Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
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:
<p><a href="javascript%3Aalert(1)">j</a></p> -
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.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | mistune | < 3.3.0 | 3.3.0 |
References
- https://github.com/lepture/mistune/releases/tag/v3.3.0
- https://github.com/lepture/mistune/commit/c7101fcbb6e8790e8e39157c5ca2238fc6dd6cbc
- https://github.com/lepture/mistune/security/advisories/GHSA-8c25-4j27-2rv3
- https://nvd.nist.gov/vuln/detail/CVE-2026-59923
- https://github.com/lepture/mistune
- https://github.com/pypa/advisory-database/tree/main/vulns/mistune/PYSEC-2026-2211.yaml
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N- Weakness (CWE)
CWE-79- Platform
- Python (PyPI)
- Package
mistune- Disclosed
- 08.07.2026
- Updated
- 11.10.2026 14:18
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-2211 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-8c25-4j27-2rv3 - CC BY 4.0
Vulnerability history: mistune
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 20
- First → latest
- 10.2017 09.2026
- Without a fix
- 1
- Exploited in attacks
- 0
Severity breakdown
- High 7
- Medium 13
Disclosures per year
- 2017: 2
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 1
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 17
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