Unfortunately public YOURLS installations are a prime target for spammers looking for a way to cloak their spammy links and make them harder for other sites to recognise as spam. Out of the box (and by design) YOURLS doesn’t have any spam protection as the developers prefer to focus on the core functions and leave extra functions to the rather excellent plugin architecture.
I’ve written a plugin which implements Google’s new reCaptcha v2 for public YOURLS sites. It’s pretty straightforward to setup and achieved a 100% reduction in spam links on my public site.
You can download the plugin from Github where full install instructions are also provided in the readme file.
Let me know in the comments below if you find this plugin useful or have any feature ideas.