Using MTA-STS to enhance email transport security and privacy

Overview

SMTP is broken by design. It comes from a time when communication partners trusted each other and the NSA was intercepting facsimiles and phone calls instead of internet traffic.

To enhance privacy, in 2002 RFC 3208 was added to the SMTP protocol. Unfortunately STARTTLS is only optional, it is not allowed to only accept encrypted connections.

The RFC states: A publicly-referenced SMTP server MUST NOT require use of the
STARTTLS extension in order to deliver mail locally
.

That is really problematic because it leaves SMTP vulnerable to MITM (Man-In-The-Middle) attacks as the connection is first established in clear text and using the STARTTLS command to establish an encrypted session. In that time window, an attacker is able to enforce a clear text communication by suppressing the STARTTLS command. DNS cache poisoning to hijack mail transfer by fake MX records is also an attack vector.

What to do to solve the problem? Well, DANE is a good solution but it requires DNSSEC to work as expected. Unfortunately DNSSEC is not available for all domains, so SMTP MTA Strict Transport Securityโ€œ (RFC 8461) was introduced.

It works similar like HTTP Strict Transport Security (HSTS). It works as “Trust on First Use” or also known as “TOFU”. The policy is announced by a DNS record (which will be cached) and will be retrieved by HTTPS.

MTA-STS is less secure than DANE, but it is a huge step forward.

Who is using it?

There are already some large scale mail providers that make use of MTA-STS. Here are a few of them:

  • Google mail
  • mail.de
  • Yahoo
  • GMX

All of them are using the testing mode in the policy.

Announce your policy

The first step is to create the DNS records needed. A TXT record is used to announce the policy and a A (or AAAA) record for the hosting of the policy. For this example I use my test domain ldelouw.ch. The data is STSv1 where v1 stand for the protocol number, where one is the first and only version at the moment. The ID is used to identify changes in the policy. It is good practice to use a timestamp in Zulu time format.

If you are using IPA for your DNS management, its very easy:

[root@ipa1 ~]# ipa dnsrecord-add ldelouw.ch _mta-sts --txt-rec="v=STSv1;id=$(date -u +'%Y%m%d%H%M%S')Z;"
  Record name: _mta-sts
  TXT record: v=STSv1;id=20181216095025Z
[root@ipa1 ~]#

Create a _smtp._tls record to announce where error reports are sent to. This is usually the postmaster of the domain. This is optional but recommended.

[root@ipa1 ~]# ipa dnsrecord-add ldelouw.ch _smtp._tls --txt-rec="v=TLSRPTv1;rua=mailto:postmaster@example.com"
  Record name: _smtp._tls
  TXT record: v=TLSRPTv1;rua=mailto:postmaster@example.com
[root@ipa1 ~]#

Be aware for the A/AAAA record(s) there is no _ (underscore) needed.

[root@ipa1 ~]# ipa dnsrecord-add ldelouw.ch mta-sts --aaaa-rec="2a01:4f8:141:14ce::9"
  Record name: mta-sts
  AAAA record: 2a01:4f8:141:14ce::9
[root@ipa1 ~]# 

The second step is to create a web server instance for https://mta-sts.<your-domain>:443. The x509 certificate must be valid (make sense, right? ๐Ÿ˜‰ ) otherwise it will not work. Free certificates can be created using Letsencrypt.org. It can also be a SAN (Subject Alternative Name). If you have a web mailer software installed on your mail server it can be reused.

In the web server instance you need to create a file containing your MTA-STS policy. The file contains the protocol version (STSv1), the mode, a list of your mail exchange servers and the maximum age caching your policy. The mode first should be testing so see if it working properly.

mkdir -p /var/www/html/mta-sts.ldelouw.ch/.well-known

cat > /var/www/html/mta-sts.ldelouw.ch/.well-known/mta-sts.txt << EOF
version: STSv1
mode: testing
mx: your-smtp-server.example.com
mx: secondary-smtp.example.com
max_age: 86400
EOF

Testing your Policy

There are two web based tests available.

  • https://aykevl.nl/apps/mta-sts/
    Does not work with IPv6 only setups, it caches DNS requests which is bad when you do some testing and need to correct DNS entries.
  • https://www.hardenize.com You need to register to be able to run multiple test. Checks a lot of related configuration parameters as well.

Fetch policies for Postfix on your SMTP Server

Postfix itself does not support MTA-STS. You need a little helper for that: postfix-mta-sts-resolver.

The software is in an early state, it lacks a bit of documentation. That probably will improve over time. The software itself works nicely, I do however don't have any experience on a large scale.

Unfortunately the stock Python version of RHEL7 is too outdated, but it will work with Python 3.6 which is available in the Software Collections.

yum -y install rh-python36-python-pip.noarch

Install postfix-mta-sts-resolver using PIP

/opt/rh/rh-python36/root/bin/pip install postfix-mta-sts-resolver

Create a user for the MTA-STS daemon:

useradd -c "Daemon for MTA-STS policy checks" mta-sts -s /sbin/nologin

Lets install a Systemd unit file:

cat > /etc/postfix-mta-sts.service << EOF
[Unit]
Description=Postfix MTA STS daemon
After=syslog.target network.target 

[Service]
Type=simple
User=mta-sts
Group=mta-sts
# This is the ExecStart path for RHEL7 using python 36 from the Software collections.
# You may use a different python interpreter on other distributions
ExecStart=/opt/rh/rh-python36/root/bin/mta-sts-daemon

[Install]
WantedBy=multi-user.target
EOF

Enable the service on system startup:

systemctl enable postfix-mta-sts.service

There is a some configuration needed for the MTA-STS daemon itself.

cat > /etc/postfix/mta-sts-daemon.yml << EOF
host: 127.0.0.1
port: 8461
cache:
  type: internal
  options:
    cache_size: 10000
default_zone:
  strict_testing: true
  timeout: 4
zones:
  myzone:
    strict_testing: false
    timeout: 4
EOF

I'm not sure what the configuration statement strict_testing means. Without setting it to true, it will not work when the policy is set to testing. Looks like it overrides the policy from testing to enforcing, handle with care!

Configuring Postfix

The last step is to let postfix know about MTA-STS. This is done with the configuration statement smtp_tls_policy_maps.

smtp_tls_policy_maps = socketmap:inet:127.0.0.1:8461:postfix

Also ensure that smtp_tls_CAfile is set to make use of your global CA-cert bundle, otherwise sending emails to TLS enabled servers will completely fail since STARTTLS is not opportunistic anymore!

smtp_tls_CAfile = /etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem

Nice to have: Increase the TLS log level to see if it is working:

smtp_tls_loglevel = 1

Testing your setup

mail:~# postmap -q ldelouw.ch socketmap:inet:127.0.0.1:8461:postfix

It should return something like:

secure match=mail.delouw.ch

Now send an email to a domain using MTA-STS and verify the Posfix log.

Dec 16 13:32:24 mail postfix/smtp[3583]: Verified TLS connection established to gmail-smtp-in.l.google.com[2a00:1450:400c:c0c::1a]:25: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)

It states Verified not just Trusted ๐Ÿ™‚

Conclusion

MTA-STS is a way that enhances security without DNSSEC. It is still in its early stage, IETF just released the first version of RFC 8461 in September 2018.

The critical point is the MTA-STS lookup to be done by the MTA. There is not much choice of software that can be used to achieve the goal of an MTA-STS capable MTA. I only made some tests with the most poplar MTA, Postfix, solutions for others like Sendmail and Exim may, or may not exist.

Another problem is the MTA-STS agent. It is a possible new attack vector for the bad guys, input sanitation for policies is key.

At the end of the day, lets give it a try and enable our MTAs out there.

Have fun ๐Ÿ™‚

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