CWE-86: Improper Neutralization of Invalid Characters in Identifiers in Web Pages

What is Improper Neutralization of Invalid Characters in Identifiers in Web Pages?

• Overview: This vulnerability occurs when a web application fails to properly neutralize or sanitize invalid characters in identifiers such as tag names or URI schemes. This can allow attackers to bypass security checks and inject malicious content, such as executing unintended scripts in the user's browser.

• Exploitation Methods:

• Attackers can exploit this by inserting invalid sequences that are not correctly neutralized, leading to the execution of unauthorized scripts.
• Common attack patterns include encoding or inserting null characters within URI schemes or tag names to evade filtering mechanisms.

• Security Impact:

• Direct consequences of successful exploitation include cross-site scripting (XSS) attacks, allowing attackers to execute malicious scripts in the context of the user's session.
• Potential cascading effects include unauthorized access to user data, session hijacking, or further propagation of malicious payloads.
• Business impact could include loss of user trust, data breaches, and potential legal liabilities due to compromised data security.

• Prevention Guidelines:

• Specific code-level fixes involve ensuring all input and identifiers are properly validated and sanitized, especially when dealing with URI schemes or HTML tags.
• Security best practices include implementing a strict content security policy (CSP) and using a whitelist approach for allowed characters.
• Recommended tools and frameworks include security libraries that offer robust input validation and escaping functions, such as OWASP's AntiSamy and HTML Sanitizer.

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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

// This code constructs HTML elements using user input without proper validation,
// allowing invalid characters in identifiers, potentially leading to XSS attacks.

function createElementFromUserInput(userInput) {
    // User input is directly used to create an element ID
    const elementId = `user-\${userInput}`;
    // Potential XSS or HTML Injection if userInput contains invalid or malicious characters
    document.body.innerHTML += `<div id="\${elementId}">User Content</div>`;
}

Explanation of the Vulnerability

In this example, user input is directly used to create an HTML element ID without any validation or sanitization. If the userInput contains special characters or malicious scripts, it can lead to Cross-Site Scripting (XSS) attacks or HTML injection, compromising the security of the web application.

How to fix Improper Neutralization of Invalid Characters in Identifiers in Web Pages?

To address this vulnerability, we need to ensure that user inputs are both validated and sanitized before being used in identifiers such as HTML element IDs. The following steps should be taken:

1. Input Validation: Ensure that the user input consists only of safe, expected characters. For HTML IDs, this typically includes alphanumeric characters, dashes, and underscores.
2. Input Sanitization: Remove or escape any invalid or potentially harmful characters.
3. Using Utility Libraries: Utilize well-tested libraries for input validation and sanitization to handle edge cases and ensure robustness.
4. Avoid Direct DOM Manipulation: Consider using frameworks or libraries that abstract away manual DOM manipulation, reducing the risk of injection vulnerabilities.

Fixed Code Example

// Fixed code with input validation and sanitization to prevent invalid characters in identifiers

function sanitizeInput(input) {
    // Allow only alphanumeric characters, dashes, and underscores
    return input.replace(/[^a-zA-Z0-9-_]/g, '');
}

function createElementFromUserInput(userInput) {
    // Sanitize user input to ensure it contains only valid characters for an ID
    const safeInput = sanitizeInput(userInput);
    const elementId = `user-\${safeInput}`;
    // Safe to use now as input is validated and sanitized
    document.body.innerHTML += `<div id="\${elementId}">User Content</div>`;
}

Explanation of the Fix

In the fixed code, we've introduced a sanitizeInput function that removes any characters that are not alphanumeric, dashes, or underscores. This ensures that the IDs generated are safe and do not allow for XSS or other injection attacks. The user input is sanitized before being used to construct the HTML element ID, thereby mitigating the risk of malicious input being executed in the browser.