Affected: npm: @xmldom/xmldom, npm: xmldom

## Summary

The package allows attacker-controlled comment content to be serialized into XML without validating or neutralizing comment breaking sequences. As a result, an attacker can terminate the comment early and inject arbitrary XML nodes into the serialized output.

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## Details

The issue is in the DOM construction and serialization flow for comment nodes.

When `createComment(data)` is called, the supplied string is stored as comment data through the generic character-data handling path. That content is kept as-is. Later, when the document is serialized, the serializer writes comment nodes by concatenating the XML comment delimiters with the stored `node.data` value directly.

That behavior is unsafe because XML comments are a syntax-sensitive context. If attacker-controlled input contains a sequence that closes the comment, the serializer does not preserve it as literal comment text. Instead, it emits output where the remainder of the payload is treated as live XML markup.

This is a real injection bug, not a formatting issue. The serializer already applies context-aware handling in other places, such as escaping text nodes and rewriting unsafe CDATA terminators. Comment content does not receive equivalent treatment. Because of that gap, untrusted data can break out of the comment boundary and modify the structure of the final XML document.

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## PoC

```js const { DOMImplementation, DOMParser, XMLSerializer } = require('@xmldom/xmldom');

const doc = new DOMI…