CVE-2026-26278 fast-xml-parser affected by DoS through entity expansion in DOCTYPE (no expansion limit)
An attacker can cause denial of service in applications using fast-xml-parser by submitting XML with unlimited entity expansion, freezing the application.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-26278?
CVE-2026-26278 is a high-severity denial of service vulnerability in NaturalIntelligence fast-xml-parser, published on 19 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-776
Which versions are affected by CVE-2026-26278?
4 open source packages ship an affected version, including fast-xml-parser. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
fast-xml-parser npm | From 4.1.3 | 4.5.4 |
fast-xml-parser npm | From 4.1.3 | 4.5.4 |
fast-xml-parser npm | From 5.0.0 | 5.3.6 |
fast-xml-parser npm | From 5.0.0 | 5.3.6 |
Products include Naturalintelligence Fast-Xml-Parser.
How do I fix CVE-2026-26278?
Upgrade fast-xml-parser to a fixed version for your release line: 4.5.4 or 5.3.6.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-26278 being exploited?
There is no known exploitation of CVE-2026-26278 in the wild and no public exploit code.
Other vulnerabilities in NaturalIntelligence fast-xml-parser
- CVE-2026-25896 fast-xml-parser has an entity encoding bypass via regex injection in DOCTYPE entity names
- CVE-2026-33036 fast-xml-parser affected by numeric entity expansion bypassing all entity expansion limits (incomplete fix for CVE-2026-26278)
- CVE-2026-25128 fast-xml-parser has RangeError DoS Numeric Entities Bug
- CVE-2024-41818 ReDOS at currency parsing fast-xml-parser
- CVE-2023-34104 Regex Injection via Doctype Entities
Sources
Also known as CLEANSTART-2026-DE27040, CLEANSTART-2026-FQ78730, DEBIAN-CVE-2026-26278, ECHO-058b-a6ce-5049, GHSA-jmr7-xgp7-cmfj, MINI-474x-5c6m-9w4f and 15 more.