CVE-2026-73569 in fast-xml-parser
Summary
by MITRE • 08/13/2026
fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From 5.9.3 until 5.10.1, src/xmlparser/OrderedObjParser.js processes multiple DOCTYPE declarations within a single XML document and passes each declaration's entities through addInputEntities(). addInputEntities() resets maxTotalExpansions and maxExpandedLength every time it is called, allowing additional DOCTYPE declarations to repeatedly reset the configured entity-expansion limits during one parse operation. A crafted XML document can then cause excessive CPU use, event-loop blocking, memory exhaustion, and process termination. This issue is fixed in version 5.10.1.
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Analysis
by VulDB Data Team • 08/13/2026
The fast-xml-parser library presents a critical vulnerability in versions 5.9.3 through 5.10.1 that fundamentally undermines XML parsing security through improper entity expansion handling. This flaw resides in the OrderedObjParser.js component where multiple DOCTYPE declarations within a single XML document trigger a dangerous cascade of entity processing operations. The vulnerability manifests when the addInputEntities() function is invoked repeatedly during parsing, as it systematically resets crucial security parameters including maxTotalExpansions and maxExpandedLength with each invocation. This resetting behavior effectively nullifies the configured entity expansion limits that should prevent excessive resource consumption during XML processing operations.
The technical implementation of this vulnerability creates a path for resource exhaustion attacks through what is known as a "billion laughs" style attack pattern. When an attacker crafts a malicious XML document containing multiple DOCTYPE declarations, each declaration triggers the addInputEntities() function which resets the expansion limits, allowing the parser to continue processing entities beyond normal constraints. This design flaw enables attackers to consume excessive CPU cycles and memory resources, leading to event-loop blocking that prevents other operations from executing while the parser remains occupied with entity expansion. The vulnerability operates at the intersection of CWE-400 (Uncontrolled Resource Consumption) and CWE-770 (Allocation of Resources Without Limits or Throttling), as it allows unlimited resource consumption through improper limit management during XML parsing operations.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially complete system compromise through denial-of-service conditions. During a successful exploitation, the parser becomes vulnerable to excessive CPU utilization that can block the entire event loop in Node.js environments, effectively halting other processing tasks while the malicious XML is being parsed. Memory exhaustion becomes another critical concern as each DOCTYPE declaration resets expansion limits, allowing unlimited entity expansion within a single parse operation. This behavior can result in process termination when system resources are exhausted, creating an attack vector that could be exploited by adversaries to disrupt services or potentially cause system instability. The vulnerability particularly affects applications that process untrusted XML input without proper validation or rate limiting mechanisms.
Mitigation strategies for this vulnerability require immediate version upgrading to 5.10.1 or later where the fix has been implemented to prevent the resetting of expansion limits during multiple DOCTYPE declaration processing. Organizations should implement comprehensive input validation and sanitization procedures for all XML data, including setting appropriate entity expansion limits at the application level regardless of library protections. Network-level defenses such as rate limiting and resource monitoring can provide additional protection layers against exploitation attempts, while regular security audits should verify that no other XML parsing components within the application stack are vulnerable to similar issues. The fix addresses the core problem by ensuring that expansion limits remain consistent throughout a single parse operation rather than being reset with each DOCTYPE declaration, aligning with industry best practices for preventing resource exhaustion attacks in XML processing environments and supporting ATT&CK technique T1496 (Resource Exhaustion) mitigation strategies.