GHSA-8C25-4J27-2RV3

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

  1. Install vulnerable version:

    pip install mistune==3.2.0

  2. Run the following code:

    import mistune

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

    print(html)

  3. Output:

    j

  4. Open the rendered HTML in a browser and click the link.

  5. 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
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{
  "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()"
}



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