The vulnerability allows an attacker to supply a markdown Include directive with a specially crafted path that contains .. sequences. The parser resolves the path on the file system without sufficient validation, enabling arbitrary file read. Additionally, any included file content is inserted into the rendered output without escaping, exposing an unchecked XSS vector even when escape=True is activated. Attackers can thus read sensitive server files and inject malicious scripts into user views.

The flaw applies to all versions of Mistune prior to 3.3.0. The vendor is lepture, and the library is publicly distributed under that name. No specific non‑patched minor releases are identified; all versions earlier than 3.3.0 contain the issue.

The CVSS score of 8.7 indicates high severity. The EPSS score is not available, and the vulnerability is not listed in CISA KEV. Because mistune is commonly used in web applications to render markdown from user data, the likely attack vector is a remote web attacker supplying malicious markdown content. If the application accepts untrusted markdown, the attacker can read arbitrary files on the server and perform XSS against other users. Local attacks would also be possible if an attacker can influence the Include directive via local files.