Summary
Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
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
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.
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.
CVE-2026-59923 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.3.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
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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Frequently Asked Questions
- What is CVE-2026-59923? CVE-2026-59923 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.
- How severe is CVE-2026-59923? CVE-2026-59923 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of mistune are affected by CVE-2026-59923? mistune (pip) versions < 3.3.0 is affected.
- Is there a fix for CVE-2026-59923? Yes. CVE-2026-59923 is fixed in 3.3.0. Upgrade to this version or later.
- Is CVE-2026-59923 exploitable, and should I be worried? Whether CVE-2026-59923 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-2026-59923 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-2026-59923? Upgrade
mistuneto 3.3.0 or later.