CVE-2026-26278 in fast-xml-parser
Summary
by MITRE • 02/19/2026
fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. In versions 4.1.3 through 5.3.5, the XML parser can be forced to do an unlimited amount of entity expansion. With a very small XML input, it’s possible to make the parser spend seconds or even minutes processing a single request, effectively freezing the application. Version 5.3.6 fixes the issue. As a workaround, avoid using DOCTYPE parsing by `processEntities: false` option.
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Analysis
by VulDB Data Team • 03/18/2026
The vulnerability identified as CVE-2026-26278 affects the fast-xml-parser library, a popular JavaScript tool for XML processing that operates without requiring C/C++ bindings or callback mechanisms. This library enables developers to validate XML documents, convert XML structures into JavaScript objects, and generate XML content from JavaScript data structures. The flaw manifests in versions 4.1.3 through 5.3.5 where the parser fails to properly constrain entity expansion during XML processing. This represents a classic denial of service vulnerability that can be exploited through malicious XML input designed to trigger excessive computational overhead.
The technical implementation of this vulnerability stems from insufficient validation of XML entity declarations within the parsing process. When the parser encounters XML with nested or recursive entity references, particularly within DOCTYPE declarations, it recursively expands these entities without adequate limits on the expansion depth or total output size. This behavior creates a potential for exponential growth in processing time and memory consumption, where minimal input can generate massive output requiring considerable computational resources. The vulnerability aligns with CWE-400, which specifically addresses unchecked resource consumption, and more broadly with CWE-129, concerning improper validation of array indices. Attackers can craft specially formatted XML documents that force the parser to perform extensive entity expansion operations, effectively causing the application to hang or become unresponsive.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise application availability and system stability. When exploited, the vulnerability can cause individual requests to consume seconds or minutes of processing time, effectively freezing the application and preventing legitimate users from accessing services. This makes it particularly dangerous in web applications where XML processing is common, such as API endpoints, data import systems, or configuration management tools. The vulnerability affects the fundamental parsing operations of the library, meaning any application using fast-xml-parser for XML processing is at risk, regardless of the specific use case. From an attacker's perspective, this represents a low-effort, high-impact vector that requires minimal input to generate significant resource consumption, making it attractive for denial of service attacks.
The remediation for CVE-2026-26278 is straightforward yet requires careful application of security best practices. Version 5.3.6 of fast-xml-parser includes the necessary fixes to properly limit entity expansion and prevent unlimited resource consumption during XML processing. Organizations should immediately update to this patched version to eliminate the vulnerability. As a temporary workaround, developers can implement the `processEntities: false` option when configuring the parser, which disables entity processing entirely and prevents the expansion behavior that leads to the vulnerability. This approach aligns with defensive programming principles and ATT&CK technique T1496, which focuses on resource exhaustion attacks. Security teams should also consider implementing input validation and rate limiting mechanisms around XML processing endpoints to provide additional layers of protection. The vulnerability demonstrates the importance of validating XML input and implementing proper resource constraints in parsing libraries, particularly those handling untrusted data from external sources.