I found my RB750 crashing due to running out of RAM this evening.Upon deeper investigation, it appeared that unauthorised access had been obtained to the router. Some firewall "drop" rules were disabled and there was a "mikrotik.php" file along with some scripts running.
I found the php file rather quickly although it took a little longer to locate the scripts.
I have upgraded from 6.38.1 to 6.42.6 (I understand there were some patched vulnerabilities) and have changed user passwords. I initially changed these after removing the file/scripts but found there was still a job running after the firmware upgrade. I subsequently changed user passwords again and have not seen it re-appear since.
Other than ensuring I lock down access to any admin interfaces from remote networks (these were a little more open than they should have been), is there anything else I can look for to ensure that I have cleared all traces of this malicious activity?