Mistune: Arbitrary File Read via Include directive path traversal

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.

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:

.. include:: ../../../example.txt

Process it using:

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.