How to prevent plaintext authentication via IMAP/POP3 and SMTP in Postfix on Plesk server?

Dmitry Bystrov

10 months ago

Question

How to prevent cleartext / plaintext authentication via IMAP/POP3 and SMTP in Postfix on Plesk server?

Answer

Note: If you don't have root access to the Plesk server via SSH, contact your service provider regarding this issue.

Click on a section to expand

Plesk with Dovecot + Postfix

Connect to the server via SSH
Enable PCI compliance to Dovecot service:

# plesk sbin pci_compliance_resolver --enable dovecot
Enable PCI compliance to Postfix service:

# plesk sbin pci_compliance_resolver --enable postfix
Edit the file /etc/postfix/main.cf adding the following line:

smtpd_tls_auth_only=yes
Open the smtpd.conf file in a text editor (in this example, we are using the vi editor) and remove "PLAIN" and "LOGIN" from mech_list:
- on CentOS/RHEL-based distributions
  
  # cat /usr/lib64/sasl2/smtpd.conf
  
  pwcheck_method: auxprop saslauthd
  auxprop_plugin: plesk
  saslauthd_path: /var/spool/postfix/private/plesk_saslauthd
  mech_list: DIGEST-MD5 CRAM-MD5
  sql_engine: intentionally disabled
  log_level: 4
- on Debian/Ubuntu-based distributions
  
  # cat /etc/postfix/sasl/smtpd.conf
  
  pwcheck_method: auxprop saslauthd
  auxprop_plugin: plesk
  saslauthd_path: /private/plesk_saslauthd
  mech_list: DIGEST-MD5 CRAM-MD5
  sql_engine: intentionally disabled
  log_level: 4
Restart mail services to apply the changes:

# service saslauthd restart && service postfix restart && service dovecot restart
Enable the RoundCube webmail to use the CRAM-MD5 auth method by adding the following lines to /usr/share/psa-roundcube/config/config.inc.php:
$config['smtp_auth_type'] = 'CRAM-MD5';
$config['imap_auth_type'] = 'CRAM-MD5';

Note: If Qmail is used, it is recommended to switch to Postfix. Forcing secure connection over SMTP in Qmail requires patching which is not supplied by Plesk.

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