CVE-2026-33036: naturalintelligence fast-xml-parser XML Entity Expansion

Vulnerability summary

CVE-2026-33036 is a high (CVSS 7.5) XML Entity Expansion vulnerability in naturalintelligence fast-xml-parser. EPSS estimates a 0.76% chance of exploitation in the next 30 days, higher than 54.4% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 20-03-2026
   PublishedVulnerability first disclosed.
2. 25-03-2026
   Last modifiedVulnerability information updated.

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.

• CWE-776Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')The product uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities.