chore(deps): update dependency markdown-it to v15.0.1 [security]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
markdown-it	15.0.0 → 15.0.1

---

markdown-it: Quadratic complexity DoS in smartquotes rule via replaceAt string operations

CVE-2026-48988 / GHSA-6v5v-wf23-fmfq

More information

Details

Summary

A quadratic time complexity vulnerability exists in markdown-it's smartquotes rule (enabled via the typographer: true option). An attacker can craft a markdown input consisting of consecutive quotation marks that causes the parser to consume excessive CPU time, leading to denial of service.

Details

The vulnerability is in the replaceAt() helper function used by the smartquotes rule in lib/rules_core/smartquotes.mjs:

function replaceAt (str, index, ch) {
  return str.slice(0, index) + ch + str.slice(index + 1)
}

When markdown-it processes a text token containing many quotation marks (either " or ') with typographer: true, the smartquotes rule iterates through each quote character and calls replaceAt() to substitute it with a typographic (curly) quote. Each call to replaceAt() creates three new string slices and concatenates them, which is an O(n) operation where n is the length of the string.

Since this is called once per quote character in the token, and there are n quote characters, the total time complexity becomes O(n^2).

The root cause is that the smartquotes rule modifies token.content in place using string slicing rather than building the result incrementally. The process_inlines() function (line 14) processes each quote in the text token, and for matching quote pairs, calls replaceAt() on both the opening and closing token's content (lines 151-152). When the entire input is a single text token of quote characters, this results in quadratic behavior.

PoC

const md = require('markdown-it');
const instance = md({ typographer: true });

// 160,000 consecutive double-quote characters
const payload = '"'.repeat(160000);

console.time('render');
instance.render(payload);
console.timeEnd('render');
// Output: render: ~21000ms (21 seconds)

// Compare with typographer disabled:
const safe = md({ typographer: false });
console.time('render-safe');
safe.render(payload);
console.timeEnd('render-safe');
// Output: render-safe: ~8ms

Measured timing on a modern system:

• 10,000 quotes: ~19ms
• 20,000 quotes: ~51ms
• 40,000 quotes: ~212ms
• 80,000 quotes: ~5,430ms
• 160,000 quotes: ~21,198ms

The scaling is clearly superlinear (quadratic), with the 80K->160K step showing a ~3.9x increase for a 2x input increase, consistent with O(n^2).

Impact

Applications that render user-supplied markdown with typographer: true are vulnerable to denial of service. An attacker can submit a relatively small payload (160KB of quote characters) that causes the server to spend over 21 seconds processing a single request. Repeated submissions can exhaust server CPU resources and prevent legitimate users from being served.

The impact is mitigated by the fact that the typographer option defaults to false and must be explicitly enabled. However, the typographer feature is commonly enabled in production applications that want smart typography, and the markdown-it documentation prominently suggests enabling it.

A suggested fix would be to replace the replaceAt() approach with an array-based or StringBuilder-style approach that collects all replacements and applies them in a single pass, reducing the time complexity to O(n).

Severity

• CVSS Score: 5.3 / 10 (Medium)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References

• https://github.com/markdown-it/markdown-it/security/advisories/GHSA-6v5v-wf23-fmfq
• https://github.com/advisories/GHSA-6v5v-wf23-fmfq

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

---

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

GHSA-253c-mchw-3w2r

More information

Details

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.

Severity

• CVSS Score: 6.3 / 10 (Medium)
• Vector String: 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

References

• https://github.com/markdown-it/markdown-it/security/advisories/GHSA-253c-mchw-3w2r
• https://github.com/markdown-it/markdown-it/commit/09fa07118dda4c953f058848f53dae88395618ca
• https://github.com/markdown-it/markdown-it/commit/aaadcfa6d817b3c5f89afb49d97c7b797a6dd4fd
• https://github.com/markdown-it/markdown-it/commit/ad70f6b7cff64bee10e42a774147112480ca0d49
• https://github.com/advisories/GHSA-253c-mchw-3w2r

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

---

Release Notes

markdown-it/markdown-it (markdown-it)

v15.0.1

Compare Source

Changed

• doc: replace oxide theme with custom one.

Fixed

• Fixed code span parsing after lookaheads for unclosed link and image labels, #​1201.
• Preserve spaces in code spans whose content consists only of spaces, #​1180.
• Preserve brackets around IPv6 address literals when normalizing links, #​1204.

Security

• Fixed quadratic complexity when replacing fuzzy links.
• Fixed quadratic complexity in scheme backscan (inline linkify rule).

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.34%. Comparing base (c36734a) to head (b3050a0).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #513   +/-   ##
=======================================
  Coverage   99.34%   99.34%           
=======================================
  Files          26       26           
  Lines        1219     1219           
=======================================
  Hits         1211     1211           
  Misses          8        8           

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