CRON session entries in logs

I'm having this in my RaspberryPi /var/log file

Feb 26 21:17:01 raspberrypi CRON[15182]: pam_unix(cron:session): session opened for user root by (uid=0)
Feb 26 21:17:01 raspberrypi CRON[15182]: pam_unix(cron:session): session closed for user root
Feb 26 21:39:01 raspberrypi CRON[15189]: pam_unix(cron:session): session opened for user root by (uid=0)
Feb 26 21:39:01 raspberrypi CRON[15189]: pam_unix(cron:session): session closed for user root

in fact... my log file is full of this CRON session opened and closed. Is this a security issue? someone is trying to access my raspberry pi?

if so .. what can I do? Is there any tool that I can install to see from what IP address is coming the attack?