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Response snapshot Oct 26, 2026, 23:07 UTC

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VulnerabilityCVE-2026-111758

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.

Published Updated

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
0.5% in the next 30 days
Sources
2 independent sources

What to do

GitHub lists first patched versions: markdown-it: 14.3.3; markdown-it: 15.0.3. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-gfr2-rv44-r23c (opens a new tab)

What happened

### Summary

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

Continue reading

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: 1. mkdir -p markdown-it-poc && cd markdown-it-poc 2. npm init -y 3. npm i markdown-it@15.0.2 4. node poc_markdown_it.js

PoC below is: ```javascript // poc_markdown_it.js // Requires: npm install markdown-it@15.0.2 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.

Affected package ranges reported by GitHub: markdown-it: >= 14.1.0, < 14.3.3 markdown-it: >= 15.0.0, < 15.0.3

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.5%

Scored Percentile 39.3%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-gfr2-rv44-r23c (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-gfr2-rv44-r23c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    NVD CVE-2026-111758 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  13. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  14. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-111758, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 22). markdown-it: Quadratic-time rendering of a single multi-line link reference definition with default options (regression since 14.0) (CVE-2026-111758). https://kevwire.com/record/CVE-2026-111758

MLA

"markdown-it: Quadratic-time rendering of a single multi-line link reference definition with default options (regression since 14.0) (CVE-2026-111758)." KEVWire, 22 Oct. 2026, kevwire.com/record/CVE-2026-111758.

Chicago

KEVWire. "markdown-it: Quadratic-time rendering of a single multi-line link reference definition with default options (regression since 14.0) (CVE-2026-111758)." Last modified October 22, 2026. https://kevwire.com/record/CVE-2026-111758.

Evidence summary (plain text)

markdown-it: Quadratic-time rendering of a single multi-line link reference definition with default options (regression since 14.0) (Oct 20, 2026).
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.

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.
CVSS 7.5 (version 3.1; evidence as of Oct 22, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-111758).
EPSS 0.5% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-111758&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-gfr2-rv44-r23c: https://github.com/advisories/GHSA-gfr2-rv44-r23c
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNTo1Mjo1NlrOAAfxiA%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNDowNjoxNFrOAAfxZw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQwODoyNzoyM1rOAAYp9A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozNTo1OFrOAAfwCQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozMDoxM1rOAAfv2A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyODowOVrOAAfv1g%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyNDoyMFrOAAfv1Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDo0NTowN1rOAAfvzQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDozMDoyNlrOAAfvxw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDowMzo1N1rOAAfvxQ%3D%3D
NVD CVE-2026-111758: https://nvd.nist.gov/vuln/detail/CVE-2026-111758
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?pubStartDate=2026-10-12T13:35:55.142Z&pubEndDate=2026-10-26T13:35:55.142Z&resultsPerPage=500&startIndex=6000
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-111758&date=2026-10-26
Record ID: CVE-2026-111758.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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