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How to restrict the field "From" mismatch for emails and prevent outbound mail spoofing?

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Yaroslav Tarasov
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Plesk Onyx for Linux kb: how-to ABT: Group B

Applicable to:

  • Plesk Onyx for Linux

Question

How to restrict the field "From" mismatch and prevent outbound mail spoofing?

Answer

Warning: This solution may disrupt mail functionality, such as Plesk notifications, Postfix sendmail wrapper, or other features.

Note: Postfix 2.1 or newer is required for the "reject_sender_login_mismatch" restriction.

  1. Connect to the server via SSH.

  2. Backup configuration files:

    # cp -a /etc/postfix/main.cf{,.bak}

  3. Change value of "smtpd_sender_restrictions" in /etc/postfix/main.cf so Postfix prevents sending messages if a logged-in mail user is different from the one written in "From" field:

    # postconf smtpd_sender_restrictions="reject_sender_login_mismatch, `postconf -h smtpd_sender_restrictions`"

  4. Add lookup key value for authentication ID in /etc/postfix/main.cf:

    # postconf smtpd_sender_login_maps=hash:/var/spool/postfix/plesk/virtual

  5. Restart Postfix:

    # service postfix restart

    Note: The setting will be applied server-wide.

This configuration can be tested with the following commands:

# touch message
# curl -k --url "smtp://127.0.0.1" --mail-from "spoofing@example.com" --mail-rcpt "jdoe2@example.com" --upload-file ./message --user 'jdoe2@example.com:password' --ssl
<...>
curl: (55) RCPT failed: 553

where "spoofing@example.com" - forged "From" field.
"jdoe2@example.com" - any recipient.
"jdoe2@example.com" - mailbox from the server.

Comments

5 comments

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  • Avatar
    Siniša Burina

    Hi!

    On Debian jessie, with postfix version 2.11.3-1+deb8u2, I'm getting the following notification in the log file, and the solution does not work:

    warning: restriction `reject_authenticated_sender_login_mismatch' ignored: no SASL support
    warning: restriction `reject_unauthenticated_sender_login_mismatch' ignored: no SASL support

    Can you please advise?

     

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  • Avatar
    Siniša Burina

    Solved!

    Here's the information, in case someone else bumps into the same issue.

    The above-mentioned restriction, if put into the main.cf file, actually gets enabled to both incoming SMTP connections and submissions, which is not what we really want. So, instead of configuring it in main.cf, put it in master.cf, submission config:

    submission inet n - - - - smtpd
     -o smtpd_enforce_tls=no
     -o smtpd_tls_security_level=may
     -o smtpd_sasl_auth_enable=yes
     -o smtpd_client_restrictions=permit_sasl_authenticated,reject
     -o smtpd_sender_restrictions=reject_authenticated_sender_login_mismatch
     -o smtpd_recipient_restrictions=$submission_recipient_restrictions
     -o smtpd_end_of_data_restrictions=$submission_end_of_data_restrictions

    Et voila! :)

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  • Avatar
    Alexandr Redikultsev

    Hi @Siniša Burina!

    Thanks for sharing!

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  • Avatar
    Siniša Burina

    Hey! What happened to my previous post? Was it dangerous in some way so you decided to remove it?

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  • Avatar
    Linus (Edited )

    If incoming connections on port 25 are blocked there is an easier solution, which doesn't disrupt mail functionality, such as Plesk notifications, Postfix sendmail wrapper, or other features.

    The solution is, just to block SASL authenticated messages instead of all messages. 


    # /etc/postfix/main.cf

    smtpd_sender_login_maps = hash:/var/spool/postfix/plesk/virtual
    smtpd_sender_restrictions = check_sender_access hash:/var/spool/postfix/plesk/blacklists, reject_authenticated_sender_login_mismatch, permit_sasl_authenticated

    Make sure to block incoming TCP connections on Port 25. Unauthenticated mails will not be checked for Sender / FROM mismatch.

     

    New SMTP behaviour:

    • SMTP (25): X Sender Login Mismatch won't be rejected
    • SMTP (SSL) (465): ✓ Sender Login Mismatch will be rejected
    • SMTP (STARTTLS) (587): ✓ Sender Login Mismatch will be rejected


    Allowed senders for SMTPS (465) and SMTP STARTTLS (587):

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