CVE-2026-26278: fast-xml-parser affected by DoS through entity expansion in DOCTYPE (no expansion limit)

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-02-19 · updated 2026-09-10
Weakness: CWE-776

Description

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.

Affected products

NaturalIntelligence fast-xml-parser: from 4.1.3 before 5.3.6 — fixed in 5.3.6

How to fix

Upgrade to a fixed version: NaturalIntelligence fast-xml-parser 5.3.6. Check the vendor advisory for mitigations.

References

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