Vulnerability-Lookup

CVE-2026-59923 (GCVE-0-2026-59923)

Vulnerability from cvelistv5 – Published: 2026-07-08 16:14 – Updated: 2026-07-08 16:41

Title

Mistune: XSS via percent-encoded javascript URI bypass in safe_url()

Summary

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.

Severity

6.1 (Medium)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-07-08 16:41 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

GitHub_M mitre root

References

3 references

URL	Tags
https://github.com/lepture/mistune/security/advis…	x_refsource_CONFIRM
https://github.com/lepture/mistune/commit/c7101fc…	x_refsource_MISC
https://github.com/lepture/mistune/releases/tag/v3.3.0	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
lepture	mistune	Affected: < 3.3.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-59923

Vulnerability from fkie_nvd - Published: 2026-07-08 17:17 - Updated: 2026-07-09 19:35

Severity

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

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/lepture/mistune/commit/c7101fcbb6e8790e8e39157c5ca2238fc6dd6cbc	Patch
security-advisories@github.com	https://github.com/lepture/mistune/releases/tag/v3.3.0	Release Notes
security-advisories@github.com	https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3	Exploit, Vendor Advisory
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3	Exploit, Vendor Advisory

Impacted products

	Vendor	Product	Version
	mistune_project	mistune	*

JSON

To clipboard

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  "affected": [
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      "affectedData": [
        {
          "product": "mistune",
          "vendor": "lepture",
          "versions": [
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              "status": "affected",
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    }
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GHSA-8C25-4J27-2RV3

Vulnerability from github – Published: 2026-07-20 21:32 – Updated: 2026-07-20 21:32

Summary

Mistune: XSS via percent-encoded javascript URI bypass in safe_url()

Details

Summary

An XSS vulnerability in Mistune allows bypassing of safe_url() protections via percent-encoded javascript URIs.

Details

The vulnerability exists in HTMLRenderer.safe_url() in Mistune.

The function is intended to block harmful URL schemes such as "javascript:" by checking the prefix of the provided URL:

_url = url.lower()
if _url.startswith(self.HARMFUL_PROTOCOLS):
    return "#harmful-link"

However, the input URL is not URL-decoded before this check. Because of this, an attacker can use percent-encoding to bypass the filter. For example:

javascript%3Aalert(1)

Since "%3A" is not decoded to ":", the check does not detect the "javascript:" scheme.

When rendered in a browser, the URL is decoded, resulting in execution of arbitrary JavaScript upon user interaction.

This effectively bypasses Mistune's built-in safe_url() protection mechanism.

PoC

Install vulnerable version:

pip install mistune==3.2.0
Run the following code:

import mistune

markdown = mistune.create_markdown() html = markdown("j")

print(html)
Output:

j
Open the rendered HTML in a browser and click the link.
The browser decodes "%3A" into ":" and executes:

javascript:alert(1)

Impact

This is a cross-site scripting (XSS) vulnerability.

An attacker can craft a malicious Markdown link that executes JavaScript in the victim's browser when clicked.

Impact includes: - Session hijacking (e.g., cookie theft) - Execution of arbitrary JavaScript in the victim's context - Potential account takeover depending on the application

This affects any application that renders user-controlled Markdown using Mistune without additional URL sanitization.

Severity

6.1 (Medium)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-59923"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-20T21:32:49Z",
    "nvd_published_at": "2026-07-08T17:17:27Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\nAn XSS vulnerability in Mistune allows bypassing of safe_url() protections via percent-encoded javascript URIs.\n\n\n### Details\nThe vulnerability exists in HTMLRenderer.safe_url() in Mistune.\n\nThe function is intended to block harmful URL schemes such as \"javascript:\" by checking the prefix of the provided URL:\n\n    _url = url.lower()\n    if _url.startswith(self.HARMFUL_PROTOCOLS):\n        return \"#harmful-link\"\n\nHowever, the input URL is not URL-decoded before this check. Because of this, an attacker can use percent-encoding to bypass the filter. For example:\n\n    javascript%3Aalert(1)\n\nSince \"%3A\" is not decoded to \":\", the check does not detect the \"javascript:\" scheme.\n\nWhen rendered in a browser, the URL is decoded, resulting in execution of arbitrary JavaScript upon user interaction.\n\nThis effectively bypasses Mistune\u0027s built-in safe_url() protection mechanism.\n\n\n\n### PoC\n1. Install vulnerable version:\n\n    pip install mistune==3.2.0\n\n2. Run the following code:\n\n    import mistune\n\n    markdown = mistune.create_markdown()\n    html = markdown(\"[j](javascript%3Aalert(1))\")\n\n    print(html)\n\n3. Output:\n\n    \u003cp\u003e\u003ca href=\"javascript%3Aalert(1)\"\u003ej\u003c/a\u003e\u003c/p\u003e\n\n4. Open the rendered HTML in a browser and click the link.\n\n5. The browser decodes \"%3A\" into \":\" and executes:\n\n    javascript:alert(1)\n\n\n### Impact\nThis is a cross-site scripting (XSS) vulnerability.\n\nAn attacker can craft a malicious Markdown link that executes JavaScript in the victim\u0027s browser when clicked.\n\nImpact includes:\n- Session hijacking (e.g., cookie theft)\n- Execution of arbitrary JavaScript in the victim\u0027s context\n- Potential account takeover depending on the application\n\nThis affects any application that renders user-controlled Markdown using Mistune without additional URL sanitization.",
  "id": "GHSA-8c25-4j27-2rv3",
  "modified": "2026-07-20T21:32:49Z",
  "published": "2026-07-20T21:32:49Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-59923"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/commit/c7101fcbb6e8790e8e39157c5ca2238fc6dd6cbc"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lepture/mistune"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/releases/tag/v3.3.0"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2211.yaml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mistune: XSS via percent-encoded javascript URI bypass in safe_url()"
}

OPENSUSE-SU-2026:21339-1

Vulnerability from csaf_opensuse - Published: 2026-07-14 10:41 - Updated: 2026-07-14 10:41

Summary

Security update for python-mistune

Severity

Moderate

Scope

10 vulnerabilities in this advisory, including CVE-2026-59923.

CVE-2026-59923

5.4 (Medium)


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

Recommended: 1 product

Open the full advisory

PYSEC-2026-2211

Vulnerability from pysec - Published: 2026-07-08 17:17 - Updated: 2026-07-13 05:49

Details

Severity

6.1 (Medium)


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

Impacted products

Name	purl
mistune	pkg:pypi/mistune

Aliases

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {},
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune",
        "purl": "pkg:pypi/mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.3.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.1.0",
        "0.2.0",
        "0.3.0",
        "0.3.1",
        "0.4",
        "0.4.1",
        "0.5",
        "0.5.1",
        "0.6",
        "0.7",
        "0.7.1",
        "0.7.2",
        "0.7.3",
        "0.7.4",
        "0.8",
        "0.8.1",
        "0.8.2",
        "0.8.3",
        "0.8.4",
        "2.0.0",
        "2.0.0a1",
        "2.0.0a2",
        "2.0.0a3",
        "2.0.0a4",
        "2.0.0a5",
        "2.0.0a6",
        "2.0.0rc1",
        "2.0.1",
        "2.0.2",
        "2.0.3",
        "2.0.4",
        "2.0.5",
        "2.1.0",
        "3.0.0",
        "3.0.0a1",
        "3.0.0a2",
        "3.0.0a3",
        "3.0.0rc1",
        "3.0.0rc2",
        "3.0.0rc3",
        "3.0.0rc4",
        "3.0.0rc5",
        "3.0.1",
        "3.0.2",
        "3.1.0",
        "3.1.1",
        "3.1.2",
        "3.1.3",
        "3.1.4",
        "3.2.0",
        "3.2.1"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-59923",
    "GHSA-8c25-4j27-2rv3"
  ],
  "details": "Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.",
  "id": "PYSEC-2026-2211",
  "modified": "2026-07-13T05:49:57.530387Z",
  "published": "2026-07-08T17:17:27.910Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/lepture/mistune/releases/tag/v3.3.0"
    },
    {
      "type": "FIX",
      "url": "https://github.com/lepture/mistune/commit/c7101fcbb6e8790e8e39157c5ca2238fc6dd6cbc"
    },
    {
      "type": "EVIDENCE",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ]
}

ubuntu-cve-2026-59923

Vulnerability from osv_ubuntu

Published

2026-07-08 17:17

Modified

2026-07-15 17:53

Summary

Details

Severity

6.1 (Medium)


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

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-59923	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-59923	REPORT
	https://github.com/lepture/mistune/security/advis…	REPORT

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-mistune",
            "binary_version": "0.7.1-1"
          },
          {
            "binary_name": "python3-mistune",
            "binary_version": "0.7.1-1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:16.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@0.7.1-1?arch=source\u0026distro=xenial"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.6-2",
        "0.7.1-1"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-mistune",
            "binary_version": "0.8.3-2"
          },
          {
            "binary_name": "python3-mistune",
            "binary_version": "0.8.3-2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:18.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@0.8.3-2?arch=source\u0026distro=bionic"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.7.4-1",
        "0.8-1",
        "0.8.1-1",
        "0.8.3-2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-mistune",
            "binary_version": "0.8.4-2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:20.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@0.8.4-2?arch=source\u0026distro=focal"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.8.4-1",
        "0.8.4-2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-mistune",
            "binary_version": "2.0.0-1+really0.8.4-1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:22.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@2.0.0-1+really0.8.4-1?arch=source\u0026distro=jammy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.8.4-4",
        "0.8.4-5",
        "2.0.0-1+really0.8.4-1"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-mistune",
            "binary_version": "3.0.2-1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:24.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@3.0.2-1?arch=source\u0026distro=noble"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.0.4-2",
        "3.0.2-1"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-mistune",
            "binary_version": "3.1.3-1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:25.10",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@3.1.3-1?arch=source\u0026distro=questing"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.0.2-2",
        "3.1.3-1"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-mistune",
            "binary_version": "3.1.4-1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:26.04:LTS",
        "name": "mistune",
        "purl": "pkg:deb/ubuntu/mistune@3.1.4-1?arch=source\u0026distro=resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.1.3-1",
        "3.1.4-1"
      ]
    }
  ],
  "aliases": [],
  "details": "Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.",
  "id": "UBUNTU-CVE-2026-59923",
  "modified": "2026-07-15T17:53:45Z",
  "published": "2026-07-08T17:17:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-59923"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-59923"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-59923"
  ]
}

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.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-59923 (GCVE-0-2026-59923)

FKIE_CVE-2026-59923

GHSA-8C25-4J27-2RV3

Summary

Details

PoC

Impact

OPENSUSE-SU-2026:21339-1

PYSEC-2026-2211

ubuntu-cve-2026-59923

Vulnerability from osv_ubuntu

Tags

Sightings

Nomenclature

Related by attack behaviour