The cPanel user shell access gives error all of a sudden :
root@server [~]# su - user
-bash: fork: Resource temporarily unavailable
The culprit seems to be cPanel and its “Shell Fork Bomb Protection”. Logging into WHM >> Security >> Security Center >> “Shell Fork Bomb Protection” and clicking “Disable Protection” resolves the issue.
Shell Fork bomb Protection will prevent users with terminal access (ssh/telnet) from using up the server’s resources and possibly crashing the server.