Unauthenticated Command Injection via kubeconfig Username in che-machine-exec

The che-machine-exec service constructs a shell command by concatenating user-controlled input (the username and namespace fields from the /exec/init and /exec/config JSON endpoints) into a double-quoted sh -c "echo "..." > ..." invocation without any escaping. When che-machine-exec is run with authentication disabled (its default), any network-reachable attacker can achieve remote code execution in containers che-machine-exec is configured to access, via shell command substitution ($(...) or backticks).