What is HTTP Response Header Injection?

Neda Ali
Reviewed by
Abey Koshy Itty
Published on
08 Nov 2023
9 min read

HTTP response header injection is a security vulnerability that occurs when an attacker can inject malicious content into the response headers returned by a web server.

This vulnerability can lead to various attacks, such as cross-site scripting (XSS), session hijacking, cache poisoning, and phishing.

The vulnerability arises when user input is not properly sanitized before being sent to the web application. This can allow an attacker to inject headers that can be used to perform a variety of malicious attacks.



How is HTTP response header injection exploited?

HTTP response header Injection can be exploited in several ways which includes:

1. Cross-site scripting (XSS)

An attacker can inject malicious JavaScript into the response header, which can then be executed by the victim’s browser. Cookies, session tokens, and other sensitive information can be stolen using this method.

2. Web cache poisoning

An attacker can inject headers that cause the web application’s response to be cached by a proxy server. This can then be used to redirect users to malicious websites or to serve them outdated or malicious content.

3. Information disclosure

An attacker can inject headers that reveal sensitive information about the web application, such as the underlying operating system or the database schema. This information can then be used to launch further attacks against the web application.

HTTP response header injection can have various impacts, depending on the context and the specific headers that are vulnerable.

What are the impacts of HTTP response header injection?

The impact of HTTP response header injection largely depends on the level of control an attacker gains over the headers, the sensitivity of the information exposed in the headers, and the attacker’s intentions

The primary consequences of this security vulnerability include:

1. Cross-Site Scripting (XSS)

If the “Set-Cookie” header or other headers used for storing user data are vulnerable to injection, an attacker could inject malicious scripts into the response.

When the user’s browser processes the response, the injected scripts may execute in the user’s context, leading to XSS attacks.

This can result in unauthorized access to sensitive information, session hijacking, or malware distribution.

2. Session hijacking

By manipulating the “Set-Cookie” header, an attacker may be able to change the session information of a user, effectively hijacking their session.

This could lead to unauthorized access to the user’s account, sensitive data, or perform actions on the user’s behalf.

3. Phishing

HTTP response header injection can be used to modify the “Location” header and redirect users to malicious websites that mimic legitimate ones.

This can trick users into divulging their credentials or personal information, leading to phishing attacks.

4. Cache poisoning

Injecting headers like “Cache-Control” or “Expires” can influence caching behavior.

Maliciously crafted headers may cause the server’s response to be cached incorrectly, leading to cache poisoning and serving unauthorized or harmful content to subsequent users.

5. Information disclosure

Certain headers, such as “Server” or “X-Powered-By,” may disclose sensitive information about the web server or technology stack being used.

This information can be used by attackers to identify potential vulnerabilities or weaknesses.

6. Security bypass

In some cases, HTTP response header injection can be used to bypass security controls or fool web application firewalls (WAFs) by modifying security-related headers.

7. HTTP response splitting

In more severe cases, HTTP response header injection can lead to HTTP response splitting attacks. This occurs when an attacker injects CRLF characters (%0D%0A) into headers, causing the server to produce multiple responses.

This can have serious consequences, such as cache poisoning, session hijacking, or sensitive data exposure.

As with any security vulnerability, it is crucial for website administrators and developers to mitigate these risks by properly validating and sanitizing user input before including it in response headers.

How do you test HTTP response header injection?

To test for HTTP response header injection, follow these steps:

1. Understand the HTTP response headers

Familiarize yourself with the HTTP response headers and their purpose. Some usual response headers include “Content-Type,” “Location,” “Set-Cookie,” and “Cache-Control.”

2. Identify user-controllable input

Identify the input fields or parameters within the web application where user-supplied data is included in the response headers. These fields could include URL query parameters, form inputs, or HTTP request headers.

3. Craft malicious payloads

Create payloads that include malicious content or exploit specific vulnerabilities. For example, include CRLF (Carriage Return Line Feed) characters (%0D%0A) to inject new lines or special characters that could alter the behavior of the headers.

4. Inject payloads

Inject the crafted payloads into the user-controllable input fields and observe the resulting response headers.

5. Analyze responses

Check if the injected content appears in the response headers as intended. Look for unexpected behavior, such as additional headers, modified headers, or injection-related errors.

6. Test for vulnerabilities

Exploit the header injection to test for specific vulnerabilities. For example, if a “Location” header is vulnerable, try redirecting users to a different domain or a phishing page. If a “Set-Cookie” header is vulnerable, attempt to manipulate session data.

7. Monitor browser behavior

Inspect the response headers in the browser developer tools or proxy tools to verify the presence of the injected content.

8. Verify impact

Assess the potential impact of vulnerability. Determine if the injected headers can lead to further attacks, such as XSS, cache poisoning, or session hijacking.

9. Mitigation and reporting

If you discover HTTP response header injection vulnerabilities, report them to the website owner or administrator following responsible disclosure guidelines. Provide details of the vulnerability, its potential impact, and possible mitigation strategies.

It’s important to note that testing for HTTP response header injection should only be performed on systems you have permission to assess. Unauthorized testing is illegal and can have severe consequences.

How do you remediate HTTP response header injection vulnerabilities?

To remediate HTTP response header injection vulnerabilities and enhance the security of your web application, consider implementing the following measures:

1. Input validation and sanitization

Thoroughly validate and sanitize all user-supplied data before including it in response headers. Use input validation to ensure that only expected and safe data is processed.

2. Context-aware output encoding

Encode user data appropriately based on the context in which it will be used in response headers. Use proper encoding methods such as URL encoding, HTML entity encoding, or header-specific encoding.

3. HTTP header restrictions

Set strict restrictions on allowed characters and lengths for various response headers. This can help prevent injection attacks by limiting the options available to attackers.

4. HTTP security headers

Implement security-related response headers, such as Content-Security-Policy (CSP), X-Content-Type-Options, X-Frame-Options, X-XSS-Protection, and others, to add an extra layer of protection against various web vulnerabilities, including injection attacks.

5. Use frameworks and libraries

Utilize secure web frameworks and libraries that handle header processing and encoding securely. These frameworks often have built-in protections against common injection attacks.

6. Security testing

Regularly conduct security testing, including penetration testing using a platform like Beagle Security, to identify and fix any vulnerabilities, including HTTP response header injection issues.

7. Content Security Policy (CSP)

Implement a strict CSP that controls the sources from which certain types of content can be loaded. This can mitigate the risk of XSS attacks that might occur due to header injection.

8. Regular updates and patching

Keep all software components, including web servers, frameworks, and libraries, up to date with the latest security patches and updates.

9. Secure development practices

Train developers on secure coding practices and promote security-aware development throughout the software development lifecycle.

By implementing these remediation measures, web application developers can significantly reduce the risk of HTTP response header injection and enhance the overall security of their web applications.

It is crucial to adopt a proactive approach to security and continuously monitor and update the application’s defenses to protect against evolving threats.

Check out Beagle Security for proactively securing your web apps and APIs with automated AI penetration testing and actionable remediation insights. Play around with our interactive demo environment or book a personalized demo today.

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Written by
Neda Ali
Neda Ali
Product Marketing Specialist
Abey Koshy Itty
Abey Koshy Itty
Marketing Manager
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