Vulnerability in Naturalintelligence Fast-Xml-Parser
CVE-2026-33349
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.
EPSS: 0.004 (37.5th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
Affected products
- Naturalintelligence Fast-xml-parser — versions 4.0.0, >= 4.0.0-beta.3, < 5.5.7
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Mitigation, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
Frequently asked questions
- What is CVE-2026-33349?
- CVE-2026-33349 is a medium-severity vulnerability in Naturalintelligence Fast-Xml-Parser, classified under Improper Validation of Specified Quantity in Input. CVSS score: 5.9/10. Published 2026-03-24.
- How severe is CVE-2026-33349?
- Medium severity. CVSS v3 base score is 5.9 out of 10.