CVE-2012-0876 in Mac OS X
Summary
by MITRE
The XML parser (xmlparse.c) in expat before 2.1.0 computes hash values without restricting the ability to trigger hash collisions predictably, which allows context-dependent attackers to cause a denial of service (CPU consumption) via an XML file with many identifiers with the same value.
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Analysis
by VulDB Data Team • 06/29/2022
The vulnerability identified as CVE-2012-0876 affects the expat XML parser library version 2.0.1 and earlier, representing a significant security flaw that impacts systems relying on XML processing. This issue stems from the hash table implementation within the xmlparse.c file where hash values are computed without adequate collision resistance mechanisms. The vulnerability is classified under CWE-327, which deals with the use of a broken or weak cryptographic algorithm, though in this case the weakness manifests in hash function design rather than cryptographic primitives. The flaw enables attackers to craft malicious XML documents that exploit the predictable hash collision patterns inherent in the parser's implementation.
The technical exploitation of this vulnerability occurs through the manipulation of XML identifiers within the parsed document structure. When the expat parser processes XML files containing numerous identifiers with identical values, it triggers a predictable hash collision scenario. This results in the hash table degenerating into a linked list structure, where each collision forces the parser to traverse increasingly longer chains of entries. The computational complexity escalates from O(1) average case to O(n) worst case, where n represents the number of colliding entries. Attackers can leverage this weakness by creating XML files with thousands or millions of identical identifiers, causing the parser to consume excessive CPU resources during processing. The vulnerability is context-dependent as it requires the specific conditions of XML parsing with repeated identifiers to manifest its effects.
The operational impact of CVE-2012-0876 presents a substantial denial of service risk to systems that process untrusted XML data. Organizations utilizing applications built on expat, including web servers, database systems, and enterprise applications, become vulnerable to resource exhaustion attacks that can severely impact system performance and availability. The attack vector is particularly concerning because XML processing is a common operation across numerous software platforms, making the vulnerability widespread in scope. Network services that accept XML input from external sources, such as web APIs, SOAP services, and configuration management systems, face elevated risk of being overwhelmed by CPU-intensive parsing operations. The vulnerability's impact is amplified in environments where XML processing occurs in resource-constrained settings or where multiple concurrent parsing operations are performed, potentially leading to complete system unresponsiveness or service disruption.
Mitigation strategies for CVE-2012-0876 primarily involve upgrading to expat version 2.1.0 or later, which implements improved hash collision resistance mechanisms. Organizations should conduct comprehensive vulnerability assessments to identify all systems utilizing affected expat versions and prioritize remediation efforts accordingly. Additional protective measures include implementing input validation controls that limit the size and complexity of XML documents processed by applications, deploying rate limiting mechanisms for XML processing requests, and establishing monitoring protocols to detect unusual CPU consumption patterns. The remediation approach aligns with ATT&CK technique T1496, which focuses on resource exhaustion attacks, emphasizing the importance of robust input validation and processing limits. System administrators should also consider implementing sandboxed XML processing environments and regular security audits to ensure continued protection against similar vulnerabilities in the expat library and other XML parsing implementations.