# 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/)*

src/fallenthrone/views/flaskdocs.py

@@ -4,7 +4,7 @@
 Gives flask's documentation on tips at http://localhost/docs
 """
 
-from flask import Blueprint
+from flask import Blueprint,safe_join
 from fallenthrone import app
 from flask import make_response, send_file
 import os
@@ -14,7 +14,7 @@
 @docs_pages.route ('/', defaults={'filename': 'index.html'})
 @docs_pages.route ('/<path:filename>')
 def docserver (filename):
-    ifile = os.path.join ("docs", "flask", filename)
+    ifile = safe_join ("docs", "flask", filename)
     return send_file (ifile)
 
 app.register_blueprint (docs_pages, url_prefix='/docs')