CVE-2026-26278

fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. In versions 4.1.3 through 5.3.5, the XML parser can be forced to do an unlimited amount of entity expansion. With a very small XML input, it’s possible to make the parser spend seconds or even minutes processing a single request, effectively freezing the application. Version 5.3.6 fixes the issue. As a workaround, avoid using DOCTYPE parsing by `processEntities: false` option.

What this means for your business

It affects Naturalintelligence Fast-Xml-Parser. It matters if your company, or a supplier that handles your data, runs it.
Code showing how to exploit it has been published, but real attacks have not been confirmed.
An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 0.8% chance of attack attempts being seen in the next 30 days, ranking above 55% of all known flaws.

What to do

1Check whether your company or your suppliers run Naturalintelligence Fast-Xml-Parser, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

Check if you’re exposed Book a call

Scoring

CVSS: 7.5 (v3.1)
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness: CWE-776
Assigned by: security-advisories@github.com

Dates

Published: 2026-02-19
Last modified: 2026-09-10
Sources: NVD

Affected products

Naturalintelligence Fast-Xml-Parser4.1.3 - 5.3.6

As listed in the NVD configuration data. Not a statement about your estate.