CVE-2026-33349: naturalintelligence fast-xml-parser

Vulnerability summary

CVE-2026-33349 is a medium (CVSS 5.9) vulnerability in naturalintelligence fast-xml-parser. EPSS estimates a 0.48% chance of exploitation in the next 30 days, higher than 39.7% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 24-03-2026
   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. 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.

• CWE-1284Improper Validation of Specified Quantity in InputThe product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties.