CVE-2022-25313 in libexpat
Summary
by MITRE • 02/18/2022
In Expat (aka libexpat) before 2.4.5, an attacker can trigger stack exhaustion in build_model via a large nesting depth in the DTD element.
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Analysis
by VulDB Data Team • 05/30/2025
The vulnerability identified as CVE-2022-25313 represents a critical stack exhaustion issue within the Expat XML parsing library, specifically affecting versions prior to 2.4.5. This flaw resides in the build_model function which processes Document Type Definitions, making it particularly dangerous for applications that handle untrusted XML input. The vulnerability stems from insufficient depth validation during DTD element processing, allowing malicious actors to craft XML documents with excessively nested elements that can overwhelm the stack memory allocation.
The technical implementation of this vulnerability involves the exploitation of recursive parsing mechanisms within the Expat library's DTD processing code. When the parser encounters deeply nested XML elements in the DTD, the build_model function recursively calls itself to process each level of nesting without adequate stack depth limits. This recursive behavior causes the stack to grow exponentially with each nested element, eventually exhausting the available stack space and leading to a stack overflow condition. The flaw maps directly to CWE-770, which addresses allocation of resources without proper limits or checks, and specifically relates to improper handling of recursive data structures in parsing operations.
The operational impact of CVE-2022-25313 extends across numerous applications and systems that rely on Expat for XML processing, including web applications, enterprise software, and security tools. Attackers can leverage this vulnerability to cause denial of service conditions by submitting malicious XML documents that trigger stack exhaustion, effectively crashing applications or rendering them unresponsive. The vulnerability is particularly concerning because XML parsing is a fundamental operation in many systems, making the attack surface broad and potentially affecting critical infrastructure components. This issue aligns with ATT&CK technique T1499.004, which involves network denial of service attacks targeting application availability through resource exhaustion.
Mitigation strategies for CVE-2022-25313 require immediate patching of affected Expat library versions to 2.4.5 or later, which includes enhanced stack depth validation and resource limits. Organizations should implement input validation measures that restrict XML document complexity and nesting levels, particularly for untrusted input sources. Additionally, deploying application-level protections such as XML parser configuration limits, memory monitoring, and resource allocation controls can help prevent exploitation. Security teams should conduct comprehensive vulnerability assessments across all systems using Expat to identify potential exposure points and implement proper monitoring for suspicious XML parsing activities that could indicate attempted exploitation of this vulnerability.