More than likely not an attack on your system but a configuration. It's possible you installed something from the web that decided to install some junk along with it or someone else using your computer installed it if that's what you're seeing
go to the spotlight (magnifying glass in the upper right hand corner)
type Activity Monitor

when the Activity Monitor application opens do a search for snapchat and if you find it:
select it from the list and click (X) n the upper left of the Activity Monitor Window and force quit the snapchat or related application
Quit out of Activity Monitor
go to the Apple Menu in the upper left
go to >System Preferences...>Users and Groups
click on your account
click on Login Items
if Snapchat is listed select it and click - to remove it.

go to Macintosh HD>Library>Launch Agents
select anything relating to snapchat and move it to the trash
go to Go in the menu bar
hold down the Option key
you will see Library added to the list of folders
go to >Launch Agents
select anything relating to snapchat and move it to the trash
btw this is a different "Launch Agents" folder I had you go to in the paragraph above
Reboot the system
For futher analysis you may post an Etrecheck report. The link is http://www.etrecheck.com