linkify-it: Quadratic-complexity DoS via the `mailto:` validator scan-loop on attacker text

Summary

linkify-it's schema-scan loop (.test() / .match(), the documented public API) invokes the mailto:
schema validator at every mailto: occurrence in the input text. For each occurrence the validator does
text.slice(pos) (an O(n) copy) and runs an email regex whose local-part class src_email_name greedily
scans the entire remaining tail (O(n)) before failing. With N mailto: occurrences that is
N × O(n) = O(n²). Because linkify-it runs on arbitrary user text (markdown-it feeds it whole documents
when linkify:true), an unauthenticated attacker can block the single-threaded event loop for many seconds
with a small input. No length bound (unlike an HTTP header).

Root cause — index.mjs + lib/re.mjs

 1// index.mjs (mailto validator) — runs at every "mailto:" hit
 2'mailto:': { validate: function (text, pos, self) {
 3  const tail = text.slice(pos)                                  // O(n) copy per hit
 4  if (!self.re.mailto) self.re.mailto = new RegExp('^' + self.re.src_email_name + '@' + self.re.src_host_strict, 'i')
 5  if (self.re.mailto.test(tail)) { ... }                        // scans the whole O(n) tail
 6  return 0
 7}}
 8// lib/re.mjs:91-93 — every char of "mailto:" (incl. ':','-',';') is in this class:
 9re.src_email_name = '[\\-;:&=\\+\\$,\\.a-zA-Z0-9_][\\-;:&=\\+\\$,\\"\\.a-zA-Z0-9_]*'

The while ((m = re.exec(text)) !== null) { …testSchemaAt… } scan loop calls the validator at each
mailto: hit; src_email_name greedily consumes the whole tail (all chars are in its class) then fails for
lack of @. http:/https: do NOT blow up — their validator requires the tail to start with //, failing
in O(1) per hit.

Proof of Concept (confirmed, linkify-it 5.0.1, Node v24)

 1const LinkifyIt = require('linkify-it');
 2const lf = new LinkifyIt();
 3lf.match('mailto:'.repeat(48000));   // ~336 KB of "mailto:mailto:…" -> seconds of blocked event loop

×~4 per 2× input ⇒ O(n²); equal-byte controls stay flat ⇒ algorithmic, not a GC/allocation artifact.
Real-world via markdown-it 14.x ({linkify:true}), md.render('mailto:'.repeat(n)): 219 KB ≈ ~5 s.
<img width="737" height="161" alt="image" src="https://github.com/user-attachments/assets/b5d390f3-68d0-4861-9c47-ad8aff0203d5" />

Impact

Reachable on arbitrary user text via the documented .test()/.match() API and through markdown-it's
linkifier — comment systems, chat, forums, wikis, note apps that render user markdown with linkify enabled.
A ~220 KB post hangs the event loop ~5 s; a few hundred KB → tens of seconds. Availability only.

Suggested remediation

Bound the email local-part per RFC 5321 (≤64) so per-hit work is O(1), and avoid the full-tail slice:

 1// lib/re.mjs — cap the greedy run:
 2re.src_email_name = '[\\-;:&=\\+\\$,\\.a-zA-Z0-9_][\\-;:&=\\+\\$,\\"\\.a-zA-Z0-9_]{0,63}'
 3// index.mjs — prefer a sticky regex anchored at `pos` over text.slice(pos).

Affected / disclosure

All versions through 5.0.1 (latest); same code on master. cve-mcp/OSV report no known vulnerability for
linkify-it. Distinct from markdown-it's own *-run ReDoS (CVE-2026-2327, different package/path) and the
recent markdown-it DoS. Reported privately; happy to test a patch against the PoC.