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Mail Server Security: Potential Vulnerabilities and Protection Methods

This article covers the topic of security for email servers. Security measures covered here will allow you to greatly increase the level of protection for your email server and prevent any attacks from succeeding.

Mail Server

1. Introduction

1.1 Challenges of server software security

With the constant development of information technology, the role of cybersecurity is getting ever bigger. While it’s impossible to imagine the modern world without constant communications over networks, almost all valuable data that’s a target for attacks is stored in various forms on servers. Not to mention that the stability of your whole system depends on servers. This is why servers are attractive targets for malicious attacks.

1.2 Email server security

Security for an email server is particularly important because email is one of the most popular means of communication and doing business. And for businesses, in particular, loss of confidential information can result in large financial losses. It’s also important for a server to run stably so that users are able to access it at any time. When a server is unstable, it can lead to the loss of customers.

1.3 Standard methods of software protection

In order to avoid data loss, problems with stability and other troubles, we need to adhere to general recommendations on how to set up an email server and monitor its security in order to detect and immediately fix any vulnerabilities. This is the main topic of this article. We’ll show examples of security measures and various problems that can arise if you don’t adhere to them. We’ll also talk about some features of certain email servers and examples of their vulnerabilities, together with general recommendations for testing.

2. Potential vulnerabilities

Every system has vulnerabilities, so every system is at risk of a cybersecurity breach. Of course, dealing with all of them is impossible, but we can greatly reduce their number. In order to do this, we need to stick to best practices when setting up email server security. These best practices and problems that can arise when you don’t follow them will be the focus of the next part of this article.

2.1 Unauthorized access to data

One widespread type of attack is when a perpetrator tries to bypass the authentication procedure in order to get access to data.

The first thing you need to do to avoid this is to establish strong requirements for the password used to access the server. This prevents the password from being cracked via brute force, which is a universal way to bypass authentication. Everything else depends on the server type. Different types of servers use different operating systems, interfaces, etc.

2.2 Threat of data leakage

Personal data is one of the key targets for hackers. When an email is sent via the internet, it goes through unprotected communication channels. Passwords, user names, and messages themselves can be intercepted. In order to prevent this, you need to encrypt both incoming and outgoing mail. SMTP, POP3, and IMAP protocols should be encrypted with SSL/TLS.

2.3 Spam

Spam is one of the biggest problems when it comes to email.

From the server security standpoint, we can divide the threat of spam into two categories:

  • Sending external spam messages to your own clients
  • Sending external spam messages to other clients (In this case, the server acts as an Open Reply.)

    2.4 Threat of malware

    Both servers and email clients are susceptible to malware. When an email server is infected, the stability of the whole system is compromised. Integrity and privacy of personal data are threatened. Malware spreads among email clients mostly thanks to infected attachments.

    Protection from malware involves both built-in tools and third-party antivirus software.

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    2.5 DoS threat

    The damage that a DoS attack can do to a mail server is hard to overstate. It leads to unreceived and unsent emails, not to mention time spent trying to restore the service. Ultimately, the reputation of the whole company suffers.

    To prevent such a threat, you need at least to limit the number of possible connections to the SMTP server. In order to cope with SMTP security issues, look into limiting the overall number of connections over time, as well as simultaneous connections.

    2.6 Server performance and stability

    When we see the words “server” and “performance”, we immediately think about load balancing.

    If your server is attacked and stops working, you need to have a plan B. In such cases, we often use a reserve server. For email servers, in particular, this is done by having two MX records for each domain.

    Email servers also have an option to use SMTP authentication. If it’s enabled, then in order to send an email to the server you need to provide an additional username and password.

    3. Detecting and analyzing vulnerabilities

    At the start, it’s necessary to design the approach that will be used to monitor mail server security. In many cases, it’s easier to look at the available solutions. Numerous companies provide cybersecurity audits. But in certain cases, it can be necessary to solve the problem yourself. How tough and formal this process should be depended on the task at hand.

    Next, we’ll look at a compromise, where we use a formal, yet flexible approach.

    3.1 Preparing the necessary documentation

    First, you need to decide what you need to check, why and how. All three questions are important because you need to cover as much ground as you can with your audit without wasting your time on unnecessary details.

    • What. Create a list of all data (usernames, contact lists, attachments, etc.) and parameters (performance, uptime, etc.) that you think you need to track and check for vulnerabilities. This list can be divided into several checklists with different areas of responsibility (server, network, operating system). Each entry in the list should be weighted according to the impact that potential problems with it may cause.
    • How. Apply two approaches:
      1. Using the previously created list of components, search for tools and utilities that can be used to check whether a particular component is vulnerable. Each component should be assigned a corresponding method of checking. All those methods will be our tests or objects for monitoring.
      2. Using the list of potential vulnerabilities, expand your list of objects to monitor with additional controls, allowing you to check for these vulnerabilities. In case some entries repeat, mark them but don’t remove them.

Why. Check the target, weight, and how much each individual check covers in order to assign a priority to it. At this stage, you can remove repeat entries from your list of components, but only if the deleted component is fully covered by some other entry or combination of entries. Otherwise, keep the repeated entry but assign it a lower priority.

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4. Example of cybersecurity audit of MS Exchange Server for Windows

4.1 Software and operating system specifics

Microsoft Exchange Server is a popular email server provided by Microsoft. Working only on Windows Server OS, Exchange Server supports both standard (SMTP, POP3, IMAP) and proprietary (MAPI, EAS) protocols.

Let’s look at specifics of Exchange Server with regard to cybersecurity. Here we’ll cover all newer versions starting with Exchange 2010 (official support for Exchange 2007 expired on April 11, 2017).

      • Edge Transport Server — This is a server for incoming and outgoing external mail. This server works great for companies with network infrastructure divided into a protected internal network and a protected perimeter, or demilitarized zone (DMZ). An Edge Transport Server is usually located inside the DMZ, while the Mailbox is located inside the private network. An Edge Transport Server provides an additional layer of defense for any messages. This way, the mail server experiences fewer external attacks. Edge Transport is optional during Exchange Server 2010 and 2016 installation but is missing from Exchange 2013.
      • Database availability group (DAG) — A component that provides high availability and recovery for data on the server. It was first introduced in Exchange 2010. DAG is a base component of the Mailbox server that ensures availability and data recovery after various incidents.
      • Spam protection is achieved via internal antispam agents. They’re available by default on Edge Transport servers starting with Exchange 2010 and can be enabled directly on the Mailbox server (in Exchange 2016)
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