CWE-97: Improper Neutralization of Server-Side Includes (SSI) Within a Web Page

What is Improper Neutralization of Server-Side Includes (SSI) Within a Web Page?

• Overview: This vulnerability occurs when a web application generates pages using user input that is not properly sanitized, allowing the input to be interpreted as a server-side include (SSI) directive. This can lead to the execution of unintended commands or scripts on the server.

• Exploitation Methods:

• Attackers can inject SSI directives into user-controllable inputs such as form fields, URLs, or headers.
• Common attack patterns include injecting SSI commands to execute arbitrary commands on the server or to retrieve sensitive data.

• Security Impact:

• Direct consequences include unauthorized command execution, data leakage, and possible server compromise.
• Potential cascading effects could involve further penetration into the server infrastructure or network.
• Business impact may involve data breaches, service disruption, loss of customer trust, and compliance violations.

• Prevention Guidelines:

• Specific code-level fixes include proper input validation and output encoding, ensuring no user input is processed as an SSI directive.
• Security best practices involve disabling SSI if not needed, and ensuring web servers are properly configured to not process SSI directives from untrusted sources.
• Recommended tools and frameworks include using web frameworks that automatically handle input sanitization and leveraging security libraries that offer input validation and output encoding functions.

Corgea can automatically detect and fix Improper Neutralization of Server-Side Includes (SSI) Within a Web Page in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

from flask import Flask, request

app = Flask(__name__)

@app.route('/page')
def render_page():
    # Vulnerable code: Directly using user input to construct a file path for SSI
    # This allows an attacker to inject malicious SSI directives or perform directory traversal
    page = request.args.get('page', 'default.html')
    with open(page, 'r') as file:
        content = file.read()
    return content

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

Explanation

In this vulnerable example, user input is directly used to determine which file to open, which can lead to security issues such as Server-Side Includes (SSI) injection and directory traversal attacks. An attacker could manipulate the page parameter to access unauthorized files or inject malicious content.

How to fix Improper Neutralization of Server-Side Includes (SSI) Within a Web Page?

To mitigate these risks, follow these security practices:

1. Input Validation: Use a whitelist to restrict the input to a predefined set of safe filenames. This ensures that only expected files can be accessed.

2. Use of Templates: Utilize a templating engine that does not support SSI or properly escapes any user input, such as Flask's render_template.

3. Directory Restrictions: Restrict file paths to a specific directory to prevent directory traversal attacks.

By implementing these controls, the application is protected against SSI injection and unauthorized file access.

Fixed Code Example

from flask import Flask, request, render_template

app = Flask(__name__)

# Define a list of allowed page names to prevent SSI injection
ALLOWED_PAGES = {'home.html', 'about.html', 'contact.html'}

@app.route('/page')
def render_page():
    # Fixed code: Validate the user input against a whitelist of allowed pages
    page = request.args.get('page', 'default.html')
    if page not in ALLOWED_PAGES:
        return "Page not found", 404
    return render_template(page)

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

Explanation

In this fixed version, we use a whitelist (ALLOWED_PAGES) to ensure only safe and expected filenames are allowed. The render_template function from Flask is used to safely render templates, which helps mitigate SSI vulnerabilities. This approach also prevents arbitrary file access and directory traversal attacks, ensuring the application remains secure.