Vulnerability · Published Mar 24, 2026 · Assigned by GitHub_M
CVE-2026-33349fast-xml-parser: Entity Expansion Limits Bypassed When Set to Zero Due to JavaScript Falsy Evaluation
Medium 5.9
In plain English
A medium-severity security flaw in fast-xml-parser from NaturalIntelligence. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | node-webfont | no fix listed | DEBIAN-CVE-2026-33349 |
| Debian:13 | node-webfont | no fix listed | DEBIAN-CVE-2026-33349 |
| Debian:14 | node-webfont | no fix listed | DEBIAN-CVE-2026-33349 |
| Ubuntu:24.04:LTS | node-webfont | no fix listed | UBUNTU-CVE-2026-33349 |
| Ubuntu:25.10 | node-webfont | no fix listed | UBUNTU-CVE-2026-33349 |
| Ubuntu:26.04:LTS | node-webfont | no fix listed | UBUNTU-CVE-2026-33349 |
| Ubuntu:26.10 | node-webfont | no fix listed | UBUNTU-CVE-2026-33349 |
| npm | fast-xml-parser | 5.5.7 | GHSA-jp2q-39xq-3w4g |
| npm | fast-xml-parser | 4.5.5 | GHSA-jp2q-39xq-3w4g |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| NaturalIntelligence | fast-xml-parser | >= 4.0.0-beta.3, < 5.5.7 |
Weakness: CWE-1284 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H