CVE-2026-33036
Naturalintelligence Fast-Xml-Parser 4.0.1 – 5.5.6
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HSummary
CVE-2026-33036 is a high-severity XML Entity Expansion (CWE-776) vulnerability in Naturalintelligence Fast-Xml-Parser. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Endpoint Denial of Service (T1499); ranked at the 45th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified map to CM-6 (Configuration Settings) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-33036 is a vulnerability in the fast-xml-parser JavaScript library, affecting versions 4.0.0-beta.3 through 5.5.5. The issue allows a bypass of entity expansion limits, such as maxTotalExpansions and maxExpandedLength, which were implemented to address CVE-2026-26278. Specifically, numeric character references like &#NNN; or &#xHH; and standard XML entities evade these limits because the replaceEntitiesValue() function in OrderedObjParser.js only enforces expansion counting for DOCTYPE-defined entities, while the lastEntities loop handling numeric and standard entities performs no counting. This results in XML entity expansion denial of service (DoS), with a CVSS v3.1 score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) and mapping to CWE-776 (Insufficiently Controlled Recursive Entity Expansion, or XXE).
An attacker can exploit this vulnerability by supplying malicious XML input containing a large number of numeric entity references, such as 1 million instances of A, to any application using the affected fast-xml-parser versions for XML processing from JavaScript objects. No special privileges are required, and exploitation occurs over the network with low complexity. Successful attacks force approximately 147MB of memory allocation and heavy CPU usage, potentially crashing the process even when developers have configured strict entity expansion limits.
The vulnerability has been fixed in fast-xml-parser version 5.5.6, as detailed in the project's security advisory (GHSA-8gc5-j5rx-235r), release notes, and the fixing commit (bd26122c838e6a55e7d7ac49b4ccc01a49999a01). Security practitioners should update to version 5.5.6 or later and review applications parsing untrusted XML input for exposure.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-13573
Vulnerability Data
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.,…
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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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Secure configuration settings can disable DTD processing or cap entity expansion in XML parsers.
Input validation can enforce limits on XML entity expansion and recursion depth before parsing occurs.
Least-functionality configuration can prohibit unsafe DTD features or external entity resolution.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly prevent introduction of improper recursive entity handling in XML code.
Hardened configuration baselines can disable or limit DTD entity expansion in XML parsers.
Identifying and recording vulnerabilities can discover XML entity expansion flaws before exploitation.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect and block XML entity-expansion vulnerabilities before deployment.
Secure SDLC mandates input validation and parser hardening that can prevent unbounded entity expansion.
Application security requirements can explicitly call for limits on XML entity recursion and DTD processing.
Secure architecture principles discourage unsafe parser configurations that enable entity expansion attacks.
Secure coding standards directly require disabling or restricting recursive entity references in XML parsers.