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Moderate severityNVD Advisory· Published Mar 24, 2026· Updated Mar 25, 2026

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

CVE-2026-33349

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.

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Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
fast-xml-parsernpm
>= 4.0.0-beta.3, < 4.5.54.5.5
fast-xml-parsernpm
>= 5.0.0, < 5.5.75.5.7

Affected products

32

Patches

Vulnerability mechanics

References

5

News mentions

0

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