CVE-2026-83613: xmldom: Quadratic-time attribute deduplication

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.