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Cross-Site Scripting, commonly abbreviated as XSS, is one of the most prevalent security vulnerabilities found in web applications today. It exploits the trust users have in a website by injecting malicious scripts—usually written in JavaScript—into pages that other users view. In essence, XSS is malware that uses JavaScript to hijack user sessions, steal information, or manipulate website behaviour.

This attack vector can compromise users, administrators, and entire systems if not addressed properly. Understanding how it works and how to defend against it is crucial system administrators and everyday users.

Types of XSS Attacks

1. Non-Persistent (Reflected) XSS

This is the most common form of XSS and typically occurs when user input is immediately returned by a web application without proper validation or encoding. Here’s how it works:

• A web page (e.g., a search box) accepts input and reflects it back in the response.
• An attacker crafts a malicious URL containing a script and sends it via email or messaging.
• When the victim clicks the link, the script executes in their browser—as if it came from the legitimate site.
• This script might capture session cookies, credentials, or other sensitive data and send it to the attacker.
• The attacker then uses this information to hijack sessions or impersonate users.

2. Persistent (Stored) XSS

Persistent XSS is more dangerous because the malicious script is permanently stored on the target server, such as in a database, comment field, or user profile. The steps typically look like this:

• An attacker posts a message (with a hidden script) to a web application like a forum or social network.
• The message is saved and later served to anyone who views the page.
• Each visitor unknowingly runs the malicious script in their browser.
• No targeted delivery is needed—anyone who views the affected content is vulnerable.
• In social networking platforms, this can rapidly propagate like a worm.

How to Protect Against XSS

• Be cautious with links: Never click on links in unsolicited emails or messages.
• Use security browser extensions: Tools like NoScript or uBlock Origin can block untrusted scripts (though they require technical understanding and configuration).
• Keep software up to date: Regularly update your browser and any plugins to patch known vulnerabilities.
• Watch for suspicious behavior: Unexpected popups, redirects, or credential prompts may be signs of an XSS attack.