CVE-2021-21366 in xmldom
Summary
by MITRE • 03/13/2021
xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. xmldom versions 0.4.0 and older do not correctly preserve system identifiers, FPIs or namespaces when repeatedly parsing and serializing maliciously crafted documents. This may lead to unexpected syntactic changes during XML processing in some downstream applications. This is fixed in version 0.5.0. As a workaround downstream applications can validate the input and reject the maliciously crafted documents.
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Analysis
by VulDB Data Team • 04/01/2021
The vulnerability identified as CVE-2021-21366 affects the xmldom JavaScript library, which provides a W3C standard-compliant XML DOM parser and serializer implementation. This library serves as a critical component in many applications that process XML documents, making it a potential attack vector for malicious actors seeking to exploit XML processing weaknesses. The flaw specifically manifests in versions 0.4.0 and earlier, where the library fails to properly maintain system identifiers, formal public identifiers, and namespace declarations during repeated parsing and serialization cycles. This technical deficiency creates a scenario where XML documents can undergo unintended syntactic transformations that may alter their structure or behavior in ways that downstream applications did not anticipate or account for.
The core technical issue stems from xmldom's improper handling of XML document metadata during processing cycles. When documents containing maliciously crafted elements are parsed and then serialized multiple times, the system identifiers, formal public identifiers, and namespace declarations are not preserved correctly. This creates a potential for XML injection attacks or data manipulation scenarios where the integrity of XML documents is compromised. The vulnerability falls under CWE-119, which addresses improper restriction of operations within a limited access scope, as it allows for unexpected behavior in XML processing that extends beyond normal operational boundaries. The flaw is particularly concerning because it can be exploited to introduce subtle changes in XML documents that may not be immediately apparent but could have significant impacts on application behavior.
The operational impact of this vulnerability extends beyond simple data corruption, potentially enabling more sophisticated attacks that leverage XML processing inconsistencies. Applications that rely on xmldom for XML handling may experience unexpected behavior when processing documents that have been manipulated to exploit this flaw. The vulnerability can be particularly dangerous in environments where XML documents are processed through multiple stages or where the output of one XML processing operation feeds into another. Attackers could craft documents that appear legitimate but, when processed through the vulnerable library, produce unexpected results that could be leveraged for privilege escalation, data exfiltration, or other malicious activities. The issue aligns with ATT&CK technique T1059.007, which covers XML and JSON injection techniques, as it allows for manipulation of XML processing behavior through crafted input.
The fix implemented in version 0.5.0 addresses the core issue by ensuring proper preservation of system identifiers, formal public identifiers, and namespace declarations during parsing and serialization operations. This update represents a critical security enhancement that restores the expected behavior of the library in handling XML documents. Organizations should immediately upgrade to version 0.5.0 or later to eliminate the risk associated with this vulnerability. As a temporary mitigation measure, downstream applications should implement robust input validation mechanisms that can identify and reject maliciously crafted documents before they reach the xmldom library. This approach aligns with defensive programming practices and helps prevent exploitation of the vulnerability while maintaining system functionality. The recommended approach of input validation serves as an additional layer of protection that can be implemented immediately, providing security benefits even while planning for the full library upgrade.