CWE-84: Improper Neutralization of Encoded URI Schemes in a Web Page

What is Improper Neutralization of Encoded URI Schemes in a Web Page?

• Overview: Improper Neutralization of Encoded URI Schemes in a Web Page occurs when a web application fails to adequately sanitize user input that includes URI encodings, allowing attackers to execute scripts hidden within these encodings.

• Exploitation Methods:

• Attackers can exploit this vulnerability by injecting malicious scripts disguised as encoded URIs.
• Common attack patterns include encoding harmful scripts using URI schemes to bypass input validation and execute code within the context of the web application.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized script execution, potentially leading to data theft or application compromise.
• Potential cascading effects can include further exploitation of the application to escalate privileges or propagate malware.
• Business impact may involve data breaches, loss of customer trust, legal consequences, and financial losses due to remediation efforts.

• Prevention Guidelines:

• Specific code-level fixes include implementing strict input validation and output encoding to ensure all user inputs are properly sanitized.
• Security best practices involve using a white-list approach for allowed URI schemes and rejecting or escaping potentially harmful encodings.
• Recommended tools and frameworks include employing security libraries that offer robust input validation and output encoding features, as well as keeping web frameworks and libraries updated to mitigate known vulnerabilities.

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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 JavaScript code demonstrates a vulnerability due to improper neutralization
// of encoded URI schemes. It accepts user input that can include encoded
// JavaScript code, which might be executed if improperly handled.

function handleUserInput(input) {
    // Assume 'input' is obtained from a query parameter or form input
    let decodedInput = decodeURIComponent(input); // Decodes the input, potentially revealing harmful scripts
    // Directly inserting user input into the page without validation or sanitization
    document.getElementById("output").innerHTML = `<a href="\${decodedInput}">Click here</a>`; // Vulnerable to XSS
}

// Example usage
handleUserInput('%3Cscript%3Ealert(%27XSS%27)%3C%2Fscript%3E'); // Malicious input that could execute JavaScript

How to fix Improper Neutralization of Encoded URI Schemes in a Web Page?

The vulnerability arises from decoding user input without proper validation and directly using it in the HTML context. This can lead to security issues like XSS (Cross-Site Scripting) if the input includes encoded scripts or malicious payloads.

To fix this issue, follow these best practices:

1. Validate Input: Ensure the input matches expected patterns (e.g., valid URLs).
2. Sanitize User Input: Use libraries or functions to sanitize inputs, removing or encoding harmful characters.
3. Use Safe API Methods: When inserting user content into the DOM, use methods that automatically handle encoding, like textContent or setAttribute.

Fixed Code Example

// Fixed code that properly validates and sanitizes user input to prevent XSS attacks.

function handleUserInput(input) {
    // Validate that the input is a valid URL
    let urlPattern = /^(https?:\/\/[^\s\$.?#].[^\s]*)\$/i;
    if (!urlPattern.test(input)) {
        console.error("Invalid URL input");
        return;
    }

    // Decode the input safely
    let decodedInput = decodeURIComponent(input);

    // Use 'textContent' to avoid direct HTML injection
    let link = document.createElement('a');
    link.textContent = "Click here";
    link.setAttribute('href', decodedInput);

    // Append the link safely
    let outputElement = document.getElementById("output");
    outputElement.innerHTML = ''; // Clear previous content
    outputElement.appendChild(link);

    console.log("User input handled safely");
}

// Example usage with safe input handling
handleUserInput('https://example.com');

Key Changes and Security Controls

• Input Validation: Introduced a regex pattern to validate that the input is a valid URL before processing it further.
• Safe Element Creation: Used document.createElement and setAttribute methods to safely handle and insert user data into the DOM.
• Clearing Previous Content: Before appending new content, the previous content is cleared to prevent any remnants of potential malicious scripts from being executed.