A path traversal flaw exists in the optional Include directive of Mistune versions prior to 3.3.0. The vulnerability allows an attacker to craft a Markdown payload that instructs the renderer to read non‑existent or unintended files on the local filesystem, including privilege‑sensitive files such as /etc/passwd. The attacker can then inject arbitrary HTML or script content into the rendered output, potentially leading to cross‑site scripting or output disclosure. The weakness is classified as CWE‑22: Path Traversal.

The flaw impacts all installations of the Mistune library provided by the Mistune project, specifically versions before 3.3.0. Any software that imports and uses Mistune to render user‑supplied Markdown is affected.

The CVSS base score of 7.1 indicates a moderate to high impact, while no EPSS data is available and the vulnerability is not listed in CISA’s KEV catalog. Exploitation requires the attacker to supply a crafted Markdown document to the Markdown.read() function; therefore, the attack is limited to systems that expose this function to untrusted input, such as web applications or APIs. As long as an attacker can submit arbitrary Markdown, the file read can be executed locally on the host running the renderer.