What Are Email Injection Attacks and How to Prevent Them?

Email injection attacks let hackers access the internals of a system to practice malicious activities. This tactic can be used to tarnish your brand’s image by sending bulk spam or phishing emails anonymously from your mail server.

Hackers often use email header injections to infect computers and servers with different types of malware. While these attacks aren’t directly dangerous to you as a website owner, they can ruin your business reputation.

Thus business owners must educate their employees about what email injection attacks are, their working procedures, and ways to prevent them. Continue reading to know all this and more.

What are Email Injection Attacks?

Let’s answer the basic question: What are email injection attacks? Well, email injections are similar to SQL injection attacks—they both exploit one common security vulnerability: Unvalidated user input fields. 

Most websites have contact forms with input fields for users to sign up to newsletters, etc. This input data often includes email headers utilized to send emails to the intended recipient. The headers are interpreted by the website server’s email library and turned into SMTP commands which the SMTP server then processes. 

However, there’s often no mechanism in place to validate and verify these headers, so hackers exploit this security vulnerability. By entering or altering email headers, cyberattackers can execute SMTP commands that allow them to send bulk spam messages or malware-infected phishing emails to unsuspecting victims. 

The worst part? These emails still originate from the website’s mail server, so they look 100% legitimate coming from a genuine website domain. 

PHP applications are more prone to such attacks; hence hackers see them as hidden treasures. PHP email injection attacks are popular as cybersecurity experts fail to trace any signs of such an attack.

Although email injections aren’t directly harmful to website owners, they can cost more than you think. Contact forms with vulnerabilities are used for spamming, phishing, injecting spyware, etc.

How Does an Email Header Injection Work?

To understand injection flaws or email injection vulnerabilities, you must know the difference between the envelope and the body of an email. The envelope forms part of the SMTP protocol as the underlying part of the message with the following commands:

• Mail From: Information about an envelope’s sender.
• RCPT To: Indicates who should receive an envelope. You can use it multiple times to send an email to many people.
• Data: Initiates the email payload consisting of email headers and the message body separated by a single line.

On the other hand, email headers aren’t part of the SMTP protocol. They’re interpreted by the email client (for display reasons)  as well as email handling libraries. An email header usually contains the following information:

• From: Shows who the visible sender is and can vary from the ‘Mail From’ content.
• To: Displays the visible recipient and can be different from ‘RCPT To’ content.

Basically, in email header injection attacks, bad actors construct malicious email headers that turn into SMTP commands, allowing them to send phishing or spam emails using your email server. They achieve this by exploiting web forms that use email headers to send hard-coded genuine-looking emails.   

These contact forms let users fill in the subject, from address, body, etc. but aren’t typically filtered or sanitized beforehand—leaving them vulnerable to malicious exploitation.

Why are Email Header Injections Dangerous?

Email header injections are dangerous because malicious actors take control of your email server and send spam or phishing messages in your company’s name. It’s easier for cyberattackers to trick victims using your official email address, which can ruin your brand reputation and put your business at risk.

How to Prevent Email Header Injection Attacks?

There aren’t any sure-shot moves to prevent email header injection attacks. So, besides filtering and validating user inputs, encode them before adding them into scripts. Companies must motivate developers to create secure codes, decreasing the number of vulnerabilities. Moreover, the following preventive measures can avert disruptions caused by email HTML injection attacks.

Use Allowlisting

Allowlisting is a cybersecurity technique that works by permitting only trusted files, applications, user inputs, etc. Allowing your system to receive all user inputs will encourage hackers to infect systems with hybrid malware or corrupted codes. 

Allowlisting mitigates different types of cyberattacks by blocking or restricting unknown activities or inputs. The approvals can be done both automatically and manually.

You can choose to filter data by context. For example, by allowing only digits for the phone number column.  

Use the Principle of Least Privilege

The principle of least privilege helps limit access only to the people who require it to do their job. Permission is only granted to users who need to read, write, or execute the files necessary to complete their tasks.

The intensity of an attack depends on who all can access your network or crucial data. Using the principle of least privilege ensures hackers can’t infiltrate your system beyond a certain point. 

You can choose to grant the level of access based on department, seniority, time of the day, occasion, etc. 

Keep your Systems Updated

Old and outdated systems are prone to various types of cyberattacks, including injecting and spreading computer worms. This is because they lack the codes to combat advanced breaching techniques. Threat actors constantly scout for vulnerabilities to attempt email header injection and other attacks.

Additionally, software updates come with better features to enhance speed and offer a better end-user experience. However, you must be careful while updating as some of them may contain malware to intercept your data. You can recognise malware by noticing mysterious disk space consumption, browser setting changes, frequent system crashes, etc. 

Regularly Back Up Your Data

Backing up your data is one of the safest ways to minimize the damage if hackers succeed in an email SQL injection attack on your company. It can also prevent ransomware attacks where malicious actors encrypt crucial data and demand hefty ransoms in exchange for the decryption key. 

Here are some ways to backup your data:

• Copy to a USB stick.
• Burn to a backup disc like CD, DVD, or Blu-Ray.
• Copy to an external hard drive.
• Get physical printouts.
• Use Network-Attached Storage or NAS.
• Subscribe to a backup service.
• Use cloud storage.

Use a Firewall

Firewalls prevent email header injection attacks by shielding your computer and network from malicious traffic. It can also help bar corrupted software from accessing a device or server via the internet. 

You can customize it to block data from specific locations, applications, ports, etc. while permitting only requisites. This way, you can block suspicious user inputs or data requests. Moreover, firewalls can spot transpiring threats even before a patch is released.

Invest in Professional Vulnerability Testing

Vulnerability testing or vulnerability assessment is a cybersecurity process that identifies and assesses vulnerabilities in an IT infrastructure. It can save your brand reputation by checking if any malicious codes are concealed in legitimate programs. 

Investing in professional vulnerability testing indirectly saves you from financial losses; ransomware attacks and lawsuits due to data breaches are common.

Final Thoughts

Email injection attacks permit hackers to conduct malicious activities using your mail server. They exploit contact form vulnerabilities to send spam messages in your business’s name.

You can prevent these attacks using the principles of least privilege, allow listing, and a firewall. It’s also advised to implement mechanisms that validate, verify, and encode data from user inputs (such as contact forms). Lastly, it’s always wise to backup good quality data to USB, cloud storage, or use paid backup services.