Support

You need to set "Enable IP Workarounds" to Off. It's not required on your server. Please note that this is not a problem, though.

This is a kind of attack which affects Joomla! versions prior to Joomla! 3.4.8 and with old, vulnerable versions of PHP (5.3 and 5.4). The attacker uses code instead of an IP address in the X-Forwarded-For HTTP header. These old versions of Joomla! on these old versions of PHP would result in arbitrary code execution. Your site is not at risk anyway since it's running on Joomla! 3.7 and PHP 7.1, both of which are not vulnerable. The attacker does not know this, hence their attempt against your site. Basically, they have zero chance of success.

Admin Tools will still catch and block that attack attempt. That's why you see a security exception record. Since the attack is based around the X-Forwarded-For HTTP header and you have "Enable IP Workarounds" (or your server is doing something to the same effect) the contents of that HTTP header are used as the visitor's IP address, therefore they are recorded as such. Again, nothing to worry about, it just means that Admin Tools is doing its job protecting you against major Joomla! threats.