How severe is GHSA-253c-mchw-3w2r?

No CVSS score has been assigned to GHSA-253c-mchw-3w2r yet. Review the advisory details and affected package list to assess your exposure.

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

GHSA-253c-mchw-3w2r affects the following packages: markdown-it (npm), markdown-it (npm). Ecosystems affected: npm.

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

Update markdown-it to 15.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-253c-mchw-3w2r is resolved across your whole dependency graph.

How do I detect GHSA-253c-mchw-3w2r in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for markdown-it, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate GHSA-253c-mchw-3w2r if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is GHSA-253c-mchw-3w2r actively exploited in the wild?

No public exploit code has been indexed for GHSA-253c-mchw-3w2r yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

When was GHSA-253c-mchw-3w2r published, and has it been updated?

GHSA-253c-mchw-3w2r was published on September 29, 2026 and was last updated on September 29, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦 npm

Not in CISA KEV

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

Fix: markdown-it/markdown-it@09fa071

GHSA-253c-mchw-3w2r is a security vulnerability in markdown-it. A fix is available for markdown-it — see the affected versions and patch details below.

Published

Sep 29, 2026

Updated

Sep 29, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Sep 29, 2026 · OSV.dev, FIRST.org (EPSS)

Real-World Exposure

2 pkgs affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

7Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream

markdown-itnpm

16.6Mdownloads / week

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
`'[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.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`markdown-it`	≥ 15.0.0&&< 15.0.1	15.0.1`npm install markdown-it@15.0.1`
📦npm	`markdown-it`	all versions	14.3.1`npm install markdown-it@14.3.1`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for markdown-it, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update markdown-it to 15.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-253c-mchw-3w2r is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

## 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_inli

O3 Security · Impact-Aware SCA

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