An OS command injection vulnerability in alseambusher/crontab-ui through 0.4.2 allows an unauthenticated remote attacker to inject arbitrary cron job entries by sending a crafted GET request to /crontab with URL-encoded newlines in the env_vars parameter. The handler appends the env_vars value directly to the crontab file string as '${envVars}\n' without stripping newline characters, enabling an attacker to insert arbitrary cron expressions that execute attacker-controlled commands.