Email injection is a cybersecurity vulnerability that targets web applications and services that send email messages based on user input. This flaw allows malicious actors to manipulate email headers and content by injecting unauthorized data into the email's structure. The vulnerability is particularly prevalent in contact forms and other web interfaces where user input is directly incorporated into email messages.

By exploiting email injection, attackers can send emails with arbitrary content to any recipient, often bypassing the intended functionality of the application. This can lead to various malicious activities, including spam and phishing attacks. The root cause of email injection lies in the improper validation and sanitization of user input before it is processed by the email-sending function.

How does Email Injection Work?

Email injection works by exploiting the way email servers process user input. Attackers insert newline characters into input fields, which are then interpreted as additional email headers or message bodies. This manipulation allows them to add or modify headers, such as "BCC" or "Reply-To," effectively altering the email's structure and content.

When a web application fails to properly sanitize user input, it becomes vulnerable to these injections. For instance, in a PHP contact form, an attacker might inject headers by sending a POST request with malicious payloads. The email library then converts these headers into SMTP commands, causing the email to be sent to unintended recipients or with altered content.

The core of the attack lies in the improper handling of user input. By appending carriage returns and additional headers, attackers can trick the email server into processing their malicious data, leading to unauthorized email transmissions. This exploitation leverages the MIME format, which uses carriage returns to separate different parts of the email, making it a potent vector for email injection attacks.

What are Examples of Email Injection?

Examples of email injection often involve manipulating contact forms to send unauthorized emails. For instance, an attacker might exploit a vulnerable PHP contact form by injecting additional headers into the email data. By sending a POST request with malicious payloads, they can add headers like "BCC" to send the email to multiple recipients or "Reply-To" to redirect responses to a different address.

Another common scenario involves manipulating the MIME format to append extra information to the email message. This can include adding new recipients or altering the message body entirely. Such exploits can turn a simple web form into a tool for sending thousands of spam or phishing emails, leveraging the application's email-sending functionality to reach a wide audience.

What are the Potential Risks of Email Injection?

Understanding the potential risks of email injection is crucial for any organization. Here are some of the key risks associated with this vulnerability:

• Compromise of Email Integrity: Attackers can alter email headers and content, leading to unauthorized modifications that compromise the integrity of the email.

• Increased Risk of Spam and Malicious Emails: Vulnerable systems can be exploited to send large volumes of spam or malicious emails, damaging the organization's reputation.

• Potential for Identity Theft: By spoofing email addresses, attackers can make phishing emails appear to come from trusted sources, increasing the risk of identity theft.

• Financial Losses Due to Fraud: Exploited systems may incur significant costs related to mitigating attacks, handling blacklisting, and addressing customer complaints.

• Legal Consequences and Compliance Issues: Unauthorized use of email systems for spam can lead to legal repercussions and compliance challenges under anti-spam laws.

How can you Protect Against Email Injection?

• Validate and Sanitize User Input: Always treat user input as untrusted. Use input filtration and output encoding to remove any potentially harmful characters.

• Use Secure Email Libraries: Opt for email libraries that are resistant to injection attacks. These libraries often require explicit recipient lists, making it harder for attackers to manipulate headers.

• Implement Whitelisting: Use a whitelist approach to filter input, allowing only specific characters and formats. This reduces the risk of malicious data being processed.

• Remove Newline Characters: Automatically strip newline characters from user input before passing it to email functions. This prevents attackers from injecting additional headers.

• Conduct Regular Security Audits: Regularly review and test your code for vulnerabilities. Security audits help identify and fix potential weaknesses before they can be exploited.