CustomSearch Rogue Browser Extension
CustomSearch is a rogue browser extension that functions as a browser hijacker.
Browser hijackers are a subcategory of potentially unwanted software that would make changes to various browser settings that can include the home page, the default search engine used when typing a search query into the address bar on a blank new tab and the content of the blank new tab itself, to name a few.
In the case of CustomSearch, it would just replace your default browser's search engine and start triggering redirects to fake search engines such as nseext dot info and customsear.ch.
This type of fake engine cannot produce its own results. It would simply grab your search query and spit out a Bing results page. This is just you using Bing search, but through a middle-man you don't really know.
You want to avoid the middle-man because third parties like this could be collecting data on your search queries and browsing history and this is a potential privacy risk. This data can be re-sold to advertising networks to help them generate better customized ads. The issue here is that this may be happening without your knowledge or consent.