Do stored XSS attacks occur when a malicious script is included in a request to a web application and then sent back to the user?

/ / Published in Cybersecurity, EITC/IS/WASF Web Applications Security Fundamentals, Cross-site scripting, Cross-Site Scripting (XSS)

Stored Cross-Site Scripting (XSS) attacks are a type of security vulnerability that occurs in web applications.

The statement "Stored XSS attacks occur when a malicious script is included in a request to a web application and then sent back to the user" is false. To understand why this is the case, it is essential to consider the mechanics of Stored XSS attacks and differentiate them from other types of XSS attacks, such as Reflected XSS.

Stored XSS, also known as Persistent XSS, is a type of attack where the malicious script is permanently stored on the target server, such as within a database, message forum, visitor log, comment field, or any other data storage mechanism. The key characteristic of Stored XSS is that the malicious code is not just included in a single request and response cycle, but rather it is saved on the server and then served to users who request the affected content.

To provide a clearer understanding, let us consider a typical scenario involving a Stored XSS attack:

1. Injection of Malicious Script: An attacker submits a form on a web application that accepts user input and stores it on the server. For example, the attacker could post a comment on a blog or a forum that includes a malicious script. The input might look something like this:

html
   <script>alert('XSS Attack!');</script>

When the attacker submits this form, the script is stored in the application's database.

2. Storage of Malicious Script: The web application, without proper input validation and sanitization, saves the malicious script in its database. This is a critical point of failure, as the application should ideally sanitize any user input before storing it to prevent such attacks.

3. Execution of Malicious Script: When other users visit the page that displays the stored content (e.g., the blog post or forum thread), the web application retrieves the data from the database and includes it in the response sent to the user's browser. Since the script is embedded in the HTML content, the user's browser executes it as part of rendering the page. This results in the malicious script running in the context of the user's session, which can lead to various harmful outcomes, such as:
– Stealing session cookies, which can allow the attacker to hijack the user's session.
– Performing actions on behalf of the user without their consent (e.g., making unauthorized transactions).
– Redirecting the user to malicious websites.
– Displaying fraudulent content to deceive the user.

In contrast, Reflected XSS (or Non-Persistent XSS) involves the immediate reflection of the malicious script back to the user as part of the response to a request. In a Reflected XSS attack, the script is not stored on the server but is instead included in the URL or form submission and reflected back in the server's response. For example:
1. Crafting a Malicious URL: An attacker crafts a URL that includes the malicious script as a parameter:

html
   http://example.com/search?q=<script>alert('XSS Attack!');</script>

2. User Interaction: The attacker tricks a user into clicking the malicious link.
3. Reflection and Execution: When the user clicks the link, the web application processes the request and includes the malicious script in the response. The user's browser then executes the script.

Given these distinctions, it is clear that Stored XSS involves the storage of the malicious script on the server and its subsequent execution when the stored content is retrieved and displayed to users. This is fundamentally different from Reflected XSS, where the script is not stored but immediately reflected back to the user.

To illustrate this with a practical example, consider a web application that allows users to post comments on articles. If the application does not properly sanitize user input, an attacker could post a comment like this:

html
Nice article! <script>alert('You have been hacked');</script>

This comment gets stored in the database. Whenever any user, including the original attacker, views the article and its comments, the script tag is included in the HTML response. The browser executes the script, and the alert message "You have been hacked" is displayed.

To mitigate Stored XSS attacks, web developers must implement robust input validation and output encoding practices. Some recommended strategies include:
– Input Validation: Validate and sanitize all user inputs to ensure they do not contain malicious scripts. This can be achieved through whitelisting acceptable input formats and rejecting any input that does not conform to these formats.
– Output Encoding: Encode output data to ensure that any potentially dangerous characters are rendered harmless by the browser. For example, converting `<` to `&lt;` and `>` to `&gt;` prevents the browser from interpreting them as HTML tags.
– Content Security Policy (CSP): Implementing a CSP can help mitigate the impact of XSS attacks by restricting the sources from which scripts can be loaded and executed.

Stored XSS attacks involve the storage of malicious scripts on the server and their execution when the stored content is retrieved and displayed to users. This is distinct from Reflected XSS, where the script is immediately reflected back to the user as part of the response to a request. Proper input validation, output encoding, and security policies are essential to prevent such vulnerabilities.

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