CWE-87: Improper Neutralization of Alternate XSS Syntax

What is Improper Neutralization of Alternate XSS Syntax?

• Overview: Improper Neutralization of Alternate XSS Syntax occurs when a product fails to correctly sanitize user input, allowing attackers to exploit alternative script syntaxes to execute malicious scripts.

• Exploitation Methods:

• Attackers can insert scripts using uncommon encodings or alternate representations to bypass standard filters.
• Common attack patterns include using different character encodings, such as Unicode, or employing uncommon scripting languages that the application fails to sanitize properly.

• Security Impact:

• Direct consequences include unauthorized script execution in the context of a user's session, leading to data theft or session hijacking.
• Potential cascading effects involve further attacks like spreading malware or defacing websites.
• Business impact includes loss of customer trust, legal ramifications, and potential financial losses due to data breaches.

• Prevention Guidelines:

• Specific code-level fixes involve comprehensive input validation and output encoding, ensuring all possible script syntaxes are neutralized.
• Security best practices include implementing a Content Security Policy (CSP) and using security libraries that automatically handle script neutralization.
• Recommended tools and frameworks such as OWASP's AntiSamy for Java or Microsoft AntiXSS Library for .NET help sanitize inputs effectively and prevent XSS attacks.

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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 takes user input from a query parameter and inserts it directly into the HTML.
// It improperly neutralizes user input that uses alternate XSS syntax, such as `<script>` tags with special characters.
const http = require('http');
const url = require('url');

http.createServer((req, res) => {
    const queryObject = url.parse(req.url, true).query;
    const userInput = queryObject.input; // User-controlled input

    // Vulnerable to XSS: Directly inserting user input into HTML
    res.writeHead(200, {'Content-Type': 'text/html'});
    res.end(`<div>User input: \${userInput}</div>`); // Dangerous line
}).listen(8080);

Explanation

In this vulnerable example, user input is taken directly from a URL query parameter and inserted into the HTML response without any sanitization or encoding. This makes the application susceptible to XSS attacks, especially if the input contains characters that can alter the HTML structure or execute scripts.

How to fix Improper Neutralization of Alternate XSS Syntax?

Fixed Code Example

// Fixed code using 'he' library to encode user input before inserting it into HTML
const http = require('http');
const url = require('url');
const he = require('he'); // Import 'he' library for HTML encoding

http.createServer((req, res) => {
    const queryObject = url.parse(req.url, true).query;
    const userInput = queryObject.input; // User-controlled input

    // Properly encoding user input to neutralize XSS
    const safeUserInput = he.encode(userInput); // Encode the input

    res.writeHead(200, {'Content-Type': 'text/html'});
    res.end(`<div>User input: \${safeUserInput}</div>`); // Safely insert encoded input
}).listen(8080);

Explanation

In the fixed code example, the he.encode() function is used to convert any potentially dangerous characters in the user input into their corresponding HTML entities. This effectively neutralizes any attempts at XSS, regardless of the syntax used, by ensuring that special characters are not interpreted as HTML or script code. This demonstrates a clear and effective way to mitigate XSS vulnerabilities using proper encoding practices.