CWE-81: Improper Neutralization of Script in an Error Message Web Page
Learn about CWE-81 (Improper Neutralization of Script in an Error Message Web Page), its security impact, exploitation methods, and prevention guidelines.
What is Improper Neutralization of Script in an Error Message Web Page?
• Overview: CWE-81 involves a vulnerability where an application fails to properly neutralize or sanitize user input that is included in error messages on web pages. This can lead to Cross-Site Scripting (XSS) attacks if attackers inject script code into error responses.
• Exploitation Methods:
- Attackers can exploit this vulnerability by crafting input that includes script elements, causing these scripts to be executed in the context of an error page.
- Common attack patterns include triggering specific error conditions (like 403 or 404 errors) with input that contains malicious script code.
• Security Impact:
- Direct consequences include unauthorized script execution in a user's browser, leading to data theft or session hijacking.
- Potential cascading effects involve spreading the attack to other users and compromising additional application components.
- Business impact may include damage to reputation, loss of customer trust, and potential legal consequences due to data breaches.
• Prevention Guidelines:
- Specific code-level fixes include output encoding all error messages and ensuring they do not contain unsanitized user input.
- Security best practices involve adopting a security-focused coding standard, performing input validation, and sanitizing all user inputs.
- Recommended tools and frameworks include using security libraries for input validation and sanitization, implementing Content Security Policy (CSP), and employing automated security testing tools to identify potential vulnerabilities.
Technical Details
Likelihood of Exploit: Not specified
Affected Languages: Not Language-Specific
Affected Technologies: Not specified
Vulnerable Code Example
Python Example
from flask import Flask, request, render_template_string
app = Flask(__name__)
@app.route('/error')
def error_page():
# Vulnerable code: User input is directly embedded in the HTML without sanitization
error_message = request.args.get('message', 'An error occurred')
return render_template_string(f"<h1>Error</h1><p>{error_message}</p>") # Direct embedding without escaping
if __name__ == '__main__':
app.run(debug=True)
Explanation
In the above code, the error_page function takes a user-supplied query parameter message and directly embeds it into an HTML template using render_template_string. This allows an attacker to inject malicious scripts into the error page, leading to Cross-Site Scripting (XSS). For instance, if an attacker sets the message to <script>alert('XSS');</script>, it would execute the script in the user's browser.
How to fix Improper Neutralization of Script in an Error Message Web Page?
To fix this vulnerability, it's essential to neutralize any user input before embedding it into HTML content. Use a templating engine that automatically escapes special characters, or manually escape them. In Flask, using render_template ensures that all inputs are escaped by default. Additionally, consider using a whitelisting approach to only allow expected values or characters.
Fixed Code Example
from flask import Flask, request, render_template
app = Flask(__name__)
@app.route('/error')
def error_page():
# Fixed code: Use render_template to automatically escape user input
error_message = request.args.get('message', 'An error occurred')
return render_template('error.html', error_message=error_message) # Safe rendering with escaping
if __name__ == '__main__':
app.run(debug=True)
Explanation
In this fixed version, the render_template method is used to render an error.html template. This method automatically escapes special characters from the error_message, preventing XSS attacks. Here's an example of what the error.html template might look like:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Error</title>
</head>
<body>
<h1>Error</h1>
<p>{{ error_message }}</p>
</body>
</html>
By using {{ error_message }} in the template, Jinja2 (the templating engine used by Flask) automatically escapes any HTML special characters, ensuring that no scripts can be injected into the web page. This approach leverages Flask's built-in protections against XSS, making your application more secure.