CVE-2026-33036: Naturalintelligence Fast-xml-parser vulnerability (CVSS 7.5)

Description

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.

CVSS 3.1 breakdown

Attack vectorNetwork

Attack complexityLow

Privileges requiredNone

User interactionNone

ScopeUnchanged

ConfidentialityNone

IntegrityNone

AvailabilityHigh

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected products

Naturalintelligence Fast-xml-parser≥ 4.0.1 < 5.5.6 · 4.0.0

Remediation

Fixed in

Naturalintelligence Fast-xml-parser≥ 5.5.6

Patch & vendor advisories

References

https://github.com/NaturalIntelligence/fast-xml-parser/commit/bd26122c838e6a55e7d7ac49b4ccc01a49999a01Patch
https://github.com/NaturalIntelligence/fast-xml-parser/releases/tag/v5.5.6ProductRelease Notes
https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-8gc5-j5rx-235rExploitMitigationVendor Advisory

Related CVEs

Recent high-severity vulnerabilities sharing a weakness type.

Frequently asked

Is CVE-2026-33036 being actively exploited?

CVE-2026-33036 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.73%.

How severe is CVE-2026-33036?

CVE-2026-33036 has a CVSS 3.1 base score of 7.5, rated high. EPSS estimates a 0.73% probability of exploitation.

Which products are affected by CVE-2026-33036?

Affected products include Naturalintelligence Fast-xml-parser.

How do I fix CVE-2026-33036?

Upgrade affected software to a fixed version, for example Naturalintelligence Fast-xml-parser 5.5.6 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-33036 against the real software inventory on every managed device, as each hourly inventory arrives. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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