XXE in Naturalintelligence Fast-Xml-Parser

CVE-2026-26278

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.

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.011 (64.6th percentile), scored .

Low probability of exploitation in the next 30 days, but still tracked. 65th percentile — 64.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-26278: held from 0.009 to 0.011.

EPSS over last 30 days · oldest: 0.009 · newest: 0.011 · change: +0.002

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-26278?
CVE-2026-26278 is a high-severity vulnerability in Naturalintelligence Fast-Xml-Parser, classified under Improper Restriction of Recursive Entity References in DTDs (XML Entity Expansion). CVSS score: 7.5/10. Published 2026-02-19.
How severe is CVE-2026-26278?
High severity. CVSS v3 base score is 7.5 out of 10.