GHSA-V245-V573-V5VM
Vulnerability from github – Published: 2026-07-21 19:06 – Updated: 2026-07-21 19:06Summary
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
// 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)
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.
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:
// 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.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 5.0.1"
},
"package": {
"ecosystem": "npm",
"name": "linkify-it"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "5.0.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-59887"
],
"database_specific": {
"cwe_ids": [
"CWE-407"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-21T19:06:09Z",
"nvd_published_at": "2026-07-08T17:17:26Z",
"severity": "HIGH"
},
"details": "### Summary\n`linkify-it`\u0027s schema-scan loop (`.test()` / `.match()`, the documented public API) invokes the `mailto:`\nschema validator at **every** `mailto:` occurrence in the input text. For each occurrence the validator does\n`text.slice(pos)` (an O(n) copy) and runs an email regex whose local-part class `src_email_name` greedily\nscans the **entire remaining tail** (O(n)) before failing. With N `mailto:` occurrences that is\n**N \u00d7 O(n) = O(n\u00b2)**. Because linkify-it runs on arbitrary user text (markdown-it feeds it whole documents\nwhen `linkify:true`), an unauthenticated attacker can block the single-threaded event loop for many seconds\nwith a small input. No length bound (unlike an HTTP header).\n\n### Root cause \u2014 `index.mjs` + `lib/re.mjs`\n```js\n// index.mjs (mailto validator) \u2014 runs at every \"mailto:\" hit\n\u0027mailto:\u0027: { validate: function (text, pos, self) {\n const tail = text.slice(pos) // O(n) copy per hit\n if (!self.re.mailto) self.re.mailto = new RegExp(\u0027^\u0027 + self.re.src_email_name + \u0027@\u0027 + self.re.src_host_strict, \u0027i\u0027)\n if (self.re.mailto.test(tail)) { ... } // scans the whole O(n) tail\n return 0\n}}\n// lib/re.mjs:91-93 \u2014 every char of \"mailto:\" (incl. \u0027:\u0027,\u0027-\u0027,\u0027;\u0027) is in this class:\nre.src_email_name = \u0027[\\\\-;:\u0026=\\\\+\\\\$,\\\\.a-zA-Z0-9_][\\\\-;:\u0026=\\\\+\\\\$,\\\\\"\\\\.a-zA-Z0-9_]*\u0027\n```\nThe `while ((m = re.exec(text)) !== null) { \u2026testSchemaAt\u2026 }` scan loop calls the validator at each\n`mailto:` hit; `src_email_name` greedily consumes the whole tail (all chars are in its class) then fails for\nlack of `@`. `http:`/`https:` do NOT blow up \u2014 their validator requires the tail to start with `//`, failing\nin O(1) per hit.\n\n### Proof of Concept (confirmed, linkify-it 5.0.1, Node v24)\n```js\nconst LinkifyIt = require(\u0027linkify-it\u0027);\nconst lf = new LinkifyIt();\nlf.match(\u0027mailto:\u0027.repeat(48000)); // ~336 KB of \"mailto:mailto:\u2026\" -\u003e seconds of blocked event loop\n```\n| input (same bytes) | 56 KB | 112 KB | 224 KB | 336 KB |\n|---|---:|---:|---:|---:|\n| **`mailto:` contiguous** | 97 ms | 357 ms | 1438 ms | 3272 ms |\n| `mailto:` space-separated | 2 ms | 3 ms | 5 ms | 8 ms |\n| `http://` contiguous | 12 ms | 17 ms | 33 ms | 49 ms |\n\n\u00d7~4 per 2\u00d7 input \u21d2 O(n\u00b2); equal-byte controls stay flat \u21d2 algorithmic, not a GC/allocation artifact.\nReal-world via markdown-it 14.x (`{linkify:true}`), `md.render(\u0027mailto:\u0027.repeat(n))`: 219 KB \u2248 ~5 s.\n\u003cimg width=\"737\" height=\"161\" alt=\"image\" src=\"https://github.com/user-attachments/assets/b5d390f3-68d0-4861-9c47-ad8aff0203d5\" /\u003e\n\n### Impact\nReachable on arbitrary user text via the documented `.test()`/`.match()` API and through markdown-it\u0027s\nlinkifier \u2014 comment systems, chat, forums, wikis, note apps that render user markdown with linkify enabled.\nA ~220 KB post hangs the event loop ~5 s; a few hundred KB \u2192 tens of seconds. Availability only.\n\n### Suggested remediation\nBound the email local-part per RFC 5321 (\u226464) so per-hit work is O(1), and avoid the full-tail slice:\n```js\n// lib/re.mjs \u2014 cap the greedy run:\nre.src_email_name = \u0027[\\\\-;:\u0026=\\\\+\\\\$,\\\\.a-zA-Z0-9_][\\\\-;:\u0026=\\\\+\\\\$,\\\\\"\\\\.a-zA-Z0-9_]{0,63}\u0027\n// index.mjs \u2014 prefer a sticky regex anchored at `pos` over text.slice(pos).\n```\n\n### Affected / disclosure\nAll versions through 5.0.1 (latest); same code on `master`. cve-mcp/OSV report no known vulnerability for\nlinkify-it. Distinct from markdown-it\u0027s own `*`-run ReDoS (CVE-2026-2327, different package/path) and the\nrecent markdown-it DoS. Reported privately; happy to test a patch against the PoC.",
"id": "GHSA-v245-v573-v5vm",
"modified": "2026-07-21T19:06:09Z",
"published": "2026-07-21T19:06:09Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/markdown-it/linkify-it/security/advisories/GHSA-v245-v573-v5vm"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59887"
},
{
"type": "WEB",
"url": "https://github.com/markdown-it/linkify-it/commit/105e5d77f7d119871d2b2d86ed208568eb3e7ffe"
},
{
"type": "PACKAGE",
"url": "https://github.com/markdown-it/linkify-it"
},
{
"type": "WEB",
"url": "https://github.com/markdown-it/linkify-it/releases/tag/5.0.2"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "linkify-it: Quadratic-complexity DoS via the `mailto:` validator scan-loop on attacker text"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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