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/rulescore/smartquotes.mjs:

javascript 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 processinlines() 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

javascript 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).

Versions of the package markdown-it from 13.0.0 and before 14.1.1 are vulnerable to Regular Expression Denial of Service (ReDoS) due to the use of the regex /\+$/ in the linkify function. An attacker can supply a long sequence of characters followed by a non-matching character, which triggers excessive backtracking and may lead to a denial-of-service condition.

Improper Neutralization of Input During Web Page Generation (XSS or 'Cross-site Scripting') vulnerability in markdown-it allows Cross-Site Scripting (XSS). This vulnerability is associated with program files lib/renderer.mjs.

This issue affects markdown-it: 14.1.0. NOTE: the Supplier does not consider this issue to be a vulnerability.

A vulnerability was found in markdown-it up to 2.x. It has been classified as problematic. Affected is an unknown function of the file lib/common/htmlre.js. The manipulation leads to inefficient regular expression complexity. Upgrading to version 3.0.0 is able to address this issue. The name of the patch is 89c8620157d6e38f9872811620d25138fc9d1b0d. It is recommended to upgrade the affected component. The identifier of this vulnerability is VDB-216852.

Impact

Special patterns with length > 50K chars can slow down parser significantly.

js const md = require('markdown-it')();

md.render(x ${' '.repeat(150000)} x \nx);

Patches

Upgrade to v12.3.2+

Workarounds

No.

References

Fix + test sample: https://github.com/markdown-it/markdown-it/commit/ffc49ab46b5b751cd2be0aabb146f2ef84986101

markdown-it before 4.1.0 does not block data: URLs.