NaturalIntelligence fast-xml-parser up to 5.5.5 replaceEntitiesValue xml entity expansion

Summaryinfo

A vulnerability labeled as problematic has been found in NaturalIntelligence fast-xml-parser up to 5.5.5. Affected by this vulnerability is the function replaceEntitiesValue. Executing a manipulation can lead to xml entity expansion. This vulnerability is tracked as CVE-2026-33036. The attack can be launched remotely. No exploit exists. The affected component should be upgraded.

Detailsinfo

A vulnerability classified as critical was found in NaturalIntelligence fast-xml-parser up to 5.5.5. This vulnerability affects the function replaceEntitiesValue. The manipulation with an unknown input leads to a xml entity expansion vulnerability. The CWE definition for the vulnerability is CWE-776. The product uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. Versions 4.0.0-beta.3 through 5.5.5 contain a bypass vulnerability where numeric character references (&#NNN;, &#xHH;) and standard XML entities completely evade the entity expansion limits (e.g., maxTotalExpansions, maxExpandedLength) added to fix CVE-2026-26278, enabling XML entity expansion Denial of Service. The root cause is that replaceEntitiesValue() in OrderedObjParser.js only enforces expansion counting on DOCTYPE-defined entities while the lastEntities loop handling numeric/standard entities performs no counting at all. An attacker supplying 1M numeric entity references like A can force ~147MB of memory allocation and heavy CPU usage, potentially crashing the process—even when developers have configured strict limits. This issue has been fixed in version 5.5.6.

The advisory is available at github.com. This vulnerability was named CVE-2026-33036 since 03/17/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Technical details are known, but there is no available exploit.

The vulnerability scanner Nessus provides a plugin with the ID 303270 (Linux Distros Unpatched Vulnerability : CVE-2026-33036), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 5.5.6 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (303270). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Vendor

• NaturalIntelligence

Name

• fast-xml-parser

Version

• 5.5.0
• 5.5.1
• 5.5.2
• 5.5.3
• 5.5.4
• 5.5.5

Website

• Product: https://github.com/NaturalIntelligence/fast-xml-parser/

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If you want to get best quality of vulnerability data, you may have to visit VulDB.

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