xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, DOMHandler.startElement in lib/dom-parser.js inserts every parsed attribute through setAttributeNode, while NamedNodeMap.setNamedItem in lib/dom.js calls the linear getNamedItem or getNamedItemNS lookup for each insertion. A well-formed element with many distinct attributes therefore requires quadratic comparisons during DOMParser.parseFromString() and can stall a Node.js event loop before application validation. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.

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

## Summary

xmldom builds the attribute collection of every parsed element by inserting attributes one at a time into a DOM `NamedNodeMap`. Each insertion first performs a **linear scan of all already-inserted attributes** to enforce the DOM uniqueness rule (no two attributes with the same qualified name / namespace+local-name). Parsing an element that carries `M` distinct attributes therefore costs `1 + 2 + … + M = O(M²)` comparisons.

Because the trigger is simply "one element with many attributes", the attack payload is a **fully well-formed XML document**. No malformed markup, no error recovery, and no non-default parser options are involved — parsing completes silently with zero `warning`/`error`/`fatalError` events. An attacker who can submit a modest, highly compressible document (a single element with tens of thousands of attributes, ~340 KB uncompressed) can consume seconds of single-threaded CPU per request, enabling an unauthenticated denial of service.

This is distinct from the known quadratic-**memory** namespace-map issue: it burns **CPU** and it does not require any namespace declarations or nesting.

## Details

The DOM content handler adds each attribute of a starting element by calling `el.setAttributeNode(attr)` in a loop:

```js // DOMHandler.startElement for (var i = 0; i < len; i++) { var namespaceURI = attrs.getURI(i); var value = attrs.getValue(i); var qName = attrs.getQName(i); var attr = doc.createAttributeNS(namespaceURI, qName); attr.value =…

undici: undici: Denial of Service via unrequested WebSocket subprotocol encoding/asn1: golang: Go encoding/asn1: Denial of Service via excessive recursion in Unmarshal ip-address: ip-address: Server-Side Request Forgery via IPv4-mapped/NAT64 IPv6 address misclassification urllib: urllib: Credential leakage via cross-origin redirects net/http: golang: Go net/http: Unencrypted HTTP/2 connections vulnerable to Denial of Service html/template: golang: Go html/template: Cross-Site Scripting via pathological input encoding/xml: golang: Go: Denial of Service via XML decoding recursion depth issue net/url: golang: golang net/url: Denial of Service from quadratic complexity in path resolution crypto/tls: golang: Golang crypto/tls: Denial of Service via indefinite KeyUpdate messages nanoid: nanoid: Denial of Service via negative size input in non-secure module functions axios: axios: Denial of Service via uncontrolled recursion in form data processing pymdown-extensions: Pymdown-extensions: Denial of Service via Regular Expression Vulnerability brace-expansion: DoS via unbounded intermediate arrays, bypassing the CVE-2026-14257 mitigation ip-address: ip-address: Inconsistent IP address parsing leads to Server-Side Request Forgery (SSRF) and trust-boundary bypass nanoid: nanoid: Predictable ID generation due to integer overflow fast-uri: fast-uri: Server-Side Request Forgery via repeated hostname percent-decoding fast-uri: fast-uri: Host confusion via skipped IDN canonicalization fast-…

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. Prior to @xmldom/xmldom versions 0.8.15 and 0.9.12, and in xmldom version 0.6.0 and earlier, DOMHandler.startElement in lib/dom-parser.js inserts every parsed attribute through setAttributeNode, while NamedNodeMap.setNamedItem in lib/dom.js calls the linear getNamedItem or getNamedItemNS lookup for each insertion. A well-formed element with many distinct attributes therefore requires quadratic comparisons during DOMParser.parseFromString() and can stall a Node.js event loop before application validation. This issue is fixed in @xmldom/xmldom versions 0.8.15 and 0.9.12; no fixed version is available for xmldom.