Cyber Resilience

CVE-2026-33349

Naturalintelligence Fast-Xml-Parser 4.0.1 – 4.5.5

Public PoC
Published
24 March 2026
Modified
26 March 2026
Patch / advisory
View advisory
CVSS Score v3.1 5.9
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS Score 0.0045 37th percentile
Risk Priority 46 floored blend · peak EPSS

Summary

CVE-2026-33349 is a medium-severity Improper Validation of Specified Quantity in Input (CWE-1284) vulnerability in Naturalintelligence Fast-Xml-Parser. Its CVSS base score is 5.9 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 37th 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 SI-10 (Information Input Validation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

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…

more

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.

CWE(s)
CWE-1284

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
T1499 Endpoint Denial of Service Impact
Adversaries may perform Endpoint Denial of Service (DoS) attacks to degrade or block the availability of services to users.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-27942Same product: Naturalintelligence Fast-Xml-Parser
CVE-2026-25128Same product: Naturalintelligence Fast-Xml-Parser
CVE-2026-26278Same product: Naturalintelligence Fast-Xml-Parser
CVE-2026-33036Same product: Naturalintelligence Fast-Xml-Parser
CVE-2026-41650Same product: Naturalintelligence Fast-Xml-Parser
CVE-2024-41818Same product: Naturalintelligence Fast-Xml-Parser
CVE-2023-34104Same product: Naturalintelligence Fast-Xml-Parser
CVE-2026-25896Same product: Naturalintelligence Fast-Xml-Parser
CVE-2024-8558Shared CWE-1284
CVE-2023-27941Shared CWE-1284

Affected Assets

naturalintelligence
fast-xml-parser
4.0.0 · 4.0.1 — 4.5.5 · 5.0.0 — 5.5.7

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V6.7.2
  • V1.4.2
  • V2.1.1
  • V2.2.1

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 requires validity checks on information inputs, directly stopping acceptance of malformed quantities before they can be used.

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.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper quantity/length validation in input handling.

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.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development can detect missing quantity validation but does not itself implement the control.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle mandates input validation for quantity fields, directly preventing CWE-1284.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly call for validation of size/length inputs, mitigating CWE-1284.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles encourage robust input handling but do not specifically address quantity validation.

A.8.28 Secure coding partial match
prevents

Secure coding standards require explicit checks on numeric quantity inputs, fully addressing CWE-1284.

References