One problem with the Joomla "community" is that while the software is nominally GPL, there are lots of paid-only add-ons, sometimes for things that should be basic features.

(long treatise about Free Software and the difference between paying for software and paying for support elided so that we can just get to the good stuff)

One such basic feature is a CAPTCHA for the Community Builder new account creation process. After much searching, I found an older (2008) plugin that works with Joomla 3.4.5 and Community Builder 2.0.12 (the latest versions at the time of writing). This plugin's license is Creative Commons by-nc-nd/2.5/in, so while it is clear I can distribute the unmodified plugin, you have to use your own judgement to determine whether you are permitted to use it on any particular site.

This plugin warns that it is not compatible with the current Joomla and CB, but it works just fine. Since the license is a "no derivatives" license, I cannot provide a modified version which doesn't display this warning.

Note that Google is now warning that the reCAPTCHA v1 API, which I believe this plugin uses, will be discontinued on March 31, 2018, so your days using plug_reCAPTCHA are numbered.

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