Do this now:
1. In DSM, click the person-shaped icon in the upper-right corner.
2. Click Personal.
3. Go to the Account tab.
4. Open Account Activity > Login History.
5. Select the suspicious login record, click Manage Trust Info, then Set to Untrusted. This automatically terminates current connections tied to that login record. [1]
Then secure the account:
- Change your password.
- Enable multi-factor authentication:
- DSM 7.0 and DSM Enterprise: 2-Factor Authentication
- DSM 6.2 and earlier: Enable 2-step verification [1]
- Also sign out of all devices: Security > Sign Me Out > Sign Out of All Devices. [2]
For the Synology Account itself:
1. Sign in to your Synology Account.
2. Go to Security > Change Password.
3. Finish the wizard. After changing the password, Synology Account signs you out of all browsers. [2]
The knowledge base also recommends:
- Use a strong password. [5]
- Turn on 2FA / Secure SignIn. [5][12][15]
Sources used: [1], [2], [5], [12], [15]