CVE-2016-3705 in libxml
Summary
by MITRE
The (1) xmlParserEntityCheck and (2) xmlParseAttValueComplex functions in parser.c in libxml2 2.9.3 do not properly keep track of the recursion depth, which allows context-dependent attackers to cause a denial of service (stack consumption and application crash) via a crafted XML document containing a large number of nested entity references.
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Analysis
by VulDB Data Team • 08/19/2022
The vulnerability identified as CVE-2016-3705 represents a critical stack consumption issue within the libxml2 library version 2.9.3, specifically affecting the xmlParserEntityCheck and xmlParseAttValueComplex functions in the parser.c file. This flaw stems from improper recursion depth tracking mechanisms that fail to adequately monitor nested entity references within XML documents. The vulnerability operates at the parsing layer of XML processing, where the library's recursive descent parser does not maintain sufficient safeguards against excessive nesting levels, creating a pathway for maliciously crafted XML content to exhaust available stack memory resources.
The technical implementation of this vulnerability exploits the fundamental parsing behavior of libxml2's XML processor, where entity references can be nested to arbitrary depths without proper depth limitation enforcement. When an attacker crafts an XML document containing a large number of nested entity references, the parser's recursive functions continuously consume stack space to process each level of nesting. The xmlParserEntityCheck function fails to maintain proper recursion depth counters during entity expansion, while xmlParseAttValueComplex similarly lacks adequate depth tracking during attribute value processing. This recursive behavior creates a stack overflow condition that ultimately leads to application crash and complete denial of service.
The operational impact of CVE-2016-3705 extends across numerous applications and systems that rely on libxml2 for XML processing, including web applications, enterprise software, and security tools. Attackers can exploit this vulnerability through context-dependent means by delivering malicious XML content to any system that processes XML input, making it particularly dangerous in web-facing applications or services that accept user-submitted XML data. The vulnerability maps directly to CWE-674 - Uncontrolled Recursion, which specifically addresses the lack of recursion depth control in parsing operations. This weakness allows attackers to consume system resources and cause application instability, effectively creating a denial of service condition that can be exploited remotely without requiring authentication or special privileges.
The exploitation of this vulnerability aligns with ATT&CK technique T1499.004 - Endpoint Denial of Service, where adversaries leverage software weaknesses to consume system resources and render services unavailable. The attack vector typically involves crafting XML documents with deeply nested entity references that trigger the recursive parsing functions, causing the application to consume excessive stack memory until system resources are exhausted. Organizations using vulnerable versions of libxml2 face significant risk in environments where XML processing occurs, particularly in web applications, XML-based APIs, and systems that process external XML data sources. The vulnerability's impact is amplified in systems where multiple XML processing operations occur simultaneously, as each concurrent request can contribute to stack exhaustion. Mitigation strategies include updating to libxml2 versions that address the recursion depth tracking issue, implementing XML size and nesting limits, and deploying application-level safeguards that monitor and restrict recursive parsing operations to prevent uncontrolled resource consumption.
This vulnerability demonstrates the critical importance of proper recursion control in parsing libraries and highlights how seemingly benign XML processing operations can become attack vectors when proper safeguards are absent. The flaw serves as a prime example of how stack-based vulnerabilities can be leveraged for denial of service attacks in widely-used software components, emphasizing the need for robust input validation and resource monitoring in XML processing systems. Organizations should prioritize updating their libxml2 installations and implementing comprehensive XML parsing security measures to prevent exploitation of this and similar recursion-based vulnerabilities.