CVE-2023-28484 Exploiting Libxml2 NULL Pointer Dereference Vulnerability

CVE-2023-28484 is a critical vulnerability found in libxml2, one of the most widely used libraries for processing XML files. This vulnerability involves a NULL pointer dereference in xmlSchemaFixupComplexType function, which can cause a segmentation fault when parsing certain invalid XSD schemas. This article provides a comprehensive guide on the exploit details, with code snippets and original references.

Vulnerability Background

Before version 2.10.4 of libxml2, parsing specific invalid XSD schemas would result in a NULL pointer dereference, causing a segmentation fault (segfault). This occurs due to improper handling of incorrect XSD schemas in the xmlSchemaFixupComplexType function of the xmlschemas.c file.

The implications of this vulnerability are grave for any applications that utilize libxml2 for XML processing. It can lead to Denial of Service (DoS) attacks, resulting in application crashes stemming from a segmentation fault.

The vulnerable code in xmlschemas.c is as follows

static int
xmlSchemaFixupComplexType(xmlSchemaParserCtxtPtr ctxt, xmlSchemaTypePtr type) {
    ...
    if ((type->contentType == XML_SCHEMA_CONTENT_ELEMENTS)
        || (type->contentType == XML_SCHEMA_CONTENT_UNKNOWN)) {
        if (type->subtypes == NULL) {
            xmlSchemaTypePtr base;

            base = type->baseType;
            if (base == NULL)
            // Critical NULL pointer dereference occurs here
                return(-1);
    ...
}

Here, the code declares a pointer variable 'base' and initializes it with the value of 'type->baseType.' However, it doesn't properly check whether 'base' is NULL before using it. This omission can lead to a NULL pointer dereference when parsing an invalid XSD schema.

To exploit this vulnerability, an attacker can construct an invalid XSD schema triggering the NULL pointer dereference segmentation fault. For example:

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">;
  <xsd:complexType name="test">
    <xsd:complexContent>
      <xsd:restriction/>
    </xsd:complexContent>
  </xsd:complexType>
</xsd:schema>

When an application processes this malformed XSD schema using libxml2, a segmentation fault will occur, causing the application to crash.

Mitigation Steps

Upgrade your libxml2 version to 2.10.4 or newer, as it includes a security fix for this vulnerability. To do this, you can visit the following link and download the latest version:

Libxml2 Official Downloads

If you can't upgrade to a patched version, it's recommended to implement strict XML schema validation before processing any XML files. This precaution can help prevent the parsing of invalid XSD schemas that trigger the vulnerability.

Conclusion

CVE-2023-28484 is a serious vulnerability with potential for severe application crashes using libxml2 library versions before 2.10.4. Implementing the mitigation measures mentioned above will ensure that your applications are protected against this exploit. Always ensure the latest security patches are applied to your software stack and regularly monitor security news for updates and advice.

Original References

1. CVE-2023-28484 - NVD Detail
2. Libxml2 Git Repository
3. Official CVE Details - CVE-2023-28484

Timeline

Published on: 04/24/2023 21:15:00 UTC
Last modified on: 05/03/2023 20:23:00 UTC