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 srcemailname 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 js // index.mjs (mailto validator) — runs at every "mailto:" hit 'mailto:': { validate: function (text, pos, self) { const tail = text.slice(pos) // O(n) copy per hit if (!self.re.mailto) self.re.mailto = new RegExp('^' + self.re.srcemailname + '@' + self.re.srchoststrict, 'i') if (self.re.mailto.test(tail)) { ... } // scans the whole O(n) tail return 0 }} // lib/re.mjs:91-93 — every char of "mailto:" (incl. ':','-',';') is in this class: re.srcemailname = '[\\-;:&=\\+\\$,\\.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; srcemailname 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) js const LinkifyIt = require('linkify-it'); const lf = new LinkifyIt(); lf.match('mailto:'.repeat(48000)); // ~336 KB of "mailto:mailto:…" -> seconds of blocked event loop | input (same bytes) | 56 KB | 112 KB | 224 KB | 336 KB | |---|---:|---:|---:|---:| | mailto: contiguous | 97 ms | 357 ms | 1438 ms | 3272 ms | | mailto: space-separated | 2 ms | 3 ms | 5 ms | 8 ms | | http:// contiguous | 12 ms | 17 ms | 33 ms | 49 ms |

×~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: js // lib/re.mjs — cap the greedy run: re.srcemailname = '[\\-;:&=\\+\\$,\\.a-zA-Z0-9][\\-;:&=\\+\\$,\\"\\.a-zA-Z0-9]{0,63}' // 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.