GHSA-6v5v-wf23-fmfq — markdown-it
MEDIUMGHSA-6v5v-wf23-fmfq is a medium-severity (CVSS 5.3) Uncontrolled Resource Consumption vulnerability in markdown-it. A fix is available for markdown-it — see the affected versions and patch details below.
markdown-it: Quadratic complexity DoS in smartquotes rule via replaceAt string operations
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for GHSA-6v5v-wf23-fmfq.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-6v5v-wf23-fmfq plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 376,715 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
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.
markdown-itnpmDescription
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).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | markdown-it | all versions | 14.2.0npm install markdown-it@14.2.0 |
Detection & mitigation playbook
Open-source dependencyDetect
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 14.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-6v5v-wf23-fmfq 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-6v5v-wf23-fmfq can be triaged on real exposure rather than presence alone.
Tailored to GHSA-6v5v-wf23-fmfq. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-6v5v-wf23-fmfq in your dependencies?
O3 Security finds GHSA-6v5v-wf23-fmfq across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.