Summary 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.

Details There is a check in DocTypeReader.js that tries to prevent entity expansion attacks by rejecting entities that reference other entities (it looks for & inside entity values). This does stop classic “Billion Laughs” payloads.

However, it doesn’t stop a much simpler variant.

If you define one large entity that contains only raw text (no & characters) and then reference it many times, the parser will happily expand it every time. There is no limit on how large the expanded result can become, or how many replacements are allowed.

The problem is in replaceEntitiesValue() inside OrderedObjParser.js. It repeatedly runs val.replace() in a loop, without any checks on total output size or execution cost. As the entity grows or the number of references increases, parsing time explodes.

Relevant code:

DocTypeReader.js (lines 28–33): entity registration only checks for &

OrderedObjParser.js (lines 439–458): entity replacement loop with no limits

PoC

js const { XMLParser } = require('fast-xml-parser');

const entity = 'A'.repeat(1000); const refs = '&big;'.repeat(100); const xml = <!DOCTYPE foo [<!ENTITY big "${entity}">]><root>${refs}</root>;

console.time('parse'); new XMLParser().parse(xml); // ~4–8 seconds for ~1.3 KB of XML console.timeEnd('parse');

// 5,000 chars × 100 refs takes 200+ seconds // 50,000 chars × 1,000 refs will hang indefinitely

Impact This is a straightforward denial-of-service issue.

Any service that parses user-supplied XML using the default configuration is vulnerable. Since Node.js runs on a single thread, the moment the parser starts expanding entities, the event loop is blocked. While this is happening, the server can’t handle any other requests.

In testing, a payload of only a few kilobytes was enough to make a simple HTTP server completely unresponsive for several minutes, with all other requests timing out.

Workaround

Avoid using DOCTYPE parsing by processEntities: false option.

Entity encoding bypass via regex injection in DOCTYPE entity names

Summary

A dot (.) in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, allowing an attacker to shadow built-in XML entities (&lt;, &gt;, &amp;, &quot;, &apos;) with arbitrary values. This bypasses entity encoding and leads to XSS when parsed output is rendered.

Details

The fix for CVE-2023-34104 addressed some regex metacharacters in entity names but missed . (period), which is valid in XML names per the W3C spec.

In DocTypeReader.js, entity names are passed directly to RegExp():

js entities[entityName] = { regx: RegExp(&${entityName};, "g"), val: val };

An entity named l. produces the regex /&l.;/g where . matches any character, including the t in &lt;. Since DOCTYPE entities are replaced before built-in entities, this shadows &lt; entirely.

The same issue exists in OrderedObjParser.js:81 (addExternalEntities), and in the v6 codebase - EntitiesParser.js has a validateEntityName function with a character blacklist, but . is not included:

js // v6 EntitiesParser.js line 96 const specialChar = "!?\\/[]$%{}^&()<>|+"; // no dot

Shadowing all 5 built-in entities

| Entity name | Regex created | Shadows | |---|---|---| | l. | /&l.;/g | &lt; | | g. | /&g.;/g | &gt; | | am. | /&am.;/g | &amp; | | quo. | /&quo.;/g | &quot; | | apo. | /&apo.;/g | &apos; |

PoC

js const { XMLParser } = require("fast-xml-parser");

const xml = <?xml version="1.0"?> <!DOCTYPE foo [ <!ENTITY l. "<img src=x onerror=alert(1)>"> ]> <root> <text>Hello &lt;b&gt;World&lt;/b&gt;</text> </root>;

const result = new XMLParser().parse(xml); console.log(result.root.text); // Hello <img src=x onerror=alert(1)>b>World<img src=x onerror=alert(1)>/b>

No special parser options needed - processEntities: true is the default.

When an app renders result.root.text in a page (e.g. innerHTML, template interpolation, SSR), the injected <img onerror> fires.

&amp; can be shadowed too:

js const xml2 = <?xml version="1.0"?> <!DOCTYPE foo [ <!ENTITY am. "'; DROP TABLE users;--"> ]> <root>SELECT FROM t WHERE name='O&amp;Brien'</root>;

const r = new XMLParser().parse(xml2); console.log(r.root); // SELECT FROM t WHERE name='O'; DROP TABLE users;--Brien'

Impact

This is a complete bypass of XML entity encoding. Any application that parses untrusted XML and uses the output in HTML, SQL, or other injection-sensitive contexts is affected.

- Default config, no special options - Attacker can replace any &lt; / &gt; / &amp; / &quot; / &apos; with arbitrary strings - Direct XSS vector when parsed XML content is rendered in a page - v5 and v6 both affected

Suggested fix

Escape regex metacharacters before constructing the replacement regex:

js const escaped = entityName.replace(/[.+?^${}()|[\]\\]/g, '\\$&'); entities[entityName] = { regx: RegExp(&${escaped};, "g"), val: val };

For v6, add . to the blacklist in validateEntityName:

js const specialChar = "!?\\/[].{}^&()<>|+";

Severity

Entity decoding is a fundamental trust boundary in XML processing. This completely undermines it with no preconditions.