CVE-2011-2188 in LuaExpat
Summary
by MITRE
LuaExpat before 1.2.0 does not properly detect recursion during entity expansion, which allows remote attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document containing a large number of nested entity references, a similar issue to CVE-2003-1564.
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Analysis
by VulDB Data Team • 02/09/2019
The vulnerability identified as CVE-2011-2188 affects LuaExpat versions prior to 1.2.0 and represents a classic example of a denial of service attack through XML entity expansion recursion. This flaw resides in the XML parsing library's handling of entity references within XML documents, specifically failing to implement proper recursion detection mechanisms during entity expansion processes. The vulnerability stems from the parser's inability to track and limit the depth of nested entity references, creating a condition where maliciously crafted XML documents can trigger unlimited recursive expansion.
The technical implementation of this vulnerability involves the exploitation of XML entity processing mechanisms where external or internal entities can reference other entities, creating potentially infinite recursive chains. When a parser encounters an entity reference that expands to another entity which in turn references the original entity, the parser should detect this recursion and terminate the process to prevent resource exhaustion. However, LuaExpat before version 1.2.0 lacks this crucial recursion detection, allowing attackers to craft XML documents containing deeply nested entity references that cause the parser to consume excessive memory and CPU resources as it attempts to resolve each level of expansion.
This vulnerability directly maps to CWE-400, which categorizes improper handling of excessive resource consumption, and specifically relates to CWE-611, concerning improper restriction of XML external entity reference. The operational impact of this vulnerability is significant as it enables remote attackers to perform denial of service attacks against systems that process XML data through LuaExpat. The attack requires minimal privileges and can be executed against any application or service that utilizes the vulnerable library, making it particularly dangerous in networked environments where XML processing is common. The resource exhaustion occurs both in memory consumption as the parser maintains expansion state and in CPU usage as the recursive expansion process continues indefinitely.
The attack vector is particularly concerning as it can be executed remotely through any XML input channel that processes documents using the vulnerable LuaExpat library. This includes web applications, XML processing services, and any system that accepts XML data for parsing and processing. The similarity to CVE-2003-1564 demonstrates the persistence of this class of vulnerability in XML parsers, indicating that despite years of awareness, proper recursion detection mechanisms are still frequently omitted from implementations. The vulnerability's exploitation results in complete system resource exhaustion, potentially leading to application crashes, system instability, or complete service unavailability for legitimate users.
Mitigation strategies for CVE-2011-2188 focus primarily on updating to LuaExpat version 1.2.0 or later, which includes proper recursion detection and limiting mechanisms. Organizations should also implement XML input validation and sanitization processes, including setting maximum entity expansion limits, implementing input size restrictions, and employing XML parsers with built-in protection mechanisms. Network-level controls such as XML firewalling and rate limiting can provide additional defense in depth. The implementation of proper input validation aligns with ATT&CK technique T1210, which involves exploitation of weaknesses in input validation, while the need for proper recursion detection relates to defensive techniques that prevent resource exhaustion attacks. System administrators should also monitor for unusual memory and CPU consumption patterns that might indicate exploitation attempts, and maintain up-to-date vulnerability assessments to identify all affected systems using the vulnerable library.