Install an antivirus scanner

sudo apt install clamav
sudo pkill -15 -x freshclam
sudo freshclam

Caution: Scanning will heavily impact the CPU of your Raspberry Pi

Install a malware checker

sudo apt install rkhunter
sudo rkhunter −−update

If the update fails, update the following file: /etc/rkhunter.conf as below:

change UPDATE_MIRRORS=0 to UPDATE_MIRRORS=1
change MIRRORS_MODE=1 to MIRRORS_MODE=0
change WEB_CMD="/bin/false" to WEB_CMD=""

Run a full automated rootkit verification check of your system with only issues highlighted:

sudo rkhunter -c --enable all --rwo --sk