Low

fast-xml-parser regex vulnerability patch could be improved from a safety perspective

Disclosed: 15.06.2023
Updated: 11.10.2026 10:22

Summary

This is a comment on https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-6w63-h3fj-q4vw and the patches fixing it.

Details

The code which validates a name calls the validator: https://github.com/NaturalIntelligence/fast-xml-parser/blob/ecf6016f9b48aec1a921e673158be0773d07283e/src/xmlparser/DocTypeReader.js#L145-L153 This checks for the presence of an invalid character. Such an approach is always risky, as it is so easy to forget to include an invalid character in the list. A safer approach is to validate entity names against the XML specification: https://www.w3.org/TR/xml11/#sec-common-syn - an ENTITY name is a Name:

[4]   NameStartChar ::= ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] |
                        [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] |
                        [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a]  NameChar ::= NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]   Name ::= NameStartChar (NameChar)*

so the safest way to validate an entity name is to build a regex to represent this expression and check whether the name given matches the regex. (Something along the lines of /^[name start char class][name char class]*$/.) There's probably a nice way to simplify the explicit list rather than typing it out verbatim using Unicode character properties, but I don't know enough to do so.

Affected software

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Platform	Package	Vulnerable versions	Fixed in
npm package	fast-xml-parser	>= 4.2.4, < 4.2.5	4.2.5

References

CVSS score -

Weakness (CWE): -
Platform: npm
Package: fast-xml-parser
Disclosed: 15.06.2023
Updated: 11.10.2026 10:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-gpv5-7x3g-ghjv - CC BY 4.0

Vulnerability history: fast-xml-parser

All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (12) in the archive

Total entries: 12
First → latest: 08.2021 07.2026
Without a fix: 0
Exploited in attacks: 0

Severity breakdown

Critical 1
High 6
Medium 3
Low 2

Disclosures per year

2021: 1
2022: 0
2023: 2
2024: 1
2025: 0
2026: 8

21.07.2026

fast-xml-parser: Repeated DOCTYPE declarations reset entity expansion limits

High CVE-2026-73569 >= 5.9.3, < 5.10.1 5.10.1
17.03.2026

fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)

High · 7.5 CVE-2026-33036 >= 5.0.0, < 5.5.6, >= 4.0.0-beta.3, < 4.5.5 5.5.6, 4.5.5
20.02.2026

fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names

Critical · 9.3 CVE-2026-25896 >= 5.0.0, < 5.3.5, >= 4.1.3, < 4.5.4 5.3.5, 4.5.4
17.02.2026

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

High · 7.5 CVE-2026-26278 >= 4.1.3, < 4.5.4, >= 5.0.0, < 5.3.6 4.5.4, 5.3.6
30.01.2026

fast-xml-parser has RangeError DoS Numeric Entities Bug

High · 7.5 CVE-2026-25128 >= 5.0.9, < 5.3.4 5.3.4
29.07.2024

fast-xml-parser vulnerable to ReDOS at currency parsing

High · 7.5 CVE-2024-41818 >= 4.3.5, < 4.4.1 4.4.1
15.06.2023

fast-xml-parser regex vulnerability patch could be improved from a safety perspective this entry

Low ghsa-gpv5-7x3g-ghjv >= 4.2.4, < 4.2.5 4.2.5
06.06.2023

fast-xml-parser vulnerable to Regex Injection via Doctype Entities

High · 7.5 CVE-2023-34104 >= 4.1.3, < 4.2.4 4.2.4

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