CRLF stands for Carriage Return (CR) and Line Feed (LF), which are control characters used to represent a new line or line break in various text-based communication protocols and file formats.
Carriage Return and Line Feed Injection (CRLF) is a type of injection attack that can cause Cross-site Scripting (XSS) attacks. This can also cause web cache poisoning among others.
This server is vulnerable to Carriage Return and Line Feed Injection. This is because the server fails to properly escape/remove CRLF characters in the user’s inputs.
This vulnerability leads to various attacks including web cache poisoning, cross user defacement, hijacking pages with sensitive user information and cross-site scripting.
Example
The below URL has been split response header.
http://www.example.beaglesecurity.com/randompage.php?page=%0d%0aContent-Type: text/html%0d%0aHTTP/1.1 200 OK%0d%0aContent-Type: text/html%0d%0a%0d%0a%3Cscript%3Ealert(1) %3C/script%3E
The user will see the following
<script>alert(10)</script>
A CRLF injection vulnerability occurs when an attacker is able to insert malicious CR and LF characters into data sent to an application.
This type of vulnerability is particularly concerning in web applications and network protocols because it can lead to various security issues, including:
1. HTTP response splitting
In web applications, an attacker might inject CRLF characters into HTTP response headers, allowing them to manipulate the response sent to a user’s browser.
This can be used for various attacks, including cache poisoning, session fixation, and cross-site scripting (XSS) attacks.
2. Email header injection
In email systems, CRLF injection can be used to inject additional headers into email messages. This can lead to email spoofing and phishing attacks.
3. HTTP parameter pollution
CRLF injection can be used to tamper with HTTP parameters and potentially manipulate server behavior or inject malicious content.
4. Server-Side Request Forgery (SSRF)
In some cases, CRLF injection can be used to trick a server into making unintended requests to internal resources, potentially exposing sensitive information or services.
What are the impacts of CRLF injection?
CRLF (Carriage Return Line Feed) injection vulnerabilities can have a range of significant impacts on web applications, network protocols, and systems.
The impacts of CRLF injection vulnerabilities can vary depending on the context and the attacker’s goals. They can range from relatively minor issues, such as cache poisoning, to severe security breaches with significant consequences, including data theft, unauthorized access, and reputational damage.
1. HTTP response splitting
Attackers can inject CRLF characters into HTTP response headers, potentially leading to response splitting attacks. This can result in:
Cache poisoning: Attacker-controlled content can be cached by intermediary proxy servers and served to other users.
Session fixation: Manipulating headers to set or fix session cookies, potentially hijacking user sessions.
Cross-Site Scripting (XSS): Injecting malicious JavaScript code into responses, leading to client-side attacks.
2. Cross-Site Scripting (XSS)
CRLF injection can be used to inject malicious scripts into web pages, leading to XSS attacks. This allows attackers to steal user data, manipulate web page content, or perform other malicious actions on behalf of the user.
3. Email header injection
In email systems, CRLF injection can be used to inject additional headers or control characters into email messages. This can result in:
Email spoofing: Manipulating headers to make an email appear as if it came from a trusted source.
Phishing: Crafting emails that trick recipients into divulging sensitive information or clicking on malicious links.
4. HTTP parameter pollution (HPP)
CRLF injection can be used to tamper with HTTP parameters and potentially manipulate server behavior or inject malicious content.
This can lead to various attacks, including unauthorized access to resources or the execution of unintended actions on the server.
5. Server-Side Request Forgery (SSRF)
In some cases, CRLF injection can be used to craft malicious URLs and trick a server into making unintended requests to internal resources or external services.
This can expose sensitive information or lead to further attacks.
6. Information disclosure
CRLF injection can lead to information disclosure by manipulating server responses or logs.
Attackers may be able to see sensitive data, such as directory structures, filenames, or database errors, which can be used for reconnaissance in further attacks.
7. Denial of Service (DoS)
In certain situations, CRLF injection can be used to craft malicious input that disrupts the normal operation of a web application or server.
For example, by injecting excessive CRLF characters, an attacker may cause a web server to become unresponsive or crash, resulting in a DoS condition.
How do you prevent CRLF vulnerability?
To prevent and mitigate CRLF (Carriage Return Line Feed) injection vulnerabilities, you need to implement security measures and best practices in your web applications and systems.
Here are steps you can take to prevent or mitigate CRLF vulnerabilities:
1. Input validation
Implement strict input validation for all user inputs and data sources.
Ensure that user-supplied data does not contain CRLF characters or other control characters that can manipulate protocol or system behavior.
2. Output encoding
Properly encode output data to prevent CRLF injection attacks.
When rendering user-controlled data in HTTP response headers or within web pages, ensure that it is properly encoded to prevent the interpretation of CRLF characters as control characters.
3. HTTP header sanitization
Sanitize and validate all user inputs that are used to construct HTTP headers, especially headers like “User-Agent,” “Referer,” “Host,” and “Cookie.”
Reject any input that contains suspicious or non-standard characters.
4. Use secure libraries and frameworks
Utilize well-established and secure libraries and frameworks for handling user input and constructing HTTP responses.
These libraries often include built-in security mechanisms to prevent CRLF injection vulnerabilities.
5. HTTP security headers
Implement security headers like “Content Security Policy (CSP)” and “X-Content-Type-Options” to help mitigate CRLF injection attacks in web applications.
6. Regular expression filters
Use regular expressions to filter and validate user input. Ensure that input data adheres to expected patterns and does not contain CRLF characters.
7. Web Application Firewall (WAF)
Deploy a Web Application Firewall (WAF) to detect and block malicious input that may contain CRLF injection attempts.
WAFs can provide an additional layer of protection against various web application vulnerabilities.
8. Security testing
Regularly test your web application for CRLF injection vulnerabilities. This can include both manual testing and automated scanning tools that look for potential injection points.
9. Security awareness and training
Educate your development team about CRLF injection risks and best practices for secure coding. Ensure that developers are aware of the potential consequences of these vulnerabilities.
10. Patch and update dependencies
Keep your web application dependencies, including libraries, frameworks, and server software, up to date.
Vulnerabilities in third-party components can sometimes be exploited to carry out CRLF injection attacks.
By implementing these preventive measures and following secure coding practices, you can significantly reduce the risk of CRLF injection vulnerabilities in your web applications and systems.
Regular security testing and ongoing monitoring are essential to ensure that your defenses remain effective over time.
