CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

What is Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')?

• Overview: This vulnerability occurs when a web application does not properly sanitize or escape user inputs before including them in web pages, allowing attackers to inject malicious scripts that execute in the context of other users' sessions.

• Exploitation Methods:

• Attackers can exploit this vulnerability by injecting scripts into web pages viewed by other users, often through input fields like search boxes, comment sections, or any form that accepts user input.
• Common attack patterns include stored XSS, where the malicious script is saved on the server, and reflected XSS, where the payload is immediately executed as part of a URL.

• Security Impact:

• Direct consequences include unauthorized actions performed on behalf of the victim, theft of cookies or session tokens, and redirection to malicious sites.
• Potential cascading effects involve further compromise of user accounts, data breaches, and exploitation of trust in the web application.
• Business impact may include damage to reputation, legal liabilities, loss of user trust, and potential financial costs from security breaches.

• Prevention Guidelines:

• Specific code-level fixes include consistently escaping user input before rendering it in HTML, JavaScript, or any other client-side context.
• Security best practices involve adopting a robust input validation and output encoding strategy, employing Content Security Policy (CSP), and keeping libraries and frameworks updated.
• Recommended tools and frameworks include using libraries such as OWASP's AntiSamy or the Java OWASP Encoder Project, and web development frameworks that offer built-in mechanisms for mitigating XSS.

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Technical Details

Likelihood of Exploit: High

Affected Languages: Not Language-Specific

Affected Technologies: Web Based

The Same Origin Policy states that browsers should limit the resources accessible to scripts running on a given web site, or "origin", to the resources associated with that web site on the client-side, and not the client-side resources of any other sites or "origins". The goal is to prevent one site from being able to modify or read the contents of an unrelated site. Since the World Wide Web involves interactions between many sites, this policy is important for browsers to enforce. When referring to XSS, the Domain of a website is roughly equivalent to the resources associated with that website on the client-side of the connection. That is, the domain can be thought of as all resources the browser is storing for the user's interactions with this particular site.

Vulnerable Code Example

const express = require('express');
const app = express();

app.get('/greet', (req, res) => {
    // Vulnerable code: Directly using user input in HTML response
    const userName = req.query.name;
    res.send(`<h1>Hello, \${userName}</h1>`);
});

app.listen(3000);

Explanation:

• Lines {6-8}: This code directly injects user input (userName) into an HTML response without any escaping or validation. This allows an attacker to inject malicious scripts by manipulating the name query parameter, leading to a Cross-Site Scripting (XSS) vulnerability.

How to fix Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')?

To mitigate XSS in JavaScript applications using Express, ensure that any user input included in HTML content is properly escaped. The escape-html library can be used to escape HTML special characters, preventing script execution.

Fixed Code Example

const express = require('express');
const escapeHtml = require('escape-html');
const app = express();

app.get('/greet', (req, res) => {
    // Fixed code: Escaping user input to prevent XSS
    const userName = req.query.name || '';
    const safeUserName = escapeHtml(userName); // Escape HTML special characters
    res.send(`<h1>Hello, \${safeUserName}</h1>`);
});

app.listen(3000);

Explanation:

• Lines {6-10}: The escapeHtml function from the escape-html library is used to escape user input, converting HTML special characters into their safe representations. This prevents any embedded scripts from executing, effectively mitigating the XSS vulnerability.
• By ensuring that user input is sanitized before being included in the HTML response, the risk of script injection attacks is significantly reduced.