Stored XSS - The Persistent Threat

Stored XSS - The Persistent Threat

Stored XSS, also called persistent XSS, represents a more dangerous variant where malicious scripts are permanently stored on the target server. These scripts are then served to every user who views the affected content, creating a one-to-many attack scenario. Common locations for stored XSS include comment sections, user profiles, forum posts, and any feature where user-generated content is saved and displayed to other users. The persistent nature of these attacks means that victims don't need to click on malicious links – simply viewing the compromised content triggers the attack.

The lifecycle of a stored XSS attack begins when an attacker identifies an input field that doesn't properly sanitize data before storage. For example, a forum that allows users to create posts with rich text formatting might not properly filter JavaScript code. An attacker could create a post containing <img src=x onerror="malicious_code()">, which appears to be a broken image but actually executes JavaScript when the browser attempts to load it. Every user who views this forum post becomes a victim, with the malicious script executing in their browser context.

Real-world stored XSS attacks have had devastating consequences. In 2005, the Samy worm on MySpace became one of the fastest-spreading viruses of all time, using stored XSS to propagate. Within just 20 hours, over one million users were infected. The worm added the text "but most of all, samy is my hero" to victims' profiles and automatically sent friend requests to the worm's creator. While relatively harmless, this attack demonstrated the massive scale that stored XSS can achieve. More malicious attacks have stolen credentials from thousands of users, distributed malware, and caused significant financial losses.