CVE-2025-1009 - Use-After-Free in XSLT Handling Hits Firefox and Thunderbird

A new browser vulnerability has come to light: CVE-2025-1009. This security hole allows a hacker to trigger a use-after-free bug by feeding specially crafted XSLT (eXtensible Stylesheet Language Transformations) data to affected software such as Firefox and Thunderbird. In simple terms, a malicious web page or attached email could cause a crash—and potentially let an attacker run code on your computer—just by loading some problematic XML.

Below, let’s break down how this bug works, see code examples, and learn how to stay safe.

What Is a Use-After-Free Bug?

A *use-after-free* bug happens when a program continues to use computer memory after it has already been released (freed) back to the system. Hackers love these kinds of bugs because they sometimes can use them to sneak in malicious code, take over your device, or get around security features.

Affected Versions

According to the official Mozilla Foundation Security Advisory (MFSA 2025-1009):

Thunderbird: Before 135 and before 128.7

If your software is *not* up-to-date with these, you’re at risk.

How the Bug Gets Triggered

Mozilla’s browsers and email software support XSLT for transforming XML data into HTML. If an attacker can feed specific, malformed XSLT data (say, via a web page or malicious email), they might be able to:

Craft XSLT and XML documents that trigger unusual memory management inside the browser.

2. Cause an object to be deleted (freed), but continue using a reference to it—leading to a use-after-free and memory corruption.

Example Flow

Suppose an attacker uses an <xsl:template> that manipulates nodes in a way which triggers a sensitive edge case in the browser’s internal handling code. Here’s a conceptual example:

<!-- Malicious XSLT snippet -->
<xsl:stylesheet version="1."
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">;
  <xsl:template match="/">
    <xsl:variable name="v">
      <xsl:copy-of select="document('attacker-controlled.xml')"/>
    </xsl:variable>
    <!-- More crafted transformations here -->
    <xsl:apply-templates select="$v"/>
  </xsl:template>
</xsl:stylesheet>

By chaining certain XSLT operations, the attacker forces the browser to prematurely free objects, then tricks it into reusing them, crashing the browser (or worse).

Proof-of-Concept (PoC) Overview

While Mozilla hasn’t published a full exploit, security researchers have shared crash triggers in bug bounty platforms and on security advisories. Due to ethical concerns, we won’t post real weaponized code, but here’s what a reduced crash trigger might look like in JavaScript and XML:

<!-- index.html -->
<iframe src="malicious.xml"></iframe>

<!-- malicious.xml -->
<?xml version="1."?>
<?xml-stylesheet type="text/xsl" href="exploit.xsl"?>
<data/>

<!-- exploit.xsl (crafted to trigger the bug) -->
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.">
  <xsl:template match="/">
    <!-- Trigger internal error state through crafted transforms -->
    <xsl:for-each select="document('attack.xml')"/>
    <!-- More crafted templates to manipulate memory -->
  </xsl:template>
</xsl:stylesheet>

By loading this combo in an old browser, researchers can trigger crashes—sometimes with memory corruption that could be exploited further.

Links to Official References

- Mozilla Foundation Security Advisory 2025-1009
- Bugzilla report #1904004 (may require access)
- CVE-2025-1009 entry at NVD (National Vulnerability Database)

How to Stay Safe

Patch as soon as possible.
If you use Firefox, Firefox ESR, or Thunderbird, check for updates and apply them immediately. Most users are protected just by keeping their software up to date.

Summary

*CVE-2025-1009* is a serious browser security bug, triggered by crafted XSLT data. By updating Firefox/Thunderbird, you protect yourself from crashes and possible code execution that this use-after-free bug might enable.

Timeline

Published on: 02/04/2025 14:15:31 UTC
Last modified on: 02/06/2025 19:28:52 UTC