A path traversal attack (also known as directory traversal) aims to a…

…ccess 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”.

The `os.path.join` call is unsafe for use with untrusted input. When the `os.path.join` call encounters an absolute path, it ignores all the parameters it has encountered till that point and starts working with the new absolute path.  Please see the example below.
```
>>> import os.path
>>> static = "path/to/mySafeStaticDir"
>>> malicious = "/../../../../../etc/passwd"
>>> os.path.join(t,malicious)
'/../../../../../etc/passwd'
```
Since the "malicious" parameter represents an absolute path, the result of `os.path.join` ignores the static directory completely. Hence, untrusted input is passed via the `os.path.join` call to `flask.send_file` can lead to path traversal attacks.

In this case, the problems occurs due to the following code :
https://github.com/HolgerGraef/MSM/blob/6dd2c9557e0285e1270c84375ebd6f8d10e422a4/app/main/views.py#L544

Here, the `path` parameter is attacker controlled. This parameter passes through the unsafe `os.path.join` call making the effective directory and filename passed to the `send_file` call attacker controlled. This leads to a path traversal attack.

The bug can be verified using a proof of concept similar to the one shown below.

```
curl --path-as-is 'http://<domain>/plugins//../../../../etc/passwd"'
```

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.

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

app/main/views.py

@@ -1,4 +1,5 @@
 from flask import render_template, redirect, request, jsonify, send_file, abort, current_app
+from werkzeug.utils import safe_join 
 from flask_login import current_user, login_required, login_user, logout_user
 from .. import db
 from .. import plugins
@@ -679,7 +680,7 @@ def swapactionorder():  # TODO: sort out permissions for this (e.g. who has the
 @login_required
 def static_file(path):
     # TODO: this looks a bit unsafe to me
-    return send_file("../plugins/" + path)
+    return send_file(safe_join('../plugins/',path))
 
 
 def str_to_bool(str):