CVE-2013-4520 in libxslt
Summary
by MITRE
xslt.c in libxslt before 1.1.25 allows context-dependent attackers to cause a denial of service (crash) via a stylesheet that embeds a DTD, which causes a structure to be accessed as a different type. NOTE: this issue is due to an incomplete fix for CVE-2012-2825.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 01/12/2022
The vulnerability described in CVE-2013-4520 represents a critical denial of service flaw within the libxslt library version 1.1.24 and earlier. This issue affects the xslt.c component of the library, which is widely used for processing xml stylesheet transformations across numerous applications and systems. The vulnerability specifically manifests when processing xml stylesheets that contain embedded Document Type Definitions, creating a scenario where memory structures are accessed with incorrect type assumptions. This fundamental type confusion error occurs because the library fails to properly validate the structure types during the parsing and transformation process, leading to unpredictable behavior when the code attempts to interpret memory locations as different data types than those originally allocated.
The technical exploitation of this vulnerability relies on crafting malicious xml stylesheets that contain embedded DTD declarations, which then trigger the type confusion bug within libxslt's internal processing mechanisms. When the library encounters such malformed input, it attempts to access memory structures using incorrect type assumptions, causing memory corruption that ultimately results in application crashes. This issue is particularly concerning because it represents an incomplete fix for CVE-2012-2825, indicating that the previous remediation efforts were insufficient to address all potential attack vectors within the same codebase. The vulnerability falls under CWE-121, which describes stack-based buffer overflow conditions, and more specifically relates to CWE-704, indicating incorrect type specification in code. From an operational perspective, this vulnerability can be exploited by context-dependent attackers who have the ability to submit xml content to systems utilizing libxslt for processing, potentially affecting web applications, xml processing services, and any system that relies on xslt transformations for content rendering or data processing.
The impact of CVE-2013-4520 extends beyond simple service disruption, as it can be leveraged to create persistent denial of service conditions that may require system restarts or application reinitialization to resolve. Systems running vulnerable versions of libxslt are at risk of experiencing crashes during xml processing operations, which could be particularly damaging in high-availability environments or mission-critical applications. Attackers can exploit this vulnerability through various vectors including web application input fields, xml file processing services, or any interface that accepts and processes xml content with xslt transformations. The vulnerability's classification under the ATT&CK framework would align with technique T1499.004, which covers network denial of service attacks, and potentially T1595.001 for reconnaissance activities targeting system vulnerabilities. Organizations should prioritize patching affected systems to version 1.1.25 or later, as this represents the first complete fix for the type confusion vulnerability. Additionally, implementing input validation measures, restricting xml content processing, and monitoring for unusual xml processing patterns can provide additional defensive layers against exploitation attempts. The vulnerability highlights the importance of thorough testing and validation of security patches, particularly when dealing with complex libraries that handle structured data processing, as incomplete fixes can leave systems vulnerable to continued exploitation.