{"items":[{"id":"NVI-00016376","kind":"vulnerability","title":"Mistune: XSS via percent-encoded javascript URI bypass in safe_url()","revision":1,"unknowns":["Customer inventory, deployment, reachability and exploitation have not been inspected."],"lifecycle":"active","identifiers":["GHSA-8c25-4j27-2rv3","CVE-2026-59923"],"plain_summary":"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.","review_status":"source_listed","control_topics":[],"public_actions":[{"summary":"Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.","source_ids":["sr-6c53edea7e49daa2581f418da474d7ab"],"action_type":"investigate","limitations":"Confirm current vendor support, compatibility and deployment evidence. This report has not tested a fix in your system.","current_supported_target":null}],"schema_version":"1.1.0","source_metrics":[{"kind":"CVSS","as_of":"2026-07-20T21:32:52.000Z","value":"6.1 medium","meaning":"GitHub source severity; exploitability and local consequence require separate evidence.","source_id":"sr-6c53edea7e49daa2581f418da474d7ab"}],"why_it_matters":"Published affected context helps an owner decide what evidence to gather. This report has not examined your application.","related_sources":[{"id":"sr-6c53edea7e49daa2581f418da474d7ab","url":"https://github.com/advisories/GHSA-8c25-4j27-2rv3","basis":"vendor_advisory","notes":"GitHub Advisory Database; CC-BY-4.0. Each advisory links its original attribution. Null normalizes fields.","title":"GHSA-8c25-4j27-2rv3","publisher":"GitHub Advisory Database","observed_at":"2026-11-03T22:35:18.117Z"}],"affected_context":[{"certainty":"source_listed","source_ids":["sr-6c53edea7e49daa2581f418da474d7ab"],"technology":"mistune","known_exclusions":[],"conditions_summary":"### Summary\nAn XSS vulnerability in Mistune allows bypassing of safe_url() protections via percent-encoded javascript URIs.\n\n\n### Details\nThe vulnerability exists in HTMLRenderer.safe_url() in Mistune.\n\nThe function is intended to block harmful URL schemes such as \"javascript:\" by checking the prefix of the provided URL:\n\n    _url = url.lower()\n    if _url.startswith(self.HARMFUL_PROTOCOLS):\n        return \"#harmful-link\"\n\nHowever, 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:\n\n    javascript%3Aalert(1)\n\nSince \"%3A\" is not decoded to \":\", the check does not detect the \"javascript:\" scheme.\n\nWhen rendered in a browser, the URL is decoded, resulting in execution of arbitrary JavaScript upon user interaction.\n\nThis effectively bypasses Mistune's built-in safe_url() protection mechanism.\n\n\n\n### PoC\n1. Install vulnerable version:\n\n    pip install mistune==3.2.0\n\n2. Run the following code:\n\n    import mistune\n\n    markdown = mistune.create_markdown()\n    html = markdown(\"j)\")\n\n    print(html)\n\n3. Output:\n\n    j\n\n4. Open the rend","public_affected_ranges":["< 3.3.0"]}],"null_published_at":"2026-11-04T10:04:27.937Z","source_updated_at":"2026-07-20T21:32:52.000Z","source_disclosed_at":"2026-07-20T21:32:49.000Z","is_synthetic_fixture":false,"null_last_reviewed_at":null,"public_change_summary":"First indexed by Null; source disclosure date is shown separately.","null_first_observed_at":"2026-11-03T22:35:18.117Z"}],"next_cursor":null,"snapshot_revision":"42594"}