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I would like to extend an existing plugin (for example FileBird), to add some features, and later publish and sell it off...

But although the plugin's source-code can be simply downloaded from WordPress.org under GPLv2, the plugin has some registration and/or billing built-in...

Is it legal to modify the plugin, where it asks for purchasing it? or even completely removing that and make a free alternative (under another name of course)?

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My first question was whether FileBird was really under GPLv2; they don't exactly go out of the way to make the license a salient. But after some digging it looks like you're right.

That given, what you propose - removing the billing and registration hooks - is completely lawful. You don't even have to use another name, though it would probably be kind of you to do so as it may reduce confusion. You will of course need to release your version under GPLv2 also, which may impact on your plans to "sell it off".

Be prepared for the possibility of opprobrium: some people release software under copyleft licences, possibly without fully understanding that that empowers people to develop the code in ways they themselves would not choose to. They then get very upset when other people try to exercise the rights they've explicitly been given. None of that, however, makes what you propose in any way unlawful.

  • Thanks, and of course selling it off would be problematic for an open-source project, although we always have the option of using Premium-marketing (like billing only for extensions, updates and...), but that requires our base-plugin to be worthy in the first place – Top-Master Mar 13 at 9:40
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    @Top-Master note that even if your extensions and updates are only distributed to paying customers, they will also have to be distributed under GPLv2, so charging for those, too, may not work very well. – MadHatter Mar 14 at 11:24
  • Is really any extension developed for a product which is published under GPLv2 forced to be under GPL too? I mean, forcing GPLv2 on plugins would be not very practical, no one would be able to create any plugin for any of such products (except when they do also use GPL as their license). for example, does that mean, we are legally NOT even allowed to develop any plugin for WordPress in the first place, except when we do publish the plugin under GPL too!!! have created another question at: opensource.stackexchange.com/questions/8080/… – Top-Master Mar 16 at 10:44

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