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Understanding CRLF Injection Vulnerability: Examples and Best Practices


Security for Everyone


In today's interconnected world, web applications have become an integral part of our lives. However, with increased functionality comes the risk of vulnerabilities. One such vulnerability is CRLF (Carriage Return Line Feed) injection. In this blog post, we will explore what CRLF injection is, how it can be exploited, and discuss best practices to mitigate this vulnerability. We will also provide some real-world examples to illustrate its potential impact.

What is CRLF Injection Vulnerability?

CRLF injection occurs when an attacker manages to insert malicious Carriage Return (CR) and Line Feed (LF) characters into a web application's output. These characters are typically used to indicate line breaks in various protocols, such as HTTP, SMTP, and FTP. Exploiting this vulnerability allows attackers to manipulate the application's behavior or inject arbitrary content into the response.

Exploitation Scenarios and Examples:

1. HTTP Response Splitting: One common scenario involves manipulating HTTP headers by injecting CRLF characters. For example, an attacker could craft a malicious URL containing CRLF characters to add extra headers or modify existing ones. Consider the following vulnerable code:

$redirectUrl = $_GET['url'];
header('Location: ' . $redirectUrl);

An attacker could exploit this code by appending CRLF characters and injecting a custom header:

The injected header would be interpreted as two separate headers, leading to a potential HTTP response splitting attack.

2. Email Header Injection: CRLF injection can also impact email systems. Attackers can exploit vulnerable email validation or formatting routines to inject additional headers or content. Here's an example of a vulnerable PHP code snippet:

$to = $_POST['to'];
$subject = $_POST['subject'];
$headers = "From: " . $_POST['from'] . "\r\n";
mail($to, $subject, "Hello", $headers);

An attacker could exploit this code by injecting CRLF characters in the "from" field and adding a custom CC header:

POST /send_email.php HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 64

The injected CC header would add an unintended recipient to the email.

Mitigation Techniques:

To protect against CRLF injection vulnerabilities, consider the following best practices:

  • Input Validation and Encoding: Always validate and sanitize user input to prevent the injection of CRLF characters. Apply appropriate encoding to ensure the characters are treated as literal data.
  • Context-Specific Output Encoding: When displaying user-supplied data, use context-specific encoding (e.g., HTML encoding, URL encoding, etc.) to prevent unintended interpretation of CRLF characters.
  • Secure Header Handling: Avoid concatenating user-controlled data directly into header values. Instead, use safe header-setting functions provided by your programming language or framework.
  • Stay Updated: Keep your software dependencies, libraries, and frameworks up to date, as vendors often release security patches to address vulnerabilities.

CRLF injection vulnerabilities pose a serious threat to web applications and can lead to various attacks, including HTTP response splitting, email header injection, and more. By understanding the risks and following best practices, developers can safeguard their applications against such attacks. Implementing proper input validation, output encoding, and secure header handling can go a long way in mitigating this vulnerability and enhancing overall application security.

To avoid CRLF injections, you should check your systems regularly. To do it, you can scan your applications using our most common CRLF Injection Vulnerability Scanner for free:

Stay vigilant, stay secure!

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