Vulnerability-Lookup

CVE-2026-48988 (GCVE-0-2026-48988)

Vulnerability from cvelistv5 – Published: 2026-06-17 20:54 – Updated: 2026-06-18 19:41

Title

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

Summary

markdown-it is a Markdown parser. Versions 14.1.1 and below contain a denial-of-service vulnerability when typographer: true is enabled, due to quadratic (O(n^2)) processing in the smartquotes rule. The issue stems from repeatedly modifying strings with replaceAt(), which performs O(n) slicing and concatenation per quote character. This can cause excessive CPU consumption when parsing quote-heavy, user-supplied markdown and may let attackers degrade or disrupt service availability. Although typographer is disabled by default, many production apps enable it for smart typography, making the issue relevant. This issue has been fixed in version 14.2.0.

Severity

5.3 (Medium)


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

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-400 - Uncontrolled Resource Consumption

Assigner

GitHub_M

References

2 references

URL	Tags
https://github.com/markdown-it/markdown-it/securi…	x_refsource_CONFIRM
https://github.com/markdown-it/markdown-it/commit…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
markdown-it	markdown-it	Affected: < 14.2.0

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-48988

Vulnerability from fkie_nvd - Published: 2026-06-17 21:16 - Updated: 2026-06-17 21:16

Severity

5.3 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/markdown-it/markdown-it/commit/9ce2087562c45d1e5ddd9f76b990f4b3fbe040e5
	security-advisories@github.com	https://github.com/markdown-it/markdown-it/security/advisories/GHSA-6v5v-wf23-fmfq

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "markdown-it",
          "vendor": "markdown-it",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 14.2.0"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "markdown-it is a Markdown parser. Versions 14.1.1 and below contain a denial-of-service vulnerability when typographer: true is enabled, due to quadratic (O(n^2)) processing in the smartquotes rule. The issue stems from repeatedly modifying strings with replaceAt(), which performs O(n) slicing and concatenation per quote character. This can cause excessive CPU consumption when parsing quote-heavy, user-supplied markdown and may let attackers degrade or disrupt service availability. Although typographer is disabled by default, many production apps enable it for smart typography, making the issue relevant. This issue has been fixed in version 14.2.0."
    }
  ],
  "id": "CVE-2026-48988",
  "lastModified": "2026-06-17T21:16:23.563",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "LOW",
          "baseScore": 5.3,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 1.4,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-06-17T21:16:23.563",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/markdown-it/markdown-it/commit/9ce2087562c45d1e5ddd9f76b990f4b3fbe040e5"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/markdown-it/markdown-it/security/advisories/GHSA-6v5v-wf23-fmfq"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-400"
        }
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      "source": "security-advisories@github.com",
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    }
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}

GHSA-6V5V-WF23-FMFQ

Vulnerability from github – Published: 2026-06-15 20:41 – Updated: 2026-06-15 20:41

Summary

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

Details

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).

Severity

5.3 (Medium)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 14.1.1"
      },
      "package": {
        "ecosystem": "npm",
        "name": "markdown-it"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "14.2.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-48988"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-15T20:41:06Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nA quadratic time complexity vulnerability exists in markdown-it\u0027s 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.\n\n### Details\n\nThe vulnerability is in the `replaceAt()` helper function used by the smartquotes rule in `lib/rules_core/smartquotes.mjs`:\n\n```javascript\nfunction replaceAt (str, index, ch) {\n  return str.slice(0, index) + ch + str.slice(index + 1)\n}\n```\n\nWhen markdown-it processes a text token containing many quotation marks (either `\"` or `\u0027`) 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.\n\nSince this is called once per quote character in the token, and there are n quote characters, the total time complexity becomes O(n^2).\n\nThe 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\u0027s content (lines 151-152). When the entire input is a single text token of quote characters, this results in quadratic behavior.\n\n### PoC\n\n```javascript\nconst md = require(\u0027markdown-it\u0027);\nconst instance = md({ typographer: true });\n\n// 160,000 consecutive double-quote characters\nconst payload = \u0027\"\u0027.repeat(160000);\n\nconsole.time(\u0027render\u0027);\ninstance.render(payload);\nconsole.timeEnd(\u0027render\u0027);\n// Output: render: ~21000ms (21 seconds)\n\n// Compare with typographer disabled:\nconst safe = md({ typographer: false });\nconsole.time(\u0027render-safe\u0027);\nsafe.render(payload);\nconsole.timeEnd(\u0027render-safe\u0027);\n// Output: render-safe: ~8ms\n```\n\nMeasured timing on a modern system:\n- 10,000 quotes: ~19ms\n- 20,000 quotes: ~51ms\n- 40,000 quotes: ~212ms\n- 80,000 quotes: ~5,430ms\n- 160,000 quotes: ~21,198ms\n\nThe scaling is clearly superlinear (quadratic), with the 80K-\u003e160K step showing a ~3.9x increase for a 2x input increase, consistent with O(n^2).\n\n### Impact\n\nApplications 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.\n\nThe 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.\n\nA 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).",
  "id": "GHSA-6v5v-wf23-fmfq",
  "modified": "2026-06-15T20:41:06Z",
  "published": "2026-06-15T20:41:06Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/markdown-it/markdown-it/security/advisories/GHSA-6v5v-wf23-fmfq"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/markdown-it/markdown-it"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "markdown-it: Quadratic complexity DoS in smartquotes rule via replaceAt string operations"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-48988 (GCVE-0-2026-48988)

FKIE_CVE-2026-48988

GHSA-6V5V-WF23-FMFQ

Summary

Details

PoC

Impact

Tags

Sightings

Nomenclature