Summary
markdown-it: Quadratic-time rendering of a single multi-line link reference definition with default options (regression since 14.0)
A link reference definition whose label or title is never closed is O(n²) in markdown-it 15.0.2 with default options. Continuation lines are pulled lazily with str += lineContent in src/rules_block/reference.ts, and the subsequent charCodeAt and slice calls flatten the V8 cons-string, copying everything collected so far; this runs before the rule decides the definition is invalid. It is a regression introduced by PR #1004 in 14.0.0: 13.0.2 processed a 313 KB input in 125 ms, while every version since takes about 11 seconds on the same input. At the sizes below, 200 KB takes 3.6 seconds, 400 KB takes 18.1 seconds, and 800 KB takes 74.0 seconds, which is roughly 4–5× per doubling. No options are required to trigger it.
Details
src/rules_block/reference.ts appends continuation lines with str += lineContent at lines 74, 83, 100, 133, and 150, then keeps indexing the accumulated string via str.charCodeAt(pos) in the label and whitespace loops and str.slice(...) in parseLinkTitle. V8 represents += as a cons-string, and the next charCodeAt or slice flattens it, copying everything collected so far, so one definition spanning n lines costs O(n²). Isolation confirms the rule is responsible: block-only parsing with inline disabled is still quadratic, md.disable('reference') makes the same input linear, and the same text without the leading [ is linear. The fix is to track positions in state.src instead of building str, or to collect line slices in an array and parse across them. At minimum, stop scanning the label after 999 characters (CommonMark §6.3) before pulling more lines, and cap title continuation. Add a pathological test with a 100k-line unclosed title and label. Open PR #1215 edits the same file but its label check runs after the label loop, so it does not bound either variant and the fix can land together with that PR.
The observed output from PoC was:
unclosed title n=100000 200009 B 3585.5 ms
unclosed title n=200000 400009 B 18107.1 ms (5.05x)
unclosed title n=400000 800009 B 73954.4 ms (4.08x)
Regression comparison at 313 KB: 13.0.2 → 125 ms, 14.1.0 → 11.4 s, 15.0.0 → 10.6 s, 15.0.2 → 10.7 s.
Proof of concept
How to execute the PoC:
- mkdir -p markdown-it-poc && cd markdown-it-poc
- npm init -y
- npm i [email protected]
- node poc_markdown_it.js
PoC below is:
// poc_markdown_it.js
// Requires: npm install [email protected]
const MarkdownIt = require('markdown-it');
const md = new MarkdownIt();
function bench(label, src) {
const t0 = process.hrtime.bigint();
md.render(src);
const t1 = process.hrtime.bigint();
console.log(
`${label.padEnd(30)} ${String(src.length).padStart(9)} B ${(Number(t1 - t0) / 1e6).toFixed(1).padStart(9)} ms`
);
}
// Unclosed title: one reference definition spread over many lines
for (const n of [100000, 200000, 400000]) {
bench(`unclosed title n=${n}`, '[a]: /u "' + 'x\n'.repeat(n));
}
// Unclosed label
for (const n of [5000, 10000, 20000]) {
bench(`unclosed label n=${n}`, '[' + 'a\n'.repeat(n));
}
Impact
This is an algorithmic-complexity denial of service (CWE-407). Any server that renders user Markdown with markdown-it is affected with default options, including comments, issues, chat, wikis, docs previews, and LLM chat UIs. A ~400 KB request body blocks the Node.js event loop for about 18 seconds and an ~800 KB body for about 74 seconds. This is the same class and attacker model as GHSA-253c, but it is reachable without any opt-in option, since the affected path is a core block rule.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-111758 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (14.3.3, 15.0.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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markdown-it to 14.3.3 or later; markdown-it to 15.0.3 or later
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Frequently Asked Questions
- What is CVE-2026-111758? CVE-2026-111758 is a high-severity uncontrolled resource consumption vulnerability in markdown-it (npm), affecting versions >= 14.1.0, < 14.3.3. It is fixed in 14.3.3, 15.0.3. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-111758? CVE-2026-111758 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of markdown-it are affected by CVE-2026-111758? markdown-it (npm) versions >= 14.1.0, < 14.3.3 is affected.
- Is there a fix for CVE-2026-111758? Yes. CVE-2026-111758 is fixed in 14.3.3, 15.0.3. Upgrade to this version or later.
- Is CVE-2026-111758 exploitable, and should I be worried? Whether CVE-2026-111758 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-111758 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-111758?
- Upgrade
markdown-itto 14.3.3 or later - Upgrade
markdown-itto 15.0.3 or later
- Upgrade