GHSA-r4rv-85jg-w4mf

### Summary A path traversal issue exists in mistune's `Include` directive when markdown files are processed using `md.read()`. A crafted include path can cause files outside the intended markdown directory to be accessed. ### Details The issue occurs in the `Include.parse()` method where user-supplied paths are joined and normalized without verifying that the resulting path remains within an expected directory. ```python relpath = self.parse_title(m) dest = os.path.join(os.path.dirname(source_file), relpath) dest = os.path.normpath(dest) ``` Because the final path is not restricted to a trusted base directory, path traversal sequences such as `../` may reference files outside the intended location. ### Proof of Concept Create a markdown file: ```markdown .. include:: ../../../example.txt ``` Process it using: ```python import mistune from mistune.directives import RSTDirective, Include md = mistune.create_markdown( plugins=[RSTDirective([Include()])] ) result, state = md.read("test.md") print(result) ``` ### Impact Applications that process untrusted markdown files with the `Include` directive enabled may allow unintended file access. The impact depends on how the feature is used and what files are accessible to the running process. ### Recommended Fix Validate the resolved path and ensure it remains within an allowed directory before opening the file.