Vulnerability Description
fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, the DocTypeReader in fast-xml-parser uses JavaScript truthy checks to evaluate maxEntityCount and maxEntitySize configuration limits. When a developer explicitly sets either limit to 0 — intending to disallow all entities or restrict entity size to zero bytes — the falsy nature of 0 in JavaScript causes the guard conditions to short-circuit, completely bypassing the limits. An attacker who can supply XML input to such an application can trigger unbounded entity expansion, leading to memory exhaustion and denial of service. This issue has been patched in version 5.5.7.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Naturalintelligence | Fast-Xml-Parser | >= 4.0.1, < 4.5.5 |
Related Weaknesses (CWE)
References
- https://github.com/NaturalIntelligence/fast-xml-parser/commit/239b64aa1fc5c5455dPatch
- https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-ExploitMitigationVendor Advisory
- https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-ExploitMitigationVendor Advisory
FAQ
What is CVE-2026-33349?
CVE-2026-33349 is a vulnerability with a CVSS score of 5.9 (MEDIUM). fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. From version 4.0.0-beta.3 to before version 5.5.7, the DocTypeReader in fast-xml-parser uses Java...
How severe is CVE-2026-33349?
CVE-2026-33349 has been rated MEDIUM with a CVSS base score of 5.9/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-33349?
Check the references section above for vendor advisories and patch information. Affected products include: Naturalintelligence Fast-Xml-Parser.