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CVE-2026-83614

npm · @xmldom/xmldom

Summary

xmldom: Quadratic-time parsing via the malformed-input recovery path — `parseElementStartPart` re-scan and `normalize()` adjacent-text merge

Advisory details

Summary

xmldom's malformed-input error-recovery path has two quadratic-time (O(n²)) behaviors that a single crafted input triggers together, so a tiny, highly compressible document (tens of KB) stalls the Node.js event loop for multiple seconds. It is reachable from DOMParser.parseFromString under default options — i.e. from unauthenticated, network-delivered XML — making this an unauthenticated denial of service. One of the two behaviors, the normalize() adjacent-text merge, is additionally reachable programmatically — via a plain normalize() call on a DOM built with adjacent text nodes, independent of the parser — so its fix must live in normalize(), not only in a parser bound.

Details

Finding A — parseElementStartPart quadratic re-scan

A < character is not a delimiter in any tag-parsing state, so parseElementStartPart scans forward character-by-character over any embedded < until it reaches the next > (or end of input), then validates the accumulated slice as a tag name and throws invalid tagName: on failure. The main loop catches this, reports an error, sets end = -1, and recovers by advancing a single character (appendText(Math.max(tagStart, start) + 1)). With a long run of < and a distant >, each of the O(n) recovery retries performs an O(n) scan plus an O(n) anchored regex validation over the growing candidate ⇒ O(n²).

Code (0.9.x, bb7a085dc5ba1eea3212388509b97bb4b4af32b9):

Code (0.8.x, e5c14802592685bb872c042c54c3f73758875c85):

Finding B — normalize() adjacent-text O(K²) merge

endDocument() calls document.normalize(). For a parent with K adjacent text nodes (produced by the one-character recovery of Finding A), normalize() performs K−1 merges. Each merge does a removeChild — which re-indexes all child nodes of the parent (O(K)) — and an appendData — which rebuilds the accumulator string this.data + text (O(K)). Total: O(K²).

Well-formed XML cannot produce adjacent text-node siblings through the parser (each text run is one node; comments, CDATA, PIs, and elements sit between runs), so the parse-path trigger for Finding B is the malformed-input recovery that emits single-character text nodes. The same O(K²) merge is, however, independently reachable via the public normalize() API on a programmatically built tree (see "Finding B is additionally reachable programmatically" below).

Code (0.9.x, bb7a085dc5ba1eea3212388509b97bb4b4af32b9):

Code (0.8.x, e5c14802592685bb872c042c54c3f73758875c85):

Finding B is additionally reachable programmatically (no parser involved)

Node.prototype.normalize() is public API on every Document/Element. A tree built entirely through the ordinary DOM API — new DOMImplementation().createDocument(...), then K× createTextNode + appendChild on one parent — reaches the same O(K²) merge when the application calls normalize(), with no parsing and no error-recovery. The parser is only one of the two callers of the vulnerable merge:

  • the parser's automatic endDocument() → document.normalize() (the parse-path trigger above), and
  • any explicit application call to the public normalize() on a tree with adjacent text nodes.

XMLSerializer does not call normalize(), so serializing an un-merged tree is O(total text), not O(K²); the O(K²) surface is exactly those two normalize() callers. Consequently a parser-side bound alone cannot remediate Finding B — the fix must live in normalize().

Affected Versions

Both findings are present across the full published @xmldom/xmldom history — both currently-maintained versions (0.8.x and 0.9.x) are affected — and across the retired unscoped xmldom line. Finding B's normalize() merge is additionally reachable programmatically: a direct normalize() call on a DOM built with adjacent text nodes hits the same O(K²) merge, independent of the parser — so, unlike Finding A, it does not require the malformed-input recovery path.

Proof of Concept

Default DOMParser, no options. The input is trivially compressible (a< / a<> repeated) and never throws — it is parsed via the recovery path.

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

// Silence the expected `error`-level recovery reports (default handler logs
// them to console.error without throwing; only fatalError throws).
console.error = function () {};

function timeParse(label, xml, mime) {
  const t0 = process.hrtime.bigint();
  new DOMParser().parseFromString(xml, mime); // completes; no exception
  const ms = Number(process.hrtime.bigint() - t0) / 1e6;
  console.log(label + '  bytes=' + Buffer.byteLength(xml) + '  time=' + ms.toFixed(1) + ' ms');
}

for (const N of [4000, 8000, 16000, 32000]) {
  // Finding A: long re-scans, O(n^2) during parse.
  timeParse('A N=' + N, '<r>' + 'a<'.repeat(N) + '</r>', 'text/xml');
  // Finding B: short re-scans (cheap parse) but K adjacent text nodes -> O(K^2) in normalize().
  timeParse('B N=' + N, '<r>' + 'a<>'.repeat(N) + '</r>', 'text/html');
  // Combined: ONE input hits both A and B under the default parser.
  timeParse('C N=' + N, '<r>' + 'a<'.repeat(N) + '</r>', 'text/xml');
}

Measured on Node v18.20.8 (absolute ms vary by host; the load-bearing fact is that doubling the input ~quadruples the time — canonical O(n²)):

Finding A, isolated ("<r>" + "a<"×N + "</r>", normalize disabled to isolate the re-scan):

N input bytes @xmldom/xmldom 0.9.10 0.8.13
2000 4007 43 ms 37 ms
4000 8007 129 ms 106 ms
8000 16007 434 ms 424 ms
16000 32007 1629 ms 1611 ms

Finding B, isolated ("<r>" + "a<>"×N + "</r>", time attributable to normalize()):

K (N) input bytes 0.9.10 0.8.13

References

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