[snakes1100]

You can use these to scan for anything suspicious.

SELECT title, phpcode, hookname, product FROM plugin WHERE phpcode LIKE '%base64%' OR phpcode LIKE '%exec%' OR phpcode LIKE '%system%' OR phpcode like '%pass_thru%' OR phpcode like '%iframe%';

SELECT styleid, title, template FROM template WHERE template LIKE '%base64%' OR template LIKE '%exec%' OR template LIKE '%system%' OR template like '%pass_thru%' OR template like '%iframe%';

Did you locate anything in the apache/nginx etc log related to that attempted post in the cms for that time stamp?