CVE-2018-14404 in libxml2
Summary
by MITRE
A NULL pointer dereference vulnerability exists in the xpath.c:xmlXPathCompOpEval() function of libxml2 through 2.9.8 when parsing an invalid XPath expression in the XPATH_OP_AND or XPATH_OP_OR case. Applications processing untrusted XSL format inputs with the use of the libxml2 library may be vulnerable to a denial of service attack due to a crash of the application.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified as CVE-2018-14404 represents a critical NULL pointer dereference flaw within the libxml2 library version 2.9.8 and earlier, specifically manifesting in the xpath.c file within the xmlXPathCompOpEval() function. This issue arises during the processing of malformed XPath expressions, particularly when encountering the XPATH_OP_AND or XPATH_OP_OR operators, creating a scenario where the application fails to properly validate input parameters before attempting to dereference a null pointer. The flaw fundamentally stems from inadequate error handling in the XPath expression evaluation engine, which is a core component of the library responsible for parsing and executing XPath queries against XML documents. The vulnerability is classified under CWE-476 as a NULL pointer dereference, which directly maps to the operational characteristics of the flaw where a program attempts to access memory through a null pointer reference, leading to immediate application termination.
The technical exploitation of this vulnerability occurs when applications that utilize libxml2 for processing XSL transformations or XML parsing receive malformed input containing invalid XPath expressions. When the xmlXPathCompOpEval() function encounters an invalid XPath expression involving logical AND or OR operations, it fails to properly validate the operands before attempting to dereference them, resulting in a segmentation fault or crash. This behavior constitutes a classic denial of service condition where an attacker can craft specific malformed XML or XSL content that triggers the NULL pointer dereference, causing the target application to terminate unexpectedly. The vulnerability is particularly concerning because it can be triggered through untrusted input streams such as user-supplied XML documents, web form submissions, or file uploads that are processed through the vulnerable libxml2 library, making it applicable across numerous web applications and services that depend on XML processing capabilities.
The operational impact of CVE-2018-14404 extends beyond simple application crashes to represent a significant threat to system availability and service integrity. When exploited successfully, this vulnerability allows attackers to perform denial of service attacks against applications that rely on libxml2 for XML processing, potentially causing service disruption for legitimate users. The vulnerability affects any application that processes XSLT transformations or XPath queries using the affected libxml2 versions, including web servers, content management systems, document processing applications, and enterprise software platforms that handle XML data. From an attacker perspective, the exploit requires minimal technical skill to craft malicious input that can trigger the crash, making it particularly dangerous in environments where applications are exposed to untrusted input from external sources. The vulnerability's impact is further amplified by the widespread adoption of libxml2 across numerous software platforms, meaning that a single exploitation can potentially affect multiple applications and services simultaneously.
Mitigation strategies for CVE-2018-14404 primarily focus on immediate remediation through library updates, with the most effective solution being the upgrade to libxml2 version 2.9.9 or later, where the NULL pointer dereference has been addressed through improved input validation and error handling mechanisms. Organizations should prioritize patching their systems and applications that utilize vulnerable versions of libxml2, particularly those handling untrusted XML or XSL input. Additional defensive measures include implementing strict input validation and sanitization protocols for all XML processing operations, employing sandboxing techniques to isolate XML parsing operations, and configuring application firewalls to filter suspicious XML content before it reaches the parsing layer. The vulnerability's characteristics align with ATT&CK technique T1499.004, which involves network denial of service attacks through resource exhaustion or application crashes, making it a significant concern for cybersecurity teams responsible for maintaining service availability and protecting against application-level attacks that target core library components.