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Public proof of conceptFix available

CVE-2026-33349

A public proof of concept exists; no exploitation attested.

  • Published2026-03-24
  • Fixed in5.5.7 · 4.5.5
  • WeaknessCWE-1284

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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.

Timeline

dates in UTC
  1. 2026-03-2420:16CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-33349?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-33349?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Naturalintelligence fast-xml-parser
BranchAffectedFixed in
5.x≥ 5.0.0, < 5.5.75.5.7
4.x≥ 4.0.1, < 4.5.54.5.5
4.x≥ 4.0.0-beta.3, < 4.5.54.5.5
4.0= 4.0.0no fix published

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • RHSA-2026:56431 Red Hat Product Security68 products fixed

    Red Hat Security Advisory: Red Hat OpenShift Data Foundation 4.18.27 security, enhancement & bug fix update

Full record on NVD

Fix

  1. Update Naturalintelligence fast-xml-parser to 5.5.7 · 4.5.5
  2. Vendor fix Red Hat Product Security RHSA-2026:56431 · 68 productsBefore applying this update, make sure all previously released errata relevant to your system have been applied. For details on how to apply this update, refer to: https://docs.redhat.com/en/documentation/red_hat_openshift_data_foundation/4.18/html/updating_openshift_data_foundation/updating-ocs-…
  3. Workaround from the vendor Red Hat Product Security RHSA-2026:56431 · 81 productsMitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • NVDCVE recordread 2026-10-23
  • CISA-ADPSSVC decisionpoc2026-03-25
  • OpenCVESSVC decisionpoc—
  • CNACVSS 3.15.9read 2026-10-23
  • CNAweakness (CWE)CWE-1284read 2026-10-23
  • FIRST EPSSEPSS score0.5 %2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Published
2026-03-24
Last Modified
2026-06-17
NVD status
Analyzed

Frequently asked questions

What is 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.

Is CVE-2026-33349 actively exploited?

A proof-of-concept exploit exists for CVE-2026-33349, but active exploitation has not been confirmed at this time.

What is the CVSS score for CVE-2026-33349?

CVE-2026-33349 has a CVSS v3 base score of 5.9 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.

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