# GHSA-253c-mchw-3w2r: Two independent quadratic paths in the `linkify: true` handling.

> An attacker can cause denial of service by submitting large markdown inputs to applications using markdown-it with linkify enabled, blocking the event loop.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Sep 2026

## What is GHSA-253c-mchw-3w2r?

GHSA-253c-mchw-3w2r is a denial of service vulnerability in markdown-it, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

## 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 |
|---|---|---|---|---|
| `'a@b.co\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 (`'a@b.co '`) takes 0.59s, one email per paragraph (`'a@b.co\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 a@b.co 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.

## Which versions are affected by GHSA-253c-mchw-3w2r?

4 open source packages ship an affected version, including markdown-it.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `markdown-it` | npm | All versions | 14.3.1 |
| `markdown-it` | npm | All versions | 14.3.1 |
| `markdown-it` | npm | From 15.0.0 | 15.0.1 |
| `markdown-it` | npm | From 15.0.0 | 15.0.1 |

## How do I fix GHSA-253c-mchw-3w2r?

Upgrade markdown-it to a fixed version for your release line: 14.3.1 or 15.0.1.

## Is GHSA-253c-mchw-3w2r being exploited?

There is no known exploitation of GHSA-253c-mchw-3w2r in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-253c-mchw-3w2r and fix it if it is affected.

What it is: GHSA-253c-mchw-3w2r is a denial of service vulnerability in markdown-it, published on 29 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-253c-mchw-3w2r in the wild and no public exploit code.

Affected packages and fixed versions:
- markdown-it (npm): affected all versions; fixed in 14.3.1
- markdown-it (npm): affected all versions; fixed in 14.3.1
- markdown-it (npm): affected from 15.0.0; fixed in 15.0.1
- markdown-it (npm): affected from 15.0.0; fixed in 15.0.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-253c-mchw-3w2r/ (Markdown: https://insights.sbomify.com/GHSA-253c-mchw-3w2r/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-253c-mchw-3w2r

---
From sbomify Insights: https://insights.sbomify.com/GHSA-253c-mchw-3w2r/
Summaries are generated and may contain mistakes; the official description above is the record.