The answer is nuanced. However, every major antivirus (Windows Defender, Malwarebytes, Avast) will flag it as "HackTool:Win32/Injector" or similar.
: Click Settings and enable Start in Secure Mode to help avoid initial detection. Extreme Injector v3.7.2 for CS- Source
The v3.7.2 update brought several refinements that solidified its reputation: The answer is nuanced
Download the v3.7.2 archive and extract it to a folder of your choice. The answer is nuanced. However
While VAC on CS: Source is not actively evolving, the VAC module still scans for known DLL signatures. Extreme Injector v3.7.2's signature has been in VAC’s database since 2015.
Counter-Strike: Source uses Valve Anti-Cheat (VAC) . Extreme Injector is a high-profile tool; using it on VAC-secured servers will almost certainly result in a permanent ban .
: Reliable but easily detected by modern anti-cheats.