Zimbra Collaboration Suite (ZCS) PostJournal service version 8.8.15 contains a critical command injection vulnerability that allows unauthenticated attackers to execute arbitrary system commands. The vulnerability stems from improper sanitization of the RCPT TO parameter via SMTP injection. Attackers can exploit this by injecting shell expansion syntax through the RCPT TO parameter to achieve remote code execution under the Zimbra service context. This vulnerability poses a high risk as it requires no authentication and allows full system command execution.