What Is Stored Cross-Site Scripting?


Larry Thompson

Stored Cross-Site Scripting (XSS) is a type of web vulnerability that occurs when an application stores user-supplied data and later displays it without proper validation. This allows an attacker to inject malicious code into the application, which is then executed by unsuspecting users.

What is Cross-Site Scripting?
Cross-Site Scripting is a common web application security flaw that allows attackers to inject malicious scripts into web pages viewed by other users. There are three main types of XSS attacks: Stored XSS, Reflected XSS, and DOM-based XSS. In this article, we’ll focus on Stored XSS.

Stored XSS Explained
Stored XSS, also known as Persistent or Type-I XSS, occurs when the injected malicious script is permanently stored on the Target server. This means that whenever a user accesses the affected page, the script is executed without any further user interaction.

How Does Stored XSS Work?

To understand how Stored XSS works, let’s consider an example scenario. Imagine a social networking website where users can post comments on each other’s profiles. The website stores these comments in a database and displays them whenever someone visits a profile.

Now, suppose an attacker posts a comment containing malicious JavaScript code as part of their comment. Since the website fails to properly validate or sanitize user input before storing it in the database, the malicious code becomes part of the stored data.

When any user visits the profile with the infected comment, their browser executes the injected script unknowingly. This can lead to various consequences such as stealing sensitive information (like session cookies), redirecting users to malicious websites, or even modifying the content of the page dynamically.

Impact of Stored XSS

The impact of a successful Stored XSS attack can be severe and wide-ranging:

  • Data Theft: Attackers can steal sensitive information, including user credentials, credit card details, and personal data, compromising user privacy.
  • Session Hijacking: By stealing session cookies, attackers can impersonate legitimate users and perform actions on their behalf.
  • Defacement or Content Modification: Attackers may modify the content of web pages to spread misinformation, defame individuals or organizations, or cause confusion among users.
  • Malware Distribution: Stored XSS can be used as a gateway to distribute malware by redirecting users to websites hosting malicious content.

Preventing Stored XSS

To prevent Stored XSS attacks, it is essential to implement proper input validation and output encoding.

Input Validation:
Always validate user input on both the client-side and server-side. This includes checking for expected data types, length restrictions, and disallowing potentially dangerous characters or scripts.

Output Encoding:
Before displaying any user-supplied data back to the user, ensure that it is properly encoded. HTML entities should be escaped to prevent browsers from interpreting them as markup. Content Security Policies (CSP) can also help mitigate XSS attacks by restricting the sources from which scripts can be loaded.


Stored Cross-Site Scripting (XSS) is a serious web vulnerability that can have severe consequences for both users and website owners. By understanding how it works and implementing proper security measures like input validation and output encoding, developers can protect their applications from this widespread threat.

Remember, security should always be an integral part of the development process. Stay vigilant and keep your applications safe from vulnerabilities like Stored XSS!

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