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Scripting and Security in Computer Networks and Web Browsers

The most common CGI function is to fill in forms; the processing script actually takes the data input by the Web surfer and sends it as e-mail to the form administrator. Discuss the different ways such a process can fall victim to an attacker.
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Asked by Owen on June 25, 2025

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When a CGI script processes user input from a web form and sends it via email, several vulnerabilities can arise:
  • Command Injection: If the CGI script constructs shell commands by concatenating user-supplied data without proper sanitization, an attacker can inject arbitrary shell commands. For example, injecting ; rm -rf / into an input field could delete files on the server. This results from inadequate input validation.
  • Email Header Injection (SMTP Injection): Attackers can inject newline characters (e.g., %0A for line feed, %0D for carriage return) into form fields used to construct email headers like 'To', 'From', or 'Subject'. This allows them to add arbitrary headers, such as BCC, or inject new email messages. This can be exploited for spamming, phishing, or information disclosure.
  • Malicious Content in Email Body: If the CGI script does not sanitize user-supplied HTML or script content before embedding it into the email body, and the administrator views this email in a client or webmail interface that renders HTML, an attacker could execute client-side scripts (Cross-Site Scripting, XSS) in the administrator's browser.
  • Denial of Service (DoS): An attacker can submit an excessive number of form requests or requests with extremely large input fields. The CGI script attempts to process and send these numerous emails, which can exhaust server resources like CPU, memory, or disk space for logs. This can overload the mail server and constitutes a Denial of Service (DoS) attack.
Sage - June 25, 2025
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