There are a lot of places where a little tiny code can be implemented that does the job!

Here are my suggested steps to take towards fixing the issue:

 - You may find out where is that code exactly written by opening your
   joomla folder(ftp, sftp) and checking your last modify date of the
   folders > files. This way you may find where is the bad code!
   
  
 - I would also recommend you to update your Joomla to latest version   
   asap!
   
 - If you are already on the latest version, I recommend you to   
   reinstall the Joomla core by going to Administrator > Components >
   Joomla Update and click on the reinstall core button!
   
      
 - If after this the issue is still present, then the issue is   
   somewhere    in the templates folder or somewhere in the 3rd party   
   modules/plugins    that are not coming with Joomla by default. If   
   there are such    extensions(modules/plugins) try disabling them from
   Joomla admin and    check if the issue is still there!
            
 - Also please check your .htaccess and see if there is any strange code
   present! if possible, remove .htaccess and after       reinstalling
   the    Joomla core, rename the htaccess.txt to       .htaccess.