xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. From 0.9.0-beta.1 until 0.9.12, HTML-mode parsing through DOMParser.parseFromString() mishandles a mixed-case closing tag for the script, style, textarea, or title raw-text elements. parseHtmlSpecialContent, selected by isHTMLRawTextElement or isHTMLEscapableRawTextElement, uses a case-sensitive indexOf() and then calls substring() with a missing-close result of negative one, causing unstable parser progression and quadratic output amplification. A small untrusted text/html document can consequently consume disproportionate CPU and memory when parsed and serialized. This issue is fixed in @xmldom/xmldom version 0.9.12.

Affected: npm: @xmldom/xmldom

## Summary

In HTML mode (`text/html`), a raw-text element (`script`, `style`, `textarea`, `title`) whose closing tag differs in case from its opening tag (e.g. `</ScRiPt>` for `<script>`) is mishandled by the parser, producing quadratic (O(n²)) output growth — a small crafted document parses and serializes into output orders of magnitude larger, exhausting CPU and memory. A modest input of tens of KB can therefore cause a denial of service in any service that parses untrusted HTML with xmldom. Only HTML mode is affected.

## Details

The parser calls `parseHtmlSpecialContent` for each raw-text element in HTML mode, matched via `isHTMLRawTextElement` / `isHTMLEscapableRawTextElement` (so all four types — `script`, `style`, `textarea`, `title` — are in scope). It searches for the element's closing tag with `source.indexOf('</' + tagName + '>', elStartEnd)`, a byte-for-byte case-sensitive match. A mixed-case closing tag never matches, so the search returns `-1`, and the following `source.substring(elStartEnd + 1, -1)` extracts text backwards from the start of the document instead of the element's content. The function then returns `-1` to the parse loop, which cannot advance normally and falls back to character-by-character reprocessing. Every raw-text element re-captures all source text preceding it, so output grows as O(n²) in the number of such elements.

### Root Cause

1. **Case-sensitive close-tag search** (`lib/sax.js:549`): `source.indexOf('</' + tagName + '>', elSt…

Affected: Ubuntu:20.04:LTS: node-xmldom, Ubuntu:22.04:LTS: node-xmldom, Ubuntu:24.04:LTS: node-xmldom, Ubuntu:26.04:LTS: node-xmldom

xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. From 0.9.0-beta.1 until 0.9.12, HTML-mode parsing through DOMParser.parseFromString() mishandles a mixed-case closing tag for the script, style, textarea, or title raw-text elements. parseHtmlSpecialContent, selected by isHTMLRawTextElement or isHTMLEscapableRawTextElement, uses a case-sensitive indexOf() and then calls substring() with a missing-close result of negative one, causing unstable parser progression and quadratic output amplification. A small untrusted text/html document can consequently consume disproportionate CPU and memory when parsed and serialized. This issue is fixed in @xmldom/xmldom version 0.9.12.