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Fencer Vulns
medium 5.9

CVE-2026-33349 json

fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation

published 2026-03-19 7 mo ago modified 2026-09-10 1 mo ago also EUVD-2026-20080 , GHSA-jp2q-39xq-3w4g

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 40%
public exploits
0
none known

Summary

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.

Patch / fix

  • npm fast-xml-parser 4.5.5, 5.5.7
  • fast-xml-parser_project fast-xml-parser 5.5.7
  • naturalintelligence fast-xml-parser 5.5.7
  • naturalintelligence fast_xml_parser 5.5.7

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-33349 updated cvss 5.9

fast-xml-parser: fast-xml-parser: Denial of Service via unbounded entity expansion due to incorrect configuration limit handling

  • Red Hat Migration Toolkit for Applications 8 mta/mta-ui-rhel9 Red Hat Advanced Cluster Security 4 advanced-cluster-security/rhacs-main-rhel8 Red Hat Data Grid 8 apicurio/apicurio-registry-ui-rhel8, apicurio/apicurio-registry-ui-rhel9 Red Hat Developer Hub 1 rhdh/backstage-community-plugin-catalog-backend-module-scaffolder-relation-processor Red Hat OpenShift AI (RHOAI) rhoai/odh-mlflow-rhel9 Red Hat OpenShift GitOps openshift-gitops-1/argocd-rhel8, openshift-gitops-1/argocd-rhel9
    +3 more products
    Red Hat OpenShift Virtualization 4 container-native-virtualization/kubevirt-console-plugin, container-native-virtualization/kubevirt-console-plugin-rhel9 Red Hat Satellite 6 satellite/iop-vulnerability-frontend-rhel9, satellite/iop-host-inventory-frontend-rhel9 Red Hat Self-service automation portal 2 ansible-automation-platform/automation-portal
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Developer Hub 1.10 rhdh/rhdh-hub-rhel9 1780930740
    fixed: vendor fix RHSA-2026:24841
  • Red Hat Openshift Data Foundation 4.18 odf4/cephcsi-rhel9 1786705347, odf4/odf-cli-rhel9 1786705938, odf4/mcg-core-rhel9 1786705558, odf4/odf-console-rhel9 1786706577, odf4/mcg-rhel9-operator 1786705646, odf4/ocs-rhel9-operator 1786705802, +13 more
    fixed: vendor fix RHSA-2026:56431
  • Red Hat Developer Hub 1.10 rhdh/rhdh-rhel9-operator 1779927546, rhdh/rhdh-operator-bundle 1780961472 Red Hat Openshift Data Foundation 4.18 odf4/mcg-operator-bundle 1786708633, odf4/ocs-operator-bundle 1786708778, odf4/odf-operator-bundle 1786709119, odf4/cephcsi-operator-bundle 1786708508, odf4/odr-hub-operator-bundle 1786709184, odf4/rook-ceph-operator-bundle 1786709254, +7 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.47% chance of exploitation in the next 30 days, higher than 40% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-jp2q-39xq-3w4g referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
npm fast-xml-parser
>=4.0.0-beta.3, <4.5.5
>=5.0.0, <5.5.7
4.5.5
5.5.7

Affected products

  • NaturalIntelligence fast-xml-parser

Classification

Weakness
  • CWE-1284 Improper Validation of Specified Quantity in Input

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.