CVE-2021-46143 in libexpat
Summary
by MITRE • 01/06/2022
In doProlog in xmlparse.c in Expat (aka libexpat) before 2.4.3, an integer overflow exists for m_groupSize.
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Analysis
by VulDB Data Team • 05/06/2025
The vulnerability identified as CVE-2021-46143 affects the Expat XML parser library, specifically within the doProlog function located in xmlparse.c. This issue represents a critical integer overflow condition that can potentially lead to memory corruption and arbitrary code execution. The flaw manifests when processing XML documents with malformed or specially crafted input that triggers an integer overflow in the m_groupSize variable, which controls the size of internal data structures used during parsing operations. The vulnerability exists in Expat versions prior to 2.4.3, making numerous applications and systems that rely on this library susceptible to exploitation.
The technical nature of this vulnerability stems from improper input validation and arithmetic handling within the XML parsing engine. When the parser encounters certain XML constructs, particularly those involving nested groups or complex parsing scenarios, the m_groupSize variable can exceed its maximum representable value, causing an integer overflow. This overflow results in the allocation of insufficient memory buffers or incorrect memory calculations, creating potential opportunities for memory corruption. The flaw aligns with CWE-190, which specifically addresses integer overflow conditions, and represents a classic example of how improper integer arithmetic can lead to security vulnerabilities in parsing libraries. The vulnerability demonstrates how XML parsers must carefully handle all input parameters to prevent arithmetic overflows that could be exploited by attackers.
The operational impact of CVE-2021-46143 extends across numerous systems and applications that depend on Expat for XML processing, including web servers, application frameworks, and enterprise software solutions. Attackers could exploit this vulnerability by crafting malicious XML documents that trigger the integer overflow condition, potentially leading to denial of service, remote code execution, or data corruption. The vulnerability is particularly concerning because XML parsing is a fundamental operation in web applications, making it a prime target for exploitation in web-based attacks. Systems using affected versions of Expat are at risk of being compromised through XML external entity (XXE) attacks or other vectorized exploitation techniques that leverage the parsing engine's weaknesses.
Mitigation strategies for CVE-2021-46143 primarily focus on upgrading to Expat version 2.4.3 or later, which contains the necessary patches to address the integer overflow condition. Organizations should prioritize updating their systems and applications that rely on Expat, ensuring that all components using the library receive the security update. Additional defensive measures include implementing strict XML input validation, limiting XML document sizes, and employing XML parsers with enhanced security features. Network-level protections such as web application firewalls and intrusion prevention systems can help detect and block malicious XML traffic. Security teams should also consider implementing monitoring and logging for XML processing activities to identify potential exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date security patches and following secure coding practices that prevent integer overflow conditions in parsing libraries, aligning with ATT&CK technique T1211 for exploitation through input validation failures.