CWE-151: Improper Neutralization of Comment Delimiters

What is Improper Neutralization of Comment Delimiters?

• Overview: This vulnerability occurs when an application does not properly handle or neutralize comment delimiter characters (such as /.../ or ), which can lead to unexpected behavior or security issues when input is passed to another component.

• Exploitation Methods:

• Attackers can inject comment delimiters into input fields, causing the application to misinterpret or alter code execution.
• Common attack patterns include injecting comment delimiters into code that is dynamically generated or interpreted, potentially leading to code injection or logic bypass.

• Security Impact:

• Direct consequences include unintended code execution or alteration of application logic.
• Potential cascading effects include data leaks, unauthorized access, or denial of service if critical code sections are commented out or altered.
• Business impact can involve loss of data integrity, breach of customer trust, and potential legal ramifications if sensitive data is exposed.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing inputs to remove or neutralize comment delimiters before processing.
• Security best practices involve using parameterized queries and avoiding dynamic code execution based on user input.
• Recommended tools and frameworks include static code analysis tools to detect improper handling of comment delimiters and input validation libraries to ensure safe input handling.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

### JavaScript Example

```javascript templateRenderer.js {3-5}
// This function takes user input and inserts it into HTML templates.
// It fails to properly escape comment delimiters, allowing an attacker to inject comments.
function renderTemplate(userInput) {
    const template = `<div>User Input: \${userInput}</div>`; // User input is directly inserted into HTML
    document.body.innerHTML += template; // This can lead to HTML injection if userInput includes: `<!--` or `-->`
    // Example of malicious input: `<!--<script>alert('XSS')</script>-->`
}

How to fix Improper Neutralization of Comment Delimiters?

Improper neutralization of comment delimiters can lead to various injection attacks, such as HTML injection or script injection. To fix this issue, it's crucial to sanitize and escape user inputs properly. In the context of HTML, this means converting characters like <, >, and & into their corresponding HTML entities (<, >, &). This prevents user input from being interpreted as HTML or JavaScript, thereby neutralizing any attempted injection of comment delimiters or other malicious code.

Steps to Fix:

1. Escape HTML Characters: Convert <, >, &, and other potentially dangerous characters into their HTML entity equivalents.
2. Use a Trusted Library: Implement a well-tested library to handle escaping, reducing the likelihood of mistakes.
3. Consistent Encoding: Ensure that the encoding of the output matches the encoding of the input.

Fixed Code Example

// This function now safely escapes user input before inserting it into HTML templates.
function escapeHtml(unsafe) {
    return String(unsafe)
        .replace(/&/g, '&')  // Converts & to &
        .replace(/</g, '&lt;')   // Converts < to &lt;
        .replace(/>/g, '&gt;')   // Converts > to &gt;
        .replace(/"/g, '&quot;') // Converts " to &quot;
        .replace(/'/g, '&#039;'); // Converts ' to &#039;
}

function renderTemplate(userInput) {
    const sanitizedInput = escapeHtml(userInput); // User input is sanitized
    const template = `<div>User Input: \${sanitizedInput}</div>`; // Safely insert sanitized input
    document.body.innerHTML += template; // User input is safely rendered without the risk of HTML injection
}

By using the escapeHtml function, we ensure that any special characters in the user input are properly escaped, neutralizing the risk of HTML comment delimiter injection and other forms of code injection. This approach enhances security by ensuring that user input is treated as plain text, not executable code.