xmldom: HTML raw-text closing-tag case mismatch causes output amplification

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 + '>', elStartEnd) does not fold case, contrary to the WHATWG HTML RAWTEXT end-tag-name rule.
2. Unguarded -1 (lib/sax.js:550): source.substring(elStartEnd + 1, elEndStart) runs even when elEndStart === -1, extracting text backwards from position 0.
3. Unstable progression (lib/sax.js:556): the function returns elEndStart (-1), driving repeated character-by-character fallback in the parse loop.

Affected Versions

Only the 0.9.x line is affected — the amplification was introduced in 0.9.0-beta.1 when parseHtmlSpecialContent was refactored, and remains through 0.9.11. The 0.8.x line is not affected: its older parseHtmlSpecialContent does not amplify, despite sharing the same case-sensitive indexOf.

Proof of Concept

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

const n = 1000;
const payload = '<html><body>' + '<script>x</ScRiPt>'.repeat(n) + '</body></html>';
const doc = new DOMParser().parseFromString(payload, 'text/html');
const out = new XMLSerializer().serializeToString(doc);
console.log(payload.length, out.length, (out.length / payload.length).toFixed(1) + 'x');
// 18026 9037063 501.3x  — an 18 KB input yields ~9 MB of output

Output size grows quadratically with the number of case-mismatched raw-text elements:

Repeats | Input len | Output len | Ratio
1       | 44        | 109        | 2.5x
100     | 1826      | 93763      | 51.3x
500     | 9026      | 2268563    | 251.3x
1000    | 18026     | 9037063    | 501.3x
2000    | 36026     | 36074063   | 1001.3x

Proof of Concept from @KarimTantawey (tested with script); the same amplification occurs for style, textarea, and title.

Impact

Small attacker payloads can force disproportionate CPU and memory usage in services that parse and serialize untrusted HTML via xmldom. The quadratic growth means a modest-sized input (tens of kilobytes) can produce output in the tens or hundreds of megabytes, potentially exhausting memory or causing timeouts.

The attack only requires HTML mode (text/html MIME type) and mixed-case closing tags for any of the four raw-text element types. No special configuration or error handler setup is needed.

Severity note

The CVSS 4.0 vector scores availability only (VA:H, with VC:N/VI:N): the flaw neither discloses nor corrupts data, but a small untrusted HTML input (tens of KB) can force output and memory in the tens to hundreds of MB, enough to exhaust a service's heap or stall its event loop. It is reachable with no authentication, configuration, or error-handler setup — only that the application parses untrusted text/html and serializes the result.

Fix Applied

The raw-text closing tag is now matched case-insensitively in HTML raw-text mode (per the WHATWG HTML RAWTEXT end-tag rule), and a missing closing tag is handled explicitly, removing the quadratic output amplification. Output for well-formed input is unchanged. Non-breaking; 0.9.x-only.