The "This Game Requires Steam" Error has occured a long time ago when CSN:Z was first launched. This is the basic walkthrough on how to fix this problem

NOTE: the following REQUIRES Windows Vista or later, users running on Windows XP cannot utilize the 1st method

1st:
Log out of steam (make sure the steam procesess is COMPLETELY terminated by right clicking on your taskbar, and by going to task manager and looking at the procesesses tab there

2nd step:
Right click on the steam icon, than run the procesess as administrator
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Not working?:
Make sure to start CSN:Z from steam, it's as simple as that. go to Library, Games (xxx), and than click on the "Counter-Strike Nexon: Zombies" and click play
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Still having issues?:
Make a Nexon EU account, than submit a ticket: http://support.csnz.nexoneu.com/Main/Default.aspx

a common question that may come to your head is, What is BlackCipher? BlackCipher, along with Xingcode are anticheats used by nexon to ensure that hackers or glitchers aren't making things too unfair for other players, alowing them to enjoy the game.

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How to fix
Disable your antivirus before playing (your antivirus and BlackCipher are conflicting with eachother) if you are worried about the security of your computer, whitelist the CSN:Z Folder, making sure your antivirus isin't detecting a false-positive.