CVE-2026-102497 in XmlSchema
Summary
by MITRE • 09/29/2026
The Apache XmlSchema walker (xmlschema-walker) doesn't detect cycles in type derivation, substitution groups, model groups or attribute groups. A malicious schema with such a cycle can make the walker recurse until the stack overflows, causing a denial of service.
Users are recommended to upgrade to version 2.3.3, which fixes this issue.
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Analysis
by VulDB Data Team • 09/29/2026
The Apache XmlSchema library serves as a foundational component for processing XML Schema Definition files within numerous Java-based enterprise applications and web services. The vulnerability identified in the xmlschema-walker module stems from an insufficient validation mechanism when traversing complex schema structures. Specifically, the walker fails to detect cyclic references during the analysis of type derivations, substitution groups, model groups, or attribute groups. In a valid XML Schema, these constructs define relationships between elements and types that must form directed acyclic graphs to ensure deterministic processing. When a malicious actor crafts an XML Schema containing circular dependencies among these structural components, the walker enters an infinite recursive loop as it attempts to resolve the type hierarchy without any cycle detection logic.
This lack of cycle detection directly leads to a stack overflow error within the Java Virtual Machine environment where the library is executed. As the recursion depth increases indefinitely due to the cyclic nature of the schema definitions, the call stack exhausts its allocated memory space. This results in an immediate termination of the application thread or process handling the XML parsing operation. The operational impact is primarily a denial of service condition for any system that processes untrusted or externally sourced XML schemas using this vulnerable version of the library. Attackers can exploit this flaw by submitting specially crafted schema files through APIs, file upload mechanisms, or configuration inputs that trigger the XmlSchema walker, thereby disrupting availability without requiring authentication in many deployment scenarios.
From a classification perspective, this vulnerability aligns with CWE-674, which describes uncontrolled recursion, and CWE-835, known as loop of death. These categories highlight failures in input validation where the application does not properly limit the depth or complexity of recursive operations based on external data. In terms of offensive security frameworks such as MITRE ATT&CK, this flaw facilitates resource exhaustion attacks, allowing adversaries to degrade service availability by consuming critical system resources like memory and CPU cycles through malformed inputs rather than exploiting code execution vulnerabilities.
To mitigate this risk, organizations relying on Apache XmlSchema must upgrade immediately to version 2.3.3 or later, where the developers have implemented proper cycle detection algorithms during schema parsing. This update ensures that cyclic references in type derivations and group definitions are identified before they trigger deep recursion. Additionally, defensive coding practices should be adopted by implementing timeout mechanisms for XML processing operations and validating input schemas against known safe patterns when possible. Security teams should also monitor application logs for stack overflow exceptions related to XML parsing components as an indicator of potential exploitation attempts in environments where the upgrade has not yet been applied.