What is CVE-2026-59926?

CVE-2026-59926 is a medium-severity cross-site scripting (XSS) vulnerability in mistune (pip), affecting versions < 3.3.0. It is fixed in 3.3.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.

Which versions of mistune are affected by CVE-2026-59926?

mistune (pip) versions < 3.3.0 is affected.

Is there a fix for CVE-2026-59926?

Yes. CVE-2026-59926 is fixed in 3.3.0. Upgrade to this version or later.

Is CVE-2026-59926 exploitable, and should I be worried?

Whether CVE-2026-59926 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-59926 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-59926 in mistune

Q: Which versions of mistune are affected by CVE-2026-59926?

mistune (pip) versions < 3.3.0 is affected.

Q: Is there a fix for CVE-2026-59926?

Yes. CVE-2026-59926 is fixed in 3.3.0. Upgrade to this version or later.

Q: Is CVE-2026-59926 exploitable, and should I be worried?

Whether CVE-2026-59926 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-59926 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-59926?

Upgrade mistune to 3.3.0 or later.

Summary

Mistune: XSS via unescaped class option in Admonition directive

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.

Impact

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

Affected versions

mistune (< 3.3.0)

Security releases

mistune → 3.3.0 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade mistune to 3.3.0 or later to resolve this vulnerability.

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-59926? CVE-2026-59926 is a medium-severity cross-site scripting (XSS) vulnerability in mistune (pip), affecting versions < 3.3.0. It is fixed in 3.3.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
Which versions of mistune are affected by CVE-2026-59926? mistune (pip) versions < 3.3.0 is affected.
Is there a fix for CVE-2026-59926? Yes. CVE-2026-59926 is fixed in 3.3.0. Upgrade to this version or later.
Is CVE-2026-59926 exploitable, and should I be worried? Whether CVE-2026-59926 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-59926 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-59926? Upgrade mistune to 3.3.0 or later.

Other vulnerabilities in mistune

CVE-IDSeverityPackage summary

CVE-2026-59929Mediummistune: Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution CVE-2026-59930Mediummistune: Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with…CVE-2026-59924Mediummistune: Mistune: Arbitrary File Read via Include directive path traversal CVE-2026-59922Highmistune: Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)CVE-2026-59923Mediummistune: Mistune: XSS via percent-encoded javascript URI bypass in safe_url()