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 Template-literal expression injection. The search value is embedded in a JavaScript template literal assigned to innerText; characters including angle brackets, quotes, backslash, and backticks are Unicode-escaped. Template interpolation syntax is not escaped, so the JavaScript engine evaluates the expression even though the final DOM assignment uses innerText.

Lab objective/setup

Open the Academy exercise at the reserved lab URL and work only in that disposable environment. The required proof is specific: Page load evaluates alert(1) and solves the lab.

What I observed

The search value is embedded in a JavaScript template literal assigned to innerText; characters including angle brackets, quotes, backslash, and backticks are Unicode-escaped. Template interpolation syntax is not escaped, so the JavaScript engine evaluates the expression even though the final DOM assignment uses innerText.

Academy objective identifies reflected input inside a template literal where angle brackets, quotes, backslashes, and backticks are escaped.
Academy objective identifies reflected input inside a template literal where angle brackets, quotes, backslashes, and backticks are escaped.

Why it is vulnerable

The application applies protection for the wrong parser or omits a control at the decisive boundary. In this case, the search value is embedded in a JavaScript template literal assigned to innerText; characters including angle brackets, quotes, backslash, and backticks are Unicode-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
`${alert(1)}`

Template interpolation syntax is not escaped, so the JavaScript engine evaluates the expression even though the final DOM assignment uses innerText.

Browser alert dialog showing `1` after the inert `${alert(1)}` interpolation expression is evaluated without terminating the template literal.
Browser alert dialog showing `1` after the inert `${alert(1)}` interpolation expression is evaluated without terminating the template literal.

Verification

Page load evaluates alert(1) and solves 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