vulnerability / NVI-00016376 / Revision 1
Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
GitHub lists this reviewed advisory for mistune. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
6.1 medium
GitHub source severity; exploitability and local consequence require separate evidence.
As of · GitHub Advisory Database
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
mistune
source listed< 3.3.0
### 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 rend
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system. GitHub Advisory Database
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-8c25-4j27-2rv3 ↗
GitHub Advisory Database · vendor advisory
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Public revision history
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- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.