NaturalIntelligence fast-xml-parser up to 5.3.5 XML Parser xml entity expansion

Summaryinfo

A vulnerability marked as problematic has been reported in NaturalIntelligence fast-xml-parser up to 5.3.5. This affects an unknown function of the component XML Parser. Performing a manipulation results in xml entity expansion. This vulnerability is cataloged as CVE-2026-26278. It is possible to initiate the attack remotely. There is no exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability classified as problematic has been found in NaturalIntelligence fast-xml-parser up to 5.3.5. This affects an unknown code of the component XML Parser. The manipulation with an unknown input leads to a xml entity expansion vulnerability. CWE is classifying the issue as 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. This is going to have an impact on availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-26278 since 02/12/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. The technical details are unknown and an exploit is not publicly available.

The vulnerability scanner Nessus provides a plugin with the ID 299641 (Linux Distros Unpatched Vulnerability : CVE-2026-26278), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 5.3.6 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 910dae5be2de2955e968558fadf6e8f74f117a77 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (299641) and CERT Bund (WID-SEC-2026-0772). Be aware that VulDB is the high quality source for vulnerability data.

Affected

• IBM App Connect Enterprise

Productinfo

Vendor

• NaturalIntelligence

Name

• fast-xml-parser

Version

• 5.3.0
• 5.3.1
• 5.3.2
• 5.3.3
• 5.3.4
• 5.3.5

License

• open-source

Website

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

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Be aware that VulDB is the high quality source for vulnerability data.

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