CWE-153: Improper Neutralization of Substitution Characters

What is Improper Neutralization of Substitution Characters?

• Overview: This vulnerability occurs when a software application receives input and fails to properly neutralize special elements that could be interpreted as substitution characters before passing them to another component. These substitution characters can alter the intended behavior of the application, leading to unintended actions or security breaches.

• Exploitation Methods:

• Attackers can inject substitution characters into input fields, which might be processed by the application in unintended ways.
• Common attack patterns include manipulating input to perform unauthorized operations, such as altering command execution or modifying data processing logic.

• Security Impact:

• Direct consequences include unauthorized access, data leakage, or execution of unintended commands.
• Potential cascading effects might involve privilege escalation or denial of service attacks.
• Business impact can involve data breaches, reputational damage, and financial losses due to exploitation of the vulnerability.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing all input data to ensure substitution characters are neutralized before processing.
• Security best practices involve employing input validation libraries and frameworks that are designed to handle encoding and escape sequences.
• Recommended tools and frameworks include using security-focused libraries and input validation tools that are regularly updated to handle new types of substitution characters and attack vectors.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

// This JavaScript function processes email templates and substitutes placeholders with user input.
// It does not properly neutralize substitution characters, which could lead to script injection vulnerabilities.

function processEmailTemplate(template, userInput) {
    // Directly substitutes user input into the template without sanitization
    return template.replace("{username}", userInput);
}

// Example of injecting a script through user input
const userEmail = processEmailTemplate("Hello, {username}!", "<script>alert('Hello!')</script>");
console.log(userEmail);

How to fix Improper Neutralization of Substitution Characters?

The vulnerability arises because the user input is directly inserted into the email template without any form of sanitization or escaping. This can lead to injection attacks, such as XSS (Cross-Site Scripting), if the input contains malicious scripts or special characters.

To fix this vulnerability, it's essential to properly escape or sanitize user inputs before using them in contexts where they might be interpreted as code or commands. In JavaScript, you can use libraries like DOMPurify to sanitize inputs or escape special characters manually before substitution.

Fixed Code Example

// Improved JavaScript function that neutralizes special characters in user input to prevent injection attacks.

function sanitizeInput(input) {
    // Simple manual sanitization by escaping special HTML characters
    return input.replace(/</g, "&lt;").replace(/>/g, "&gt;")
                .replace(/"/g, "&quot;").replace(/'/g, "&#x27;")
                .replace(/\//g, "&#x2F;");
}

function processEmailTemplate(template, userInput) {
    // Sanitize user input before substitution
    const sanitizedInput = sanitizeInput(userInput);
    return template.replace("{username}", sanitizedInput);
}

// The sanitized input prevents script execution
const userEmail = processEmailTemplate("Hello, {username}!", "<script>alert('Hello!')</script>");
console.log(userEmail);

In the fixed version, the sanitizeInput function is introduced to escape HTML special characters in the user input, effectively neutralizing any potential script injection attempts. This practice ensures that user-generated content is safely inserted into HTML, mitigating the risk of XSS and similar attacks.