XXE in Naturalintelligence Fast-Xml-Parser
CVE-2026-33036
fast-xml-parser allows users to process XML from JS object without C/C++ based libraries or callbacks. Versions 4.0.0-beta.3 through 5.5.5 contain a bypass vulnerability where numeric character references (&#NNN;, &#xHH;) and standard XML entities completely evade the entity expansion limits (e.g., maxTotalExpansions, maxExpandedLength) added to fix CVE-2026-26278, enabling XML entity expansion Denial of Service. The root cause is that replaceEntitiesValue() in OrderedObjParser.js only enforces expansion counting on DOCTYPE-defined entities while the lastEntities loop handling numeric/standard entities performs no counting at all. An attacker supplying 1M numeric entity references like A can force ~147MB of memory allocation and heavy CPU usage, potentially crashing the process—even when developers have configured strict limits. This issue has been fixed in version 5.5.6.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.008 (54.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 54th percentile — 54.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.006 · newest: 0.008 · change: +0.002
Affected products
- Naturalintelligence Fast-Xml-Parser — versions 4.0.0, >= 4.0.0-beta.3, < 5.5.6
Weakness classification (CWE)
References
- github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-8gc5-j5… (Exploit, Mitigation, Vendor Advisory)
- github.com/NaturalIntelligence/fast-xml-parser/commit/bd26122c838e6a55e7d7ac49b… (Patch)
- github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.5.6 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-33036?
- CVE-2026-33036 is a high-severity vulnerability in Naturalintelligence Fast-Xml-Parser, classified under Improper Restriction of Recursive Entity References in DTDs (XML Entity Expansion). CVSS score: 7.5/10. Published 2026-03-20.
- How severe is CVE-2026-33036?
- High severity. CVSS v3 base score is 7.5 out of 10.