Mistune CVE-2026-59925

Exploitation requires only that the target application pass attacker-controlled text through Mistune's default inline parser - the vulnerable emphasis handling is active in default Mistune parsing, so no special plugin, renderer, or non-default option must be enabled. The concrete trigger is a Markdown payload containing long sequences of well-formed double-asterisk () or triple-asterisk (*) emphasis pairs around a character. The vector is unauthenticated and remote (PR:N, UI:N per CVSS). The main limiting factor is exposure rather than configuration: the flaw is only reachable where untrusted input is rendered, so applications that render only trusted, first-party Markdown are not practically exploitable, and input-size caps or parse timeouts placed ahead of the parser also limit the achievable impact. Affected only on Mistune versions below 3.3.0.

The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, base 7.5) cleanly matches the described flaw: network-reachable, no authentication, no user interaction, and impact confined to availability with no confidentiality or integrity effect. This is a genuine but bounded risk - it is a single-request CPU-exhaustion DoS, not code execution or data exposure. No EPSS score, KEV entry, POC, or SSVC decision data was provided in the input, so exploitation probability cannot be quantified from signals; the assessment rests on the CVSS vector and description alone. The practical priority depends entirely on exposure: services that parse attacker-supplied Markdown (comments, chat, issue trackers, API bodies) inherit the full unauthenticated-DoS risk, whereas deployments that only render trusted first-party Markdown face little real-world danger despite the high base score. Because exploitation requires no special privileges and the trigger is trivial to construct, this should be treated as a real priority for any internet-facing Markdown-rendering endpoint.

An attacker submits a comment, chat message, or API request containing a crafted Markdown string with a long sequence of /* emphasis pairs to any endpoint that renders Markdown with a vulnerable Mistune version. The server's parser enters quadratic processing, consuming excessive CPU on a single unauthenticated request (AV:N/AC:L/PR:N/UI:N); repeating the request across connections can exhaust worker threads and take the service offline. No public POC is referenced in the provided data.