Mistune prior to version 3.3.0 contains an algorithmic‑complexity vulnerability in its inline link parser that triggers a quadratic time (O(N^2)) operation when processing long runs of unmatched '[' characters. An unauthenticated attacker can send a small Markdown payload that causes the parser to take more than 30 seconds to complete for a 16 KB input, exhausting CPU resources of rendering workers. The result is a loss of service availability for any application that relies on Mistune for Markdown rendering over HTTP or other user‑controllable interfaces.

The affected product is the Mistune Markdown parser for Python. Any project that includes Mistune 3.2.x or earlier versions is impacted. Organizations that deploy Mistune within web services, content management systems, chat platforms or other user‑facing applications should verify the installed version. Versions 3.3.0 and later are not affected.

With a CVSS score of 8.7 the risk of exploitation is high. The EPSS score is currently unavailable, and the vulnerability is not listed in the CISA KEV catalog, but the lack of authentication and trivial payload construction make the attack easier to launch. An attacker can send the exploit payload remotely to any endpoint that transmits user‑supplied Markdown to Mistune, causing the rendering process to become CPU‑bound. Because the attacker only needs to supply a long sequence of '[' characters, the exploitation conditions are minimal and the probability of attack is significant in exposed environments.