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.
Published · last modified .
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.
EPSS exploit prediction
EPSS: 0.005 (39.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 40th percentile — 39.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.000
Affected products
- Naturalintelligence Fast-Xml-Parser — versions 4.0.0, >= 4.0.0-beta.3, < 5.5.7
Weakness classification (CWE)
References
- github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-jp2q-39… (Exploit, Mitigation, Vendor Advisory)
- github.com/NaturalIntelligence/fast-xml-parser/commit/239b64aa1fc5c5455ddebbbb5… (Patch)
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.