CVE-2008-3281 in libxml2
Summary
by MITRE
libxml2 2.6.32 and earlier does not properly detect recursion during entity expansion in an attribute value, which allows context-dependent attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document.
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Analysis
by VulDB Data Team • 08/16/2019
The vulnerability identified as CVE-2008-3281 affects libxml2 versions 2.6.32 and earlier, representing a critical security flaw in XML parsing functionality that has significant implications for system stability and resource management. This issue specifically targets the entity expansion mechanism within XML documents, where the library fails to properly detect recursive entity references during attribute value processing. The vulnerability operates under the Common Weakness Enumeration category CWE-121, which classifies it as a buffer overflow condition, though in this case the overflow manifests as resource exhaustion rather than direct memory corruption. The flaw exists in the XML parser's handling of entity references, particularly when these entities are expanded within attribute values, creating a scenario where recursive references can cause infinite loops or excessive resource consumption.
The technical implementation of this vulnerability exploits the recursive nature of XML entity references, where entities can reference other entities, potentially creating circular dependencies that the parser fails to detect. When an XML document contains nested entity references within attribute values, the libxml2 parser continues to expand these entities without proper recursion depth checking or cycle detection mechanisms. This allows malicious actors to craft XML documents that contain carefully constructed entity references designed to trigger infinite expansion loops. The parser's inability to terminate expansion when recursion is detected leads to progressive consumption of both memory and cpu resources, as the system continuously processes the same entity references without reaching a termination condition. This behavior aligns with the ATT&CK technique T1499.004, which describes resource exhaustion attacks targeting system resources through malformed input processing.
The operational impact of CVE-2008-3281 extends beyond simple denial of service, as it can be leveraged to create sustained resource exhaustion conditions that may affect system availability and performance. Systems processing untrusted XML content using vulnerable libxml2 versions become susceptible to attacks that can consume all available memory or cpu cycles, effectively rendering applications unresponsive or causing system crashes. The vulnerability is particularly dangerous in web applications, xml processing services, or any system that accepts and processes external xml documents, as attackers can craft payloads that cause progressive resource consumption over time. Organizations running applications that utilize vulnerable libxml2 versions face significant risk of service disruption, especially in environments where xml processing is common or where applications are exposed to untrusted input sources.
Mitigation strategies for this vulnerability require immediate patching of affected libxml2 installations to versions that include proper recursion detection and cycle breaking mechanisms. System administrators should prioritize updating all affected systems and verify that xml processing libraries have been upgraded to versions that address this specific recursion detection flaw. Additionally, implementing input validation measures such as xml size limits, entity reference restrictions, and resource monitoring can provide additional defense in depth layers. The vulnerability demonstrates the importance of proper recursion handling in parsing libraries, as highlighted by the CWE-121 classification, which emphasizes the need for robust boundary checking and cycle detection in parsing operations. Organizations should also consider implementing xml schema validation and restricting entity expansion in applications that process external xml content to prevent exploitation of similar parsing vulnerabilities.