Executive brief
fast-xml-parser is a widely-used JavaScript library for parsing and processing XML documents. The library contains a flaw where numeric entity references (such as A) bypass configured entity expansion limits, allowing attackers to trigger denial of service through excessive memory and CPU consumption. An application developer believing they are protected by strict entity limits can still be compromised.
Technical details
The vulnerability is a CWE-776 XML Entity Expansion Denial of Service. The root cause is incomplete remediation of CVE-2026-26278: the replaceEntitiesValue() function in OrderedObjParser.js tracks entity expansion for DOCTYPE-defined entities but not for numeric character references (&#NNN; and &#xHH;) or standard XML entities, which are processed through a separate code path. An attacker who can provide XML input can craft documents with massive numbers of numeric entity references—100,000 references produce 500,000 characters despite maxExpandedLength=100, and 1M references consume ~147MB. Network vector, no authentication required. Patches available in versions 4.5.5 and 5.5.6.
Affected products
- NaturalIntelligence fast-xml-parser >=4.0.0-beta.3, <=5.5.5
Timeline
- 2026-03-17: disclosed
- 2026-03-17: patched: versions 4.5.5 and 5.5.6