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Response snapshot Oct 27, 2026, 03:06 UTC

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VulnerabilityCVE-2026-111766

markdown-it: Quadratic-time denial of service in inline HTML parsing with html enabled

With `html: true`, markdown-it takes quadratic time to parse repeated unterminated HTML constructs inside a paragraph. This affects HTML comments (` `, `?>`, `>`, and `]]>`. When the terminator is absent, matching fails.

At a glance

Exploited
Not known
Severity
3.7 Low (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

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-vfgh-jcmw-g37q (opens a new tab)

What happened

### Summary

With `html: true`, markdown-it takes quadratic time to parse repeated unterminated HTML constructs inside a paragraph. This affects HTML comments (` `, `?>`, `>`, and `]]>`.

When the terminator is absent, matching fails. Parsing eventually reaches the next opener and scans an overlapping suffix again. Repeating this across the paragraph produces quadratic total work. For these inputs, the problem is repeated scanning across separate matches.

The payload must reach inline parsing. Prefixing it with `x ` keeps it inside a paragraph. Starting a line directly with one of these constructs instead invokes HTML block parsing, which does not exhibit the same repeated inline scans for these payloads.

Continue reading

The existing `unclosed '.repeat(n)], 'HTML block': [mdHtml, '<!--'.repeat(n)], 'Plain text': [mdHtml, 'x ' + 'abcd'.repeat(n)] }

for (const [name, [md, src]] of Object.entries(controls)) { console.log({ control: name, ms: measure(md, src) }) }

const repoPayload = '</' + '<!--'.repeat(100000) console.log({ payload: 'Existing pathological test', htmlFalseMs: measure(mdDefault, repoPayload), htmlTrueMs: measure(mdHtml, repoPayload) }) ```

Measurements from the original report on `markdown-it@15.0.2`, Node.js 20:

| Construct | 8,000 repetitions | 64,000 repetitions | | --- | ---: | ---: | | Comment | 170 ms | 8,799 ms | | Processing instruction | 26 ms | 1,566 ms | | Declaration | 93 ms | 5,628 ms | | CDATA | 119 ms | 4,179 ms |

An eightfold increase in repetitions produces approximately 35–60 times the execution time, consistent with quadratic growth becoming dominant.

For the comment payload at 64,000 repetitions:

- HTML disabled: 25 ms. - Closed comments: 29 ms. - Payload at the start of an HTML block: 1 ms. - Plain text: 1 ms.

The existing pathological-test payload with 100,000 comment openers took 33 ms with HTML disabled and 21,030 ms with HTML enabled. A separate comment payload with 128,000 repetitions, approximately 500 KiB, took 32,847 ms.

Exact timings depend on the runtime and hardware.

### Impact

This is an availability issue affecting applications that render attacker-controlled Markdown with `html: true`, including applications using the `commonmark` preset.

On Node.js, a synchronous `render()` call can occupy a CPU core and block the event loop for seconds or tens of seconds, delaying other requests handled by the same process. Repeated submissions can cause denial of service. Browser applications can similarly become unresponsive when rendering on the main thread.

Sanitizing the generated HTML afterwards does not prevent the issue because the expensive processing occurs during Markdown rendering.

The default configuration, with HTML disabled, is unaffected by these payloads. Disabling HTML or limiting input size can reduce exposure while a fix is prepared.

The reported affected release is `15.0.2`; the full affected version range has not been established.

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

Scores and how they were measured

CVSS base severity

3.7

Version 3.1As of

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

Measurement source (opens a new tab)

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

EPSS probability

0.4%

Scored Percentile 36.8%

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-vfgh-jcmw-g37q (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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, 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-vfgh-jcmw-g37q, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-111766, 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 20). markdown-it: Quadratic-time denial of service in inline HTML parsing with html enabled (CVE-2026-111766). https://kevwire.com/record/CVE-2026-111766

MLA

"markdown-it: Quadratic-time denial of service in inline HTML parsing with html enabled (CVE-2026-111766)." KEVWire, 20 Oct. 2026, kevwire.com/record/CVE-2026-111766.

Chicago

KEVWire. "markdown-it: Quadratic-time denial of service in inline HTML parsing with html enabled (CVE-2026-111766)." Last modified October 20, 2026. https://kevwire.com/record/CVE-2026-111766.

Evidence summary (plain text)

markdown-it: Quadratic-time denial of service in inline HTML parsing with html enabled (Oct 20, 2026).
With `html: true`, markdown-it takes quadratic time to parse repeated unterminated HTML constructs inside a paragraph. This affects HTML comments (` `, `?>`, `>`, and `]]>`.

When the terminator is absent, matching fails. Parsing eventually reaches the next opener and scans an overlapping suffix again. Repeating this across the paragraph produces quadratic total work. For these inputs, the problem is repeated scanning across separate matches.
CVSS 3.7 (version 3.1; evidence as of Oct 20, 2026; https://github.com/advisories/GHSA-vfgh-jcmw-g37q).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-111766&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-vfgh-jcmw-g37q: https://github.com/advisories/GHSA-vfgh-jcmw-g37q
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
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-111766&date=2026-10-26
Record ID: CVE-2026-111766.
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 27, 2026, 01:40 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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