markdown-it linkify: true has two quadratic paths, so a few hundred KB of markdown blocks the event loop for tens of seconds

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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/rulescore/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/rulesinline/linkify.ts runs state.pending.match(SCHEMERE) 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),…

Published by the source

29 Sep 2026, 17:57 UTC

Last modified by the source

2 Nov 2026, 18:55 UTC

Source last checked

2 Nov 2026, 20:00 UTC

1. Revision 6 · amended · 2 Nov 2026

   Amended from ghsa record GHSA-253c-mchw-3w2r

2. Revision 5 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-253c-mchw-3w2r

3. Revision 4 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-253c-mchw-3w2r

4. Revision 3 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

5. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-253c-mchw-3w2r

6. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-253c-mchw-3w2r

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# Patch Next advisory brief: GHSA-253c-mchw-3w2r

Advisory: markdown-it linkify: true has two quadratic paths, so a few hundred KB of markdown blocks the event loop for tens of seconds
IDs: GHSA-253c-mchw-3w2r
Record: https://patchnext.com/advisories/ghsa-253c-mchw-3w2r (revision 6, source checked 2 Nov 2026, 20:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: markdown-it 15.0.1, 14.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm markdown-it: affected >= 15.0.0, <= 15.0.0; < 14.3.1; fixed in 15.0.1, 14.3.1

## Suggested inspection (read-only)
- `npm ls markdown-it --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on markdown-it.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-253c-mchw-3w2r (GitHub-reviewed advisory GHSA-253c-mchw-3w2r lists affected ranges: npm markdown-it >=15.0.0 =0 <14.3.1)
- Verified: https://osv.dev/vulnerability/GHSA-253c-mchw-3w2r (OSV record GHSA-253c-mchw-3w2r lists affected ranges: npm markdown-it >=15.0.0 =0 <14.3.1 (fixed))

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