CVE-2026-0989 in libxml2
Summary
by MITRE • 01/15/2026
A flaw was identified in the RelaxNG parser of libxml2 related to how external schema inclusions are handled. The parser does not enforce a limit on inclusion depth when resolving nested <include> directives. Specially crafted or overly complex schemas can cause excessive recursion during parsing. This may lead to stack exhaustion and application crashes, creating a denial-of-service risk.
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Analysis
by VulDB Data Team • 06/29/2026
The vulnerability identified as CVE-2026-0989 resides within the RelaxNG parser component of libxml2, a widely deployed XML parsing library that forms the backbone of numerous applications across multiple platforms and operating systems. This flaw specifically targets the handling of external schema inclusions, where the parser fails to implement proper safeguards against excessive recursion during the resolution of nested <include> directives. The absence of inclusion depth limits creates a condition where maliciously crafted XML schemas can trigger unlimited recursive parsing operations, ultimately leading to stack exhaustion and application instability.
From a technical perspective, the vulnerability manifests as a lack of recursion depth control within the RelaxNG parsing logic, which directly maps to CWE-674, the weakness category for "Uncontrolled Recursion." The parser processes nested schema inclusions without maintaining a counter or limit on the depth of nested includes, allowing attackers to construct schemas that create deeply nested structures. When the parser attempts to resolve these inclusions, each recursive level consumes stack space, and without bounds enforcement, the stack can quickly become exhausted, resulting in segmentation faults or application crashes. This behavior represents a classic denial-of-service vector where computational resources are consumed in a manner that prevents legitimate processing.
The operational impact of this vulnerability extends beyond simple application crashes, as it can affect any system that relies on libxml2 for XML processing, including web servers, database systems, content management platforms, and security appliances. Attackers can exploit this weakness by submitting specially crafted XML documents containing deeply nested include directives, causing the target application to consume excessive memory and CPU resources during parsing. The vulnerability is particularly concerning because it can be triggered through various input vectors, including file uploads, network requests, or any processing of external XML content. The denial-of-service condition can be sustained, potentially allowing attackers to consume system resources over time and render services unavailable to legitimate users.
Mitigation strategies for CVE-2026-0989 should focus on implementing immediate defensive measures while longer-term solutions involve updating to patched versions of libxml2. Organizations should consider implementing input validation and size limits on XML content processed by applications using libxml2, particularly for external schemas or documents from untrusted sources. Network-level controls such as rate limiting and content filtering can help reduce the impact of exploitation attempts. Additionally, application developers should implement proper error handling and resource management to prevent stack overflow conditions from causing system-wide failures. The ATT&CK framework categorizes this vulnerability under T1499.004 for "Network Denial of Service" and T1059.007 for "Command and Scripting Interpreter: Python," as exploitation may involve crafting malicious XML payloads that can be processed through various scripting interfaces. Organizations should monitor for any signs of exploitation attempts and maintain updated threat intelligence regarding similar vulnerabilities in XML processing libraries.