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
```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.src_email_name + '@' + self.re.src_host_strict, '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.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)
```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.src_email_name = '[\\-;:&=\\+\\$,\\.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.
