Synology NAS systems are - along with QNAPs - currently the target of a wide brute-force attack. A botnet tries to break in via weak passwords and infects the system with ransomware. Once infected, it encrypts all files and data.
This affects systems which are reachable over the internet (open firewall ports / NAT).
To protect yourself you should
- activate the DoS protection including account blocking
- apply strong password rules to all users
- create a new admin account with a strong password and disable the standard „admin“ account
More informations:
https://www.synology.com/en-global/company/news/article/2019JulyRansomware