CVE-2026-25896: fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names

Severity: critical (CVSS 9.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-02-20 · updated 2026-09-10
Weakness: CWE-185

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. From 4.1.3to before 5.3.5, a dot (.) in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, allowing an attacker to shadow built-in XML entities (<, >, &, ", ') with arbitrary values. This bypasses entity encoding and leads to XSS when parsed output is rendered. This vulnerability is fixed in 5.3.5.

Affected products

NaturalIntelligence fast-xml-parser: from 4.1.3 before 5.3.5 — fixed in 5.3.5

How to fix

Upgrade to a fixed version: NaturalIntelligence fast-xml-parser 5.3.5. Check the vendor advisory for mitigations.

References

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