^{CVE-2021-38084:}Command Injection Vulnerability in Courier Mail Server POP3 Component

Overview

The vulnerability affects the POP3 component of Courier Mail Server before version 1.1.5. The issue stems from improper handling of command pipelining in relation to the STLS command, which is used to initiate TLS encryption for a POP3 session. When a client sends an STLS command to start encrypted communication, an attacker positioned between the client and server can inject additional plaintext commands before the encryption takes effect. These injected commands would then be executed within the context of the encrypted session, potentially leading to unauthorized access to email content, account compromise, or other malicious activities. This attack subverts the security expectations of users who believe their entire session is encrypted and protected.

Remediation

To address this vulnerability, system administrators should:

1. Update Courier Mail Server to version 1.1.5 or later, which contains fixes for this vulnerability.
2. If immediate updating is not possible, consider implementing network-level protections such as:
   • Using a VPN or other secure tunneling mechanism to protect POP3 traffic
   • Implementing network monitoring to detect potential command injection attempts
   • Restricting POP3 access to trusted networks only
3. Consider implementing additional authentication mechanisms beyond password authentication
4. Monitor server logs for unusual POP3 session behaviors that might indicate exploitation attempts
5. Consider using alternative secure email protocols if updates cannot be applied

References

1. Original vulnerability discussion thread: https://sourceforge.net/p/courier/mailman/courier-imap/thread/cone.1382574216.483027.8082.1000%40monster.email-scan.com/#msg31555583
2. Additional information about the vulnerability: https://sourceforge.net/p/courier/mailman/message/37329216/
3. CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component: https://cwe.mitre.org/data/definitions/74.html
4. Courier Mail Server project page: https://www.courier-mta.org/