CVE-2025-24928 - Stack-based Buffer Overflow in Libxml2: A Deep Dive into xmlSnprintfElements in valid.c

The Common Vulnerabilities and Exposures (CVE) database is known for cataloging security vulnerabilities in software around the world. CVE-2025-24928 is a recent vulnerability that affects the widely used xml parsing library, libxml2. In this post, we'll discuss the details of this vulnerability, showcase code snippets from the affected library, and explain how to exploit it.

What is libxml2?

Libxml2 is a popular C library that aids in parsing XML documents. It's used in a variety of applications and platforms, including both commercial and open-source projects. The affected versions for this vulnerability include libxml2 versions before 2.12.10 and 2.13.x's versions before 2.13.6.

The Vulnerability: Stack-based Buffer Overflow in xmlSnprintfElements

The vulnerability occurs in the xmlSnprintfElements function in valid.c within libxml2. The stack-based buffer overflow makes it possible for an attacker to execute arbitrary code on the target system given that the attacker can influence the input of the xmlSnprintfElements function.

To successfully exploit the vulnerability, an attacker would have to provide an untrusted document or untrusted Document Type Definition (DTD) for DTD validation. This vulnerability shares similarities with CVE-2017-9047.

The code snippet below shows the affected function in valid.c

/*
 * xmlSnprintfElements:
 * @buf:  the buffer to store the string
 * @size:  the number of bytes available
 * @nodes:  the list of nodes
 *
 * Saves the string representation of the given list of nodes in the given @buf
 * with a size limit of @size.
 */
static void
xmlSnprintfElements(char *buf, int size, const xmlElementContent *content)
{
    if (size <= 1)
        return;

    if (content == NULL) {
        *buf = ;
        return;
    }
    
    //...
}

A stack-based buffer overflow occurs when the buffer _buf_ (which is of a fixed size) is filled beyond its capacity due to a lack of proper bounds checking. The function xmlSnprintfElements contains such an issue, which can be exploited by providing an input XML file with a large number of elements.

To understand the vulnerability better, let's take a closer look at the source code of libxml2, available through its official repository:

- libxml2-2.12.10
- libxml2-2.13.6

Notice that the buffer size _size_ is not checked against the length of the input content. As a result, an attacker can provide a carefully crafted input file that overflows the buffer, potentially allowing the execution of arbitrary code.

Mitigation Measures

To safeguard against this vulnerability, it's essential to upgrade to the latest version of libxml2. The following versions have already addressed the issue:

libxml2 2.13.6 or later

Additionally, ensure that you only perform DTD validation on trusted documents or DTDs to minimize the attack surface.

Conclusion

CVE-2025-24928 is a critical vulnerability that allows an attacker to exploit a stack-based buffer overflow in libxml2. Upgrading to the latest version of libxml2 and ensuring you only perform DTD validation on trusted documents or DTDs are the best methods to protect against this vulnerability. By understanding the root cause, we can learn how to identify similar vulnerabilities and protect our systems from potential attacks.

Timeline

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