CVE-2003-1564 in libxml2
Summary
by MITRE
libxml2, possibly before 2.5.0, does not properly detect recursion during entity expansion, which allows context-dependent attackers to cause a denial of service (memory and CPU consumption) via a crafted XML document containing a large number of nested entity references, aka the "billion laughs attack."
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Analysis
by VulDB Data Team • 08/16/2019
The vulnerability identified as CVE-2003-1564 represents a critical denial of service flaw within the libxml2 XML parsing library that affected versions prior to 2.5.0. This vulnerability specifically targets the library's entity expansion mechanism, which is a fundamental component of XML processing that allows for the definition and reuse of content through entity references. The flaw enables attackers to craft malicious XML documents that exploit the library's inadequate recursion detection capabilities during entity expansion processes. This issue falls under the category of CWE-400, which classifies improper handling of exceptional conditions, particularly focusing on resource exhaustion vulnerabilities that can lead to system instability.
The technical implementation of this vulnerability stems from the libxml2 library's failure to properly track and limit recursive entity references during XML parsing operations. When an XML document contains deeply nested entity references, the parser should detect and terminate such recursive expansions to prevent infinite loops and excessive resource consumption. However, the vulnerable versions of libxml2 lacked adequate recursion detection mechanisms, allowing attackers to construct XML documents with thousands or millions of nested entity references. Each entity reference expansion consumes memory and processing cycles, leading to exponential resource consumption that can quickly exhaust system resources and render the affected application or system unusable.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can severely compromise system availability and performance. Attackers can exploit this weakness by submitting carefully crafted XML documents to applications that utilize libxml2 for XML processing, causing significant memory allocation and CPU consumption that may lead to system crashes, application hangs, or complete system exhaustion. This vulnerability is particularly dangerous in web applications, enterprise systems, and any environment where XML processing is prevalent, as it can be exploited through various attack vectors including file uploads, API requests, and web form submissions. The attack pattern known as the "billion laughs attack" demonstrates how relatively small XML documents can trigger massive resource consumption through recursive entity expansion, making this vulnerability particularly effective for resource exhaustion attacks.
Mitigation strategies for CVE-2003-1564 require immediate remediation through library updates to versions 2.5.0 or later where proper recursion detection has been implemented. Organizations should also implement XML parsing restrictions, including setting limits on entity expansion depth and total entity size, which aligns with the defensive techniques outlined in the ATT&CK framework under the 'Resource Exhaustion' tactic. Network-based protections such as XML firewalls and content filtering systems can help detect and block malicious XML payloads before they reach vulnerable applications. Additionally, implementing proper input validation and sanitization measures, along with monitoring for unusual resource consumption patterns, provides layered defense against exploitation attempts. The vulnerability's classification under CWE-400 and its exploitation patterns align with common attack methodologies documented in cybersecurity frameworks, emphasizing the importance of proper exception handling and resource management in software development practices.