CVE-2026-33036: fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-20 · updated 2026-06-17
Weakness: CWE-776

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.

Affected products

NaturalIntelligence fast-xml-parser: from 4.0.1 before 5.5.6; 4.0.0, 4.0.0-beta3, 4.0.0-beta4, 4.0.0-beta5, 4.0.0-beta6, 4.0.0-beta7 (+1) — fixed in 5.5.6

How to fix

Upgrade to a fixed version: NaturalIntelligence fast-xml-parser 5.5.6. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-33036 or the next vulnerability affects your stack