CVE-2026-59926: Mistune Cross-Site Scripting Vulnerability

MODERATE
Advisory Identifier: GHSA-g97x-gvcm-x72h
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CWE-79
Published
Advisory Summary

CVE-2026-59926 is a MODERATE cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Mistune libraries in the PyPI ecosystem. The issue involves xss via unescaped class option in admonition directive. Remediation is available by upgrading affected packages to version 3.3.0.

CVE-2026-59926 at a Glance

CVE Identifier CVE-2026-59926
GHSA Identifier GHSA-g97x-gvcm-x72h
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Mistune
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix CVE-2026-59926

To remediate this vulnerability, upgrade all affected Mistune dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

mistune fixed: 3.3.0 
pip install mistune>=3.3.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show mistune

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-59926, inspect your installed packages using pip:

pip show mistune

Analyze the terminal output to locate any version of Mistune in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.3.0), your application is affected.

Note: The pip show mistune command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. In Mistune, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

In `src/mistune/directives/admonition.py`, the `render_admonition()` function concatenates the `:class:` option directly into the HTML class attribute without escaping (lines 63-68). This allows attribute injection and XSS even when `HTMLRenderer(escape=True)` is used. The directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.

CVE-2026-59926 Affected Mistune Versions

1 component(s)
Fixed Versions:
3.3.0
Product / Package Ecosystem Affected Versions Fixed Version
mistune
pypi/mistune
PyPI
< 3.3.0
3.3.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/lepture/mistune/security/advisories/GHSA-g97x-gvcm-x72h
NVD — CVE-2026-59926 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-59926
Security Fix Commit FIX COMMIT
https://github.com/lepture/mistune/commit/a3cb6e5655308797e8be021d6c7b5bab13cbace2
Mistune Source Repository REPOSITORY
https://github.com/lepture/mistune
Mistune Release RELEASE
https://github.com/lepture/mistune/releases/tag/v3.2.1
Mistune Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2214.yaml

Frequently Asked Questions

What is CVE-2026-59926?
CVE-2026-59926 is a security vulnerability in Mistune involving XSS via unescaped class option in Admonition directive under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Mistune versions are affected?
Affected versions include releases prior to 3.3.0 across affected packages: mistune.
What version fixes CVE-2026-59926?
Upgrading to 3.3.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is mistune affected?
Yes, mistune is affected in releases prior to 3.3.0. Upgrade your dependency to 3.3.0 or later.
What CWE is associated with CVE-2026-59926?
CVE-2026-59926 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check PyPI dependencies for CVE-2026-59926?
Run the dependency tree command: 'pip show mistune'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-g97x-gvcm-x72h",
  "modified" : "2026-07-20T21:34:14Z",
  "published" : "2026-07-20T21:34:14Z",
  "aliases" : [ "CVE-2026-59926" ],
  "summary" : "Mistune: XSS via unescaped class option in Admonition directive",
  "details" : "In `src/mistune/directives/admonition.py`, the `render_admonition()` function concatenates the `:class:` option directly into the HTML class attribute without escaping (lines 63-68).\n\nThis allows attribute injection and XSS even when `HTMLRenderer(escape=True)` is used.\n\nThe directive name parameter is safe (validated against whitelist), but the class option comes from raw user input.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mistune"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.3.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/security/advisories/GHSA-g97x-gvcm-x72h"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-59926"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/commit/a3cb6e5655308797e8be021d6c7b5bab13cbace2"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lepture/mistune"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lepture/mistune/releases/tag/v3.2.1"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2214.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T21:34:14Z",
    "nvd_published_at" : "2026-07-08T17:17:28Z"
  }
}