Nothing to worry about. This kind of attack affected all versions of Joomla released prior to Christmas 2015. Me and my developer, Davide, fixed that in Joomla itself back then. Some idiot is trying to use this very old and no longer working attack against your site. It's about as effective a method of penetration as firing a kid's squirt gun against a concrete wall. Just ignore them, they will go away.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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