GHSA-wh4c-j3r5-mjhp xmldom: XML injection via unsafe CDATA serialization allows attacker-controlled markup insertion
Summary`@xmldom/xmldom` allows attacker-controlled strings containing the CDATA terminator `]]>` to be inserted into a `CDATASection` node. During serialization, `XMLSerializer` emitted the CDATA content verbatim without rejecting or safely splitting the terminator. As a result, data intended to remain text-only became **active XML markup** in the serialized output, enabling XML structure
injection and downstream business-logic manipulation.The sequence `]]>` is not allowed inside CDATA content and must be rejected or safely handled during serialization. ([MDN Web Docs](https://developer.mozilla.org/))Attack surface`Document.createCDATASection(data)` is the most direct entry point, but it is not the only one. The WHATWG DOM spec intentionally does not validate `]]>` in mutation methods — only `createCDATASection` carries that guard. The following paths therefore also allow `]]>` to enter a CDATASection node and reach the serializer:`CharacterData.appendData()``CharacterData.replaceData()``CharacterData.insertData()`Direct assignment to `.data`Direct assignment to `.textContent`(Note: assigning to `.nodeValue` does **not** update `.data` in this implementation — the serializer reads `.data` directly — so `.nodeValue` is not an exploitable path.)Parse pathParsing XML that contains a CDATA section is **not** affected. The SAX parser's non-greedy `CDSect` regex stops at the first `]]>`, so parsed CDATA data never contains the terminator.ImpactIf an application uses `xmldom` to generate "trusted" XML documents that embed **untrusted user input** inside CDATA (a common pattern in exports, feeds, SOAP/XML integrations, etc.), an attacker can inject additional XML elements/attributes into the generated document.This can lead to:Integrity violation of generated XML documents.Business-logic injection in downstream consumers (e.g., injecting `<approved>true</approved>`,  `<role>admin</role>`, workflow flags, or other security-relevant elements).Unexpected privilege/workflow decisions if downstream logic assumes injected nodes cannot appear.This issue does **not** require malformed parsers or browser behavior; it is caused by serialization producing attacker-influenced XML markup.Root Cause (with file + line numbers)**File:** `lib/dom.js`1. No validation in `createCDATASection``createCDATASection: function (data)` accepts any string and appends it directly.**Lines 2216–2221** (0.9.8)2. Unsafe CDATA serializationSerializer prints CDATA sections as:without handling `]]>` in the data.**Lines 2919–2920** (0.9.8)Because CDATA content is emitted verbatim, an embedded `]]>` closes the CDATA section early and the remainder of the attacker-controlled payload is interpreted as markup in the serialized XML.Proof of Concept — Fix A: `createCDATASection` now throwsOn patched versions, passing `]]>` directly to `createCDATASection` throws `InvalidCharacterError` instead of silently accepting the payload:Expected output on patched versions:Proof of Concept — Fix B: mutation vector now safeOn patched versions, injecting `]]>` via a mutation method (`appendData`, `replaceData`, `.data =`, `.textContent =`) no longer produces injectable output. The serializer splits the terminator so the result round-trips as safe text:Expected output on patched versions:Fix AppliedBoth mitigations were implemented:Option A — Strict/spec-aligned: reject `]]>` in `createCDATASection()``Document.createCDATASection(data)` now throws `InvalidCharacterError` (per the [WHATWG DOM spec](https://dom.spec.whatwg.org/#dom-document-createcdatasection)) when `data` contains `]]>`. This closes the direct entry point.Code that previously passed a string containing `]]>` to `createCDATASection` and relied on the silent/unsafe behaviour will now receive `InvalidCharacterError`. Use a mutation method such as `appendData` if you intentionally need `]]>` in a CDATASection node's data (the serializer split in Option B will keep the output safe).Option B — Defensive serialization: split the terminator during serialization`XMLSerializer` now replaces every occurrence of `]]>` in CDATA section data with the split sequence `]]]]><![CDATA[>` before emitting. This closes all mutation-vector paths that Option A alone cannot guard, and means the serialized output is always well-formed XML regardless of how `]]>` entered the node.Update — 2026-04-xx (0.9.10 / 0.8.13)`splitCDATASections` is deprecatedThe CDATA split behavior introduced as Option B of this fix (replacing `]]>` with`]]]]><![CDATA[>` during serialization) is **deprecated** as of 0.9.10 / 0.8.13.This release introduces a `requireWellFormed` option on `XMLSerializer.serializeToString()`. When `{ requireWellFormed: true }` is passed as the second argument, the serializer throws `InvalidStateError` if CDATA section data contains `]]>` — this is the spec-aligned behavior (W3C DOM Parsing and Serialization, `require well-formed` flag) and the recommended migration path going forward.
The split behavior is now controlled by an explicit `splitCDATASections` option (default `true`, preserving the current behavior). The three serialization behaviors are:
| `requireWellFormed` | `splitCDATASections` | Behavior ||---|---|---|| `false` (default) | `true` (default) | Split `]]>` → `]]]]><![CDATA[>` (current behavior, deprecated) || `true` | — (ignored) | Throw `InvalidStateError` — spec-aligned, recommended |\ `false` | `false` | Emit verbatim — same as pre-0.9.9 behavior |`requireWellFormed: true` takes precedence: the split path is unreachable when it is set.MigrationReplace any reliance on the default split behavior with an explicit opt-in:
CVSS v3.1 score: 7.5