins & plugins from the terms of the GPL:
This file forms part of the Roundcube Webmail Software for which the
following exception is added: Plug-ins and Skins which merely
make function calls to the Roundcube Webmail Software, and for that
purpose include it by reference shall not be considered
modifications of the software.
If you wish to use this file in another project or create a modified version that will not be part of the Roundcube Webmail Software, you may remove the exception above and use this source code under the original version of the license.
This shall give a maximum flexibility to skin designers and plugin developers to choose any other license for their work or even to create commercial or proprietary components without being forced to publish them according to the strong copy-left requirements of the GPL.
The proposal and the following discussion about this license clarification can be read in our mailing list archive.
Request for Agreement
Because the copyright in Roundcube is not consolidated, making these updates requires the agreement of all contributors to Roundcube.
So if you have in the past contributed to Roundcube, we would very much like to ask for your explicit agreement with this path forward. We believe this primarily represents a clarification and an adjustment to what we practiced over the past years.
But of course we cannot force anyone to agree, so for those who do not agree to this license update we still offer the choice to have their code removed from Roundcube.
Please contact us in case you have serious objections against the GPLv3+ with exceptions for skins & plugins and you want your contributed code to be removed.