GHSA-v245-v573-v5vm

### 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.