Check your Browser Extension Preferences  for pop-ups, as sometimes when you download things and click Agree, then you are inadvertently allowing certain applications to install malware onto your computer.
 

To remove extensions on Safari :

1. Open Safari .
    
2. Click on Safari   > Preferences.
   
    
3. Click on Extensions.
   
    
4. Remove any Unwanted or Suspicious Extensions Appearing in the list on the left.