What Is a Header Injection Attack?

Definition: Header Injection Attack

A Header Injection Attack is a type of web security vulnerability where an attacker manipulates HTTP headers by injecting malicious input. This attack exploits weaknesses in web applications that fail to properly validate user-supplied input before including it in HTTP response headers.

Header injection attacks can lead to various security risks, including session hijacking, cross-site scripting (XSS), cache poisoning, and HTTP response splitting. Attackers use these techniques to manipulate server responses, steal sensitive information, or redirect users to malicious sites.

Understanding Header Injection Attacks

HTTP headers play a crucial role in web communication. They provide metadata about the request and response, such as content type, cookies, caching directives, and security policies. When an application fails to sanitize input before including it in HTTP headers, attackers can inject arbitrary data, altering the behavior of the web response.

How Header Injection Works

1. An attacker submits crafted input that includes malicious characters (e.g., newline characters like %0A, %0D, or \r\n).
2. The vulnerable application fails to sanitize the input and includes it directly in HTTP response headers.
3. Injected headers alter the response, allowing the attacker to manipulate browser behavior, inject scripts, or redirect users to another site.

Common Attack Scenarios

• HTTP Response Splitting: Injecting additional headers to manipulate web responses.
• Session Hijacking: Injecting custom cookies to steal session data.
• Cross-Site Scripting (XSS): Injecting malicious scripts via headers.
• Cache Poisoning: Manipulating cache control headers to serve malicious content.

Techniques Used in Header Injection Attacks

1. HTTP Response Splitting

• An attacker injects newline characters (\r\n) to split the HTTP response into multiple headers.
• The browser interprets the injected headers as part of the server response, leading to security issues.

Example of HTTP Response Splitting Attack

GET /index.php?redirect=http://attacker.com%0D%0ASet-Cookie:sessionID=malicious HTTP/1.1<br>Host: victim.com<br>

If the application includes redirect as part of the response headers without proper validation, the injected Set-Cookie header modifies the session, allowing the attacker to hijack user accounts.

2. Cookie Injection

• Attackers inject fake Set-Cookie headers to modify user sessions or bypass authentication.

Example of Cookie Injection Attack

GET /profile HTTP/1.1<br>Host: victim.com<br>User-Agent: Mozilla/5.0%0D%0ASet-Cookie:authToken=malicious HTTP/1.1<br>

• If the server does not sanitize headers, the attacker’s custom authentication token is stored, granting them unauthorized access.

3. Cross-Site Scripting (XSS) via Header Injection

• If the application reflects header input into the response body, attackers can execute scripts in the victim’s browser.

Example of XSS via Header Injection

GET / HTTP/1.1<br>Host: victim.com<br>Referer: http://legit.com%0D%0AContent-Length: 0%0D%0AContent-Type: text/html%0D%0A%0D%0A<script>alert('Hacked!')</script><br>

If the application echoes headers into the response without encoding them properly, it can execute the script in the user’s browser, leading to session hijacking or phishing attacks.

4. Cache Poisoning

• Attackers manipulate cache-related headers to serve malicious responses to other users.
• If a proxy or CDN caches a tampered response, subsequent users receive the attacker-controlled page.

Example of Cache Poisoning Attack

GET /page.php HTTP/1.1<br>Host: victim.com<br>X-Forwarded-For: 192.168.1.1%0D%0ACache-Control: public, max-age=3600<br>

If the web server improperly processes input, the attacker modifies caching behavior, causing users to receive malicious content from the cache instead of the legitimate page.

Impacts of Header Injection Attacks

A successful header injection attack can lead to serious security breaches, including:

• Session Hijacking – Attackers steal user sessions by injecting malicious cookies.
• Data Leakage – Sensitive information is exposed through manipulated headers.
• Malicious Redirects – Victims are redirected to phishing or malware-infected sites.
• Security Policy Bypass – Security headers like Content-Security-Policy (CSP) and X-Frame-Options are altered.
• Cache Poisoning – Attackers modify cache rules to deliver harmful content.

How to Prevent Header Injection Attacks

1. Validate and Sanitize User Input

• Ensure all input that influences headers is validated against a strict allowlist.
• Remove or encode newline characters (\r, \n, %0D, %0A) to prevent response splitting.

Example in PHP:

header("Location: " . filter_var($_GET['redirect'], FILTER_SANITIZE_URL));<br>

2. Use HTTP Headers Securely

• Set security headers such as: X-Frame-Options: DENY X-XSS-Protection: 1; mode=block Content-Security-Policy: default-src 'self'
• Avoid reflecting user input in response headers.

3. Implement Input Encoding

• Convert user input into a safe format to neutralize harmful characters.

Example in Python:

import urllib.parse<br>safe_input = urllib.parse.quote(user_input, safe='')<br>

4. Use a Web Application Firewall (WAF)

• Deploy WAF rules to detect and block header injection attempts.
• Monitor logs for suspicious header manipulation.

5. Restrict Response Headers

• Prevent untrusted sources from modifying HTTP headers.
• Use content security policies (CSPs) to mitigate cross-site scripting risks.

Tools for Detecting Header Injection Attacks

1. Burp Suite

• Used for manually testing header vulnerabilities and response splitting attacks.

2. OWASP ZAP

• An automated security scanner that detects header manipulation risks.

3. ModSecurity (Web Application Firewall)

• Blocks malicious HTTP requests at the server level.

4. Nikto Scanner

• Identifies misconfigurations and insecure HTTP headers.

Real-World Examples of Header Injection Exploits

1. Twitter Redirect Vulnerability (2018)

A header injection vulnerability allowed attackers to manipulate the Location header, redirecting users to malicious sites.

2. Web Cache Poisoning Attack on GitHub (2020)

An attacker used header injection to manipulate cache rules, causing GitHub’s CDN to serve attacker-controlled content.

3. API Gateway Exploits (Multiple Cases)

Several cloud API gateways were found vulnerable to header injection, allowing attackers to bypass authentication and inject malicious payloads.

Conclusion

A Header Injection Attack is a serious web security risk that can manipulate server responses, hijack user sessions, and bypass security mechanisms. By exploiting improperly validated input, attackers can introduce malicious headers, leading to HTTP response splitting, XSS, cache poisoning, and other critical vulnerabilities.

Preventing header injection requires proper input validation, secure HTTP headers, and proactive security monitoring. Organizations should implement strict sanitization measures and use security tools like Web Application Firewalls (WAFs) to mitigate these risks effectively.

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