Mistune, a pure‑Python Markdown parser, has a flaw in its inline_parser that causes an inefficient quadratic‑time scan when parsing text that includes long runs of correctly matched double or triple asterisk emphasis markers around a single character. Each possible opening marker is examined and the parser searches forward for a matching closing marker, resulting in CPU usage that grows roughly with the square of the input length. An attacker can supply a small Markdown string containing such a pattern and trigger disproportionate processing time, leading to a denial‑of‑service condition for the process rendering the Markdown.

All releases of Lepture Mistune before version 3.3.0 are affected. Any application that imports or uses Mistune to render user‑supplied Markdown is potentially exposed, including web frameworks, static site generators, CMS plugins, or APIs. The vulnerability lies in the inline_parser module, so any component that triggers that code during rendering could be abused. (These types of systems are inferred from common usage of Mistune.)

The CVSS score of 7.5 indicates a high severity vulnerability, while the EPSS score of less than 1 % suggests it is rarely exploited in the wild. The flaw is not listed in CISA’s KEV catalog. An attacker does not need privileged access; sending a malicious Markdown payload to any endpoint that forwards text to Mistune is sufficient. Based on the description, the likely attack vector is a crafted Markdown document sent to a public or authenticated endpoint that performs server‑side rendering, which will cause the parsing engine to consume excessive CPU resources.