CVE-2026-48801: LinkifyIt#match scan loop has quadratic algorithmic complexity

LinkifyIt.prototype.match — the package’s primary public API — has O(N²) algorithmic complexity for inputs containing many fuzzy links or emails. This is not a regex backtrack bug; it’s a structural issue in the JS-level scan loop that re-slices the input and re-runs unanchored regex searches on progressively shorter tails, N times.

64 KB of "a@b.com\n" repeated burns ~2.5 s of single-threaded CPU; 128 KB takes ~10 s. Doubling the input quadruples the time — textbook O(N²).

The same cost passes through markdown-it (linkify:true) unmodified. Any service that synchronously renders untrusted Markdown with linkify enabled on a request hot-path (forums, comments, chat, wikis, AI chat UIs) inherits a worker-process DoS triggerable by a tens-of-KB request body.