Make sure your Steam account actually owns the modern version of CS:S, opt out of betas, verify files, and avoid cracked/old servers.
Your Steam account does not own a valid Counter-Strike: Source license for the version you’re trying to play. This usually happens with old retail (disc) keys or third-party keys. Make sure your Steam account actually owns the
If you bought a key from a third-party site (G2A, Kinguin, etc.) for $2, it’s almost certainly a stolen or region-locked key. Steam will reject it for internet play. Refund it if you can. If you bought a key from a third-party
SteamUser_2004
FIXED: "CD Key Authentication Invalid" on internet servers (CSS) etc.) for $2
Hope this saves someone the headache.
I know this is an old error, but I just ran into it after digging out my old copy of CS:S, and it took me an hour to fix. If you’re getting when trying to join internet servers (but LAN works fine), here’s what’s actually happening and how to solve it.