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

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).