Authorization disclaimer. This walkthrough is limited to the intentionally vulnerable PortSwigger Web Security Academy lab linked below. Test only systems you own or are explicitly authorized to assess.

Overview

This lab isolates HTML entity decoding inside stored onclick. The stored website appears inside JavaScript in an anchor onclick; angle brackets/double quotes are HTML-encoded and literal single quotes/backslashes are escaped. The HTML parser decodes ' to a quote before the event-handler JavaScript is compiled, creating an executable subtraction expression.

Lab objective/setup

Open the Academy exercise at the reserved lab URL and work only in that disposable environment. The required proof is specific: Clicking the stored author name invokes alert(1) and completes the lab.

What I observed

The stored website appears inside JavaScript in an anchor onclick; angle brackets/double quotes are HTML-encoded and literal single quotes/backslashes are escaped. The HTML parser decodes ' to a quote before the event-handler JavaScript is compiled, creating an executable subtraction expression.

Academy objective requiring stored XSS from the comment author name when its generated `onclick` handler is activated.
Academy objective requiring stored XSS from the comment author name when its generated `onclick` handler is activated.

Why it is vulnerable

The application applies protection for the wrong parser or omits a control at the decisive boundary. In this case, the stored website appears inside JavaScript in an anchor onclick; angle brackets/double quotes are HTML-encoded and literal single quotes/backslashes are escaped. Browser parsing therefore preserves the attacker-controlled behavior instead of reducing the value to inert text or a safe URL.

Walkthrough in the authorized PortSwigger lab

First reproduce the recorded page or request and inspect the exact reflected, stored, or DOM-generated context described above. Then supply the following lab payload as data, not as active markup in this article:

1
`http://foo?'-alert(1)-'`

The HTML parser decodes ' to a quote before the event-handler JavaScript is compiled, creating an executable subtraction expression.

Browser alert dialog showing `1` after the entity-encoded apostrophes are decoded inside the stored anchor event handler and the author name is clicked.
Browser alert dialog showing `1` after the entity-encoded apostrophes are decoded inside the stored anchor event handler and the author name is clicked.

Verification

Clicking the stored author name invokes alert(1) and completes the lab. This distinguishes execution from a payload merely appearing in source or being stored without reaching the required event or sink.

Impact and defensive lesson

In a real application, equivalent XSS could act with the victim’s same-origin privileges, expose readable data, or perform authenticated actions. Fix the data flow at its context: use text-only DOM APIs, JavaScript- or attribute-aware encoding, strict URL-scheme allowlists, and maintained sanitization where HTML is unavoidable. HttpOnly cookies and a restrictive CSP reduce consequences but do not repair the injection flaw.

References