# Absolute Path Traversal due to incorrect use of send_file call

A path traversal attack (also known as directory traversal) aims to access files and directories that are stored outside the web root folder. By manipulating variables that reference files with “dot-dot-slash (../)” sequences and its variations or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system including application source code or configuration and critical system files. This attack is also known as “dot-dot-slash”, “directory traversal”, “directory climbing” and “backtracking”.

## Root Cause Analysis

Passing untrusted input to `flask.send_file`can lead to path traversal attacks.

In this case, the problems occurs due to the following code :
https://github.com/piaoyunsoft/bt_lnmp/blob/fa49519b04586a00e76c105e7ce1da36eadf6922/www/server/panel/BTPanel/__init__.py#L858

Here, the `filename` parameter is attacker controlled and is used as the filename passed to the `send_file` call. This leads to a path traversal attack.

## Remediation

This can be fixed by preventing flow of untrusted data to the vulnerable `send_file` function. In case the application logic necessiates this behaviour, one can either use the `flask.safe_join` to join untrusted paths or replace `flask.send_file` calls with `flask.send_from_directory` calls.

## References
* [OWASP Path Traversal](https://owasp.org/www-community/attacks/Path_Traversal)
* github/securitylab#669

### This bug was found using *[CodeQL by Github](codeql.github.com/)*

sphere/gui/gui_server.py

@@ -1,17 +1,20 @@
 import flask
 from furl import furl
 from datetime import date
+import os
 import pathlib
 from .. import db_drivers
 
 
 app = flask.Flask(__name__)
 
+
 @app.route('/')
 def index():
     users = app._db_driver.get_users()
     return flask.render_template('index.html', users=users)
 
+
 @app.route('/<int:user_id>')
 def user(user_id):
     user = app._db_driver.get_users(user_id=user_id)
@@ -20,28 +23,32 @@ def user(user_id):
         return flask.render_template('user.html', user=user, snapshots=snapshots)
     return 'wrong page', 404
 
+
 @app.route('/<int:user_id>/<int:snapshot_id>')
 def snapshot(user_id, snapshot_id):
     user = app._db_driver.get_users(user_id=user_id)
     if not user:
         return 'wrong page', 404
-    snapshot = app._db_driver.get_snapshots(user_id=user_id, snapshot_id=snapshot_id)
+    snapshot = app._db_driver.get_snapshots(
+        user_id=user_id, snapshot_id=snapshot_id)
     if not snapshot:
         return 'wrong page', 404
     return flask.render_template('snapshot.html', user=user, snapshot=snapshot)
 
+
 @app.route('/uploads/<path:path>')
 def download_file(path):
     return flask.send_file(
-            '../../' + path,
-            attachment_filename=f'{path}.jpg',
-            mimetype='image/jpg')
+        flask.safe_join(os.path.join(os.path.dirname(
+            os.path.abspath(__file__)), '../../'), path),
+        attachment_filename=f'{path}.jpg',
+        mimetype='image/jpg')
 
 
 def run_server(host='0.0.0.0', port=8080, database_url='mongodb://0.0.0.0:27017/'):
     """
     Runs the GUI server on host, port that connects to the database.
-    
+
     :param host: The GUI's host address, defaults to '0.0.0.0'
     :type host: str, optional
     :param port: The GUI's port number, defaults to 8080
@@ -50,6 +57,6 @@ def run_server(host='0.0.0.0', port=8080, database_url='mongodb://0.0.0.0:27017/
     :type database_url: str
     """
     f = furl(database_url)
-    db, db_host, db_port = f.scheme, f.host, f.port 
+    db, db_host, db_port = f.scheme, f.host, f.port
     app._db_driver = db_drivers[db].Driver(db_host, db_port)
-    app.run(host=host, port=port)
+    app.run(host=host, port=port)