GHSA-253c-mchw-3w2r

GHSA-253c-mchw-3w2r · markdown-it linkify: true has two quadratic paths, so a few hundred KB of markdown blocks the event loop for tens of seconds

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## Summary Two independent quadratic paths in the `linkify: true` handling. Both are in markdown-it's own code rather than in linkify-it, which stays linear on both payloads. `src/rules_core/linkify.ts` calls `arrayReplaceAt` once per linkified text token, and that rebuilds the whole `children` array each time. A paragraph of N soft-broken lines is one inline token with about 2N children, so you get N rebuilds over a 2N array. Schema-less emails are what reach it. A `http://` link gets consumed by the inline rule first and never arrives as a text token, so those stay linear. `src/rules_inline/linkify.ts` runs `state.pending.match(SCHEME_RE)` at every `://` in the source. `state.pending` only gets truncated once a link is actually produced, so an unregistered scheme leaves it growing and every `://` rescans the lot. ## Proof of concept Clean install of 15.0.0 from npm, `new MarkdownIt({ linkify: true }).render(payload)`, Node 24. | N | 5,000 | 10,000 | 20,000 | 40,000 | |---|---|---|---|---| | `'[email protected]\n'.repeat(N)`, 34KB to 273KB | 1.1s | 4.7s | 19.6s | 89s | | `'a://'.repeat(N)`, 20KB to 156KB | 0.25s | 0.76s | 3.1s | 15.0s | Doubling the input roughly quadruples the time in both. With `linkify: false` the same inputs run in 25 to 131ms and stay flat. Controls for the first one, all at N=20000: putting every email in a single text token (`'[email protected] '`) takes 0.59s, one email per paragraph (`'[email protected]\n\n'`) takes 0.64s, and soft-broken lines with nothing linkifiable take 42ms. So it needs many children AND many of them linkifying. For the second, replaying just the `SCHEME_RE` calls against the same growing prefixes with no markdown-it involved accounts for 11.7s of the 15s. Ordinary prose does it too. `'ping [email protected] ok\n'.repeat(20000)` is 293KB and takes 31s. ## Caveat `linkify` is off by default, so this only reaches apps that turn it on. ## Impact Availability only. A few hundred KB of fairly ordinary markdown pins one core for tens of seconds, and because it's quadratic it degrades quickly with size. Nothing is read, written or executed.

CVSS 6.3 · Medium · Fix available

Published

2026-09-29T17:57:56Z

Updated

2026-09-29T18:15:04.692324591Z

Stack: node

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