Mail Injection in PHP: Attacks and Prevention

Email remains the cornerstone of digital communication, and it is unlikely to be replaced anytime soon. Despite the emergence of instant messaging, social media, and mobile chat apps, these technologies have complemented rather than replaced the traditional email. However, email is not without its vulnerabilities, particularly to mail injection attacks.

PHP Mail Injection: Attacks and Prevention
PHP Mail Injection. Image generated by ChatGPT using DALL-E

Mail injection is a significant risk that can lead to the unauthorized sending of bulk messages and compromise security. In this blog post, I will discuss how developers can write more secure and efficient code to prevent mail injection, ensuring emails remain a safe and reliable communication tool.

When email is a part of your application, the responsibilities that comes with it is crucial. Effective email management includes validating and cleaning inputs, using allowlists for operations, and restricting actions. Keeping logs and monitoring activity can help spot problems, and rate limiting how many emails you send can prevent your domain from being marked as spam.

Mail Header Injection

When using PHP’s native mail() function, understanding its method signature is important to preventing mail injection attacks:

The parameters $to, $subject, and $message are straightforward. However, the parameters $additional_headers and $additional_params are critical in the context of email header injection. It is important to note that without proper filtering or validation, the $to parameter can be exploited for spam or phishing.

Additional headers can be set in $additional_headers, which are – as the name suggests – appended to the email header. Typical examples of email headers are From:, Reply-To: or CC:. If an attacker wants to set a CC: header, for example, the value must simply be inserted after the colon and separated with a CRLF character (usually \r\n, but some email servers also accept \n). If the user’s input is not checked or not checked enough, it can be misused to send emails. For example, several spam emails can be sent in this way (by injecting several email addresses into the To:, CC: and/or BCC: header) or the Reply-To: header can be set to obtain sensitive information.

Shell Injection mit PHP mail()

Another critical point of concern with PHP’s mail() function is the misuse of its last parameter, $additional_parameters. This parameter plays a pivotal role when PHP needs to interact with external mail services or local software, such as a Mail Transfer Agent (MTA).

When PHP communicates with an MTA, it can execute shell commands, a functionality extended by $additional_parameters. This allows for the passing of additional parameters to the MTA. While escaping these parameters can reduce risks, it’s crucial to remember that savvy attackers can still manipulate them to add program flags and execute unauthorized actions.

Vulnerable code

The above code is vulnerable to attacks as the last parameter is read from $_GET and is not filtered or validated before being passed to mail(). An attacker can set additional flags and thus manipulate the behavior of the MTA. For example, configurations in sendmail can be changed with the parameter -O or the path of a log file can be specified with -X.

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    Malicious code

    If the data field is set as for the vulnerable code shown above, the following happens: PHP places a shell in /var/www/html (which is the web root in the default configuration). file_with_sensitive_information.php contains sensitive log data, which can be changed or falsified with PHP code. In this way, an attacker with access to file_with_sensitive_information.php can execute any malicious code on the web server.

    Using PHP mail() securely

    First things first: there is no magic formula that could prevent malicious misuse of mail(). Rather, a critical analysis of all parameters should be carried out. The following rules of thumb are generally considered a good approach:

    • Parameter $to: do not accept direct input from the user,
    • Parameter $subject: relatively safe, usually poses no risk,
    • Parameter $message: (external) resources such as CSS or JavaScript can pose a potential security risk; the content should be sanitized
    • Parameter $additional_headers: remove all \r, \n and \r\n characters, allow predefined headers only
    • Parameter $additional_parameters: do not accept any input from the user, allow predefined headers only

    If one of the above parameters is to process or use user input, I recommend using a restrictive filter (allowlisting). For instance, for additional CC or BCC headers, this technique involves validating email addresses against a preset list of approved recipients or domains before they are added to the outgoing emails. Alongside basic validations, such as checking if a string conforms to the structure of a valid email address, allowlists offer an additional layer of security. They can be applied both at the individual receiver level and at the domain level, enhancing the overall integrity of email communications.

    Under no circumstances should you simply use escaping when using $additional_parameters, as this is implemented differently on different platforms and valid parameters can pose a risk. This variability across platforms means that techniques effective on one system may not be secure on another, potentially opening up vulnerabilities. Additionally, even legitimately intended parameters, when manipulated, can expose systems to unexpected security risks, underscoring the need for robust safeguards beyond simple escaping.

    I also recommend saving mails in a queue in order to send them on a time-controlled basis. This is usually unavoidable from a performance point of view, as sending via SMTP or HTTP takes a certain amount of time, which can have a negative impact on the user experience. On the other hand, you can use this to track which emails have been sent and stop them if necessary. Further, I recommend a so-called rate limiter, which only sends a certain number of emails in general or specifically to a certain address, with certain content or a combination of all of these.

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    In conclusion, it is essential to exercise caution when utilizing PHP’s mail() function to secure against vulnerabilities like mail injection. If you’re interested in learning more about how to prevent mail injections, subscribe to my newsletter. As a new subscriber, you will receive a PDF that summarizes the best practices for securing your email communications.

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