Safari's default Search Engine hijacked by Yahoo

It may just be malware that has hi-jacked your browser with Yahoo! Quit Safari.

Download and run the free MalwareBytes for Mac, and see if it detects, and offers to remove anything for you. Then back to Safari.

1. Press and hold the Shift key while launching Safari this prevents prior crap from loading
2. Safari menu : Clear History… all history : Clear History.
   1. Because you launched using the shift key, this now clears all cookies too.
3. Safari Preferences : Advanced
   1. [√] Show Develop menu in menu bar
   2. Press option+command+e to clear your browser cache
4. Safari Prefernces : Extensions
   1. Remove any extension you did not install yourself
5. Quit Safari

Now, you can relaunch Safari and determine if the Yahoo! invasion is over.

Good that you solved it with the first three steps.

Malwarebytes is a product that many of us have been recommending for several years, and install ourselves. It is unneeded for those of us that only download software from original, trusted vendors, and not from any other source, such as spoof download buttons, or download aggregation sites like macupdate, softonic, CNET, etc.

Malwarebytes is for that succinct purpose - detecting and removing adware/malware. It specifically is not anti-virus, or we would not recommend it, aside from the fact that there are no viruses on the Mac. A virus is a self-replicating entity that actively infects other devices on your network, and beyond. Windows viruses do just that, but cannot run on the Mac.