CWE-644: Improper Neutralization of HTTP Headers for Scripting Syntax

What is Improper Neutralization of HTTP Headers for Scripting Syntax?

• Overview: Improper Neutralization of HTTP Headers for Scripting Syntax (CWE-644) refers to a vulnerability where web scripting syntax is not correctly neutralized in HTTP headers, allowing attackers to inject scripts that could be executed in a user's browser.

• Exploitation Methods:

• Attackers exploit this by injecting malicious scripts into HTTP headers, which are then processed by web browser components capable of executing scripts.
• Common techniques include Cross-Site Scripting (XSS) attacks and HTTP response splitting, where attackers manipulate HTTP headers to execute malicious scripts.

• Security Impact:

• Direct consequences include unauthorized script execution in the client's browser, leading to data theft, session hijacking, or unauthorized actions.
• Potential cascading effects include further exploitation of compromised sessions, spreading malware, or conducting phishing attacks.
• Business impact can involve reputational damage, legal consequences, and financial losses due to compromised user data and trust.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing all user inputs that are placed in HTTP headers, ensuring that no scripting syntax is allowed.
• Security best practices involve implementing Content Security Policy (CSP), input validation, and output encoding to neutralize potentially harmful scripts.
• Recommended tools and frameworks include using secure libraries and frameworks that automatically handle HTTP header security, such as OWASP's security libraries and employing automated security testing tools to identify vulnerabilities.

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Technical Details

Likelihood of Exploit: High

Affected Languages: Not Language-Specific

Affected Technologies: Web Based

Vulnerable Code Example

from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/set-header')
def set_header():
    # Vulnerable: Directly using user input in HTTP header without sanitization
    user_agent = request.args.get('user_agent')
    # User input is directly inserted into the response header
    response = make_response("Header set!")
    response.headers['User-Agent'] = user_agent
    return response

if __name__ == '__main__':
    app.run()

Explanation:

• Vulnerability: The code accepts a user_agent parameter from the request and directly uses it in an HTTP header. If an attacker injects malicious scripts in the user_agent, it can be executed by components that parse these headers, like some browser extensions or plugins. This can lead to script injection attacks.

How to fix Improper Neutralization of HTTP Headers for Scripting Syntax?

To fix this vulnerability, it's crucial to sanitize and validate all user inputs before including them in HTTP headers. This ensures that any special characters that might be used for injection are properly neutralized. Use a library or package that safely handles HTTP headers, or employ strict validation checks to allow only safe characters.

Fixed Code Example

from flask import Flask, request, make_response
import re  # Import regular expression library for input validation

app = Flask(__name__)

@app.route('/set-header')
def set_header():
    user_agent = request.args.get('user_agent', '')
    
    # Fix: Validate and sanitize user input
    # Only allow alphanumeric characters and some common punctuation
    safe_user_agent = re.sub(r'[^a-zA-Z0-9 .-]', '', user_agent)
    
    response = make_response("Header set!")
    response.headers['User-Agent'] = safe_user_agent
    return response

if __name__ == '__main__':
    app.run()

Explanation:

• Fix Implemented:
  • Input Validation/Sanitization: A regular expression is used to remove any characters that are not alphanumeric or common punctuation (e.g., spaces, dots, and hyphens) from the user_agent. This reduces the risk of script injection by ensuring only safe characters are included.
  • Safe Assignment: The sanitized input is then safely assigned to the User-Agent header, mitigating the risk of malicious script execution.

By implementing input validation and sanitization, we ensure that the data used in the HTTP headers is free from potentially malicious scripts or unintended characters, thus mitigating the risk of exploitation.