CWE-82: Improper Neutralization of Script in Attributes of IMG Tags in a Web Page

What is Improper Neutralization of Script in Attributes of IMG Tags in a Web Page?

• Overview: CWE-82 refers to a vulnerability where a web application fails to properly neutralize or sanitize scripting elements within the attributes of HTML IMG tags, such as the src attribute. This can lead to execution of malicious scripts in the user's browser, often resulting in cross-site scripting (XSS) attacks.

• Exploitation Methods:

• Attackers can inject malicious scripts into the values of IMG tag attributes, like the src attribute, which are then executed when the page loads in a user's browser.
• Common attack patterns include crafting IMG tags with src attributes that contain JavaScript code or other scripts designed to execute harmful actions when rendered.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized execution of scripts in the user's browser, leading to data theft, session hijacking, and unauthorized actions on behalf of the user.
• Potential cascading effects involve further spreading of malicious code, compromising other users or systems connected to the affected application.
• Business impact may include loss of user trust, damage to brand reputation, legal liabilities, and financial losses due to data breaches.

• Prevention Guidelines:

• Specific code-level fixes include ensuring that any user input or dynamic data included in IMG tag attributes is properly sanitized and encoded to neutralize potential script elements.
• Security best practices involve implementing a robust input validation and output encoding strategy, especially for content that will be rendered in a user's browser.
• Recommended tools and frameworks include using libraries and frameworks that provide built-in protections against XSS, such as Content Security Policy (CSP), and utilizing security-focused development practices and tools for code scanning and vulnerability detection.

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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 is a simplified example of a web application that dynamically inserts an image element.
// The src attribute is constructed using untrusted user input, leading to a security vulnerability.

function insertImage(userInput) {
    // User input directly placed into the src attribute of an IMG tag
    const imgTag = `<img src="\${userInput}" alt="User Image">`; // Vulnerability: Direct insertion of user input
    document.getElementById('image-container').innerHTML = imgTag; // Vulnerability: Potential XSS via innerHTML
}

// Example of a malicious input that an attacker might use
insertImage('javascript:alert("XSS")'); // Malicious input exploiting the vulnerability

How to fix Improper Neutralization of Script in Attributes of IMG Tags in a Web Page?

To fix the vulnerability, the code must ensure that any user input used in attributes such as the src attribute of an IMG tag is properly sanitized and validated. This involves:

1. Sanitization: Remove or neutralize any potentially harmful content from user input to prevent script execution.
2. Validation: Ensure that the input strictly conforms to the expected format, such as URLs for image sources.
3. Using Safe Methods: Avoid directly injecting user input into HTML. Instead, use safe DOM methods to create and set attributes.

Fixed Code Example

// This fixed version of the code uses proper input sanitization and validation.
// It also utilizes DOM manipulation methods to safely insert the image.

function insertSafeImage(userInput) {
    // Simple URL validation function to ensure input is a valid URL
    function isValidURL(url) {
        try {
            const parsedUrl = new URL(url);
            return parsedUrl.protocol === 'http:' || parsedUrl.protocol === 'https:'; // Validate only HTTP/HTTPS URLs
        } catch (e) {
            return false;
        }
    }

    // Validate the user input
    if (isValidURL(userInput)) { // Ensure the input is a valid and safe URL
        // Create a new Image element using safe DOM methods
        const imgElement = document.createElement('img');
        imgElement.src = userInput; // Safe assignment of validated URL
        imgElement.alt = 'User Image';

        // Safely append the image to the container
        const imageContainer = document.getElementById('image-container');
        imageContainer.innerHTML = ''; // Clear any previous content to prevent residual XSS
        imageContainer.appendChild(imgElement); // Safe insertion of the image element
    } else {
        console.error('Invalid URL provided'); // Log an error if the URL is invalid
    }
}

// Example of valid input
insertSafeImage('https://example.com/image.png'); // Safe input example

In the fixed code:

• Line {8}: Validates the input to ensure it's a proper URL using the isValidURL function, and ensures the URL uses a safe protocol (HTTP/HTTPS).
• Line {10}: Uses the createElement method to safely create an IMG element and assigns the src attribute only after validation.
• Line {15}: Appends the created image element to the DOM using appendChild, ensuring that only safe, validated content is inserted. The container is cleared before appending to prevent any leftover malicious content.