Path traversal check

app/app.py

@@ -12,7 +12,7 @@ from flask_swagger_ui import get_swaggerui_blueprint
 from translatehtml import translate_html
 from werkzeug.utils import secure_filename
 
-from app import flood, remove_translated_files
+from app import flood, remove_translated_files, security
 from app.language import detect_languages, transliterate
 from .api_keys import Database
 from .suggestions import Database as SuggestionsDatabase
@@ -621,10 +621,15 @@ def create_app(args):
         Download a translated file
         """
         if args.disable_files_translation:
-            abort(403, description="Files translation are disabled on this server.")
+            abort(400, description="Files translation are disabled on this server.")
-
+        
-
         filepath = os.path.join(get_upload_dir(), filename)
+        try:
+            checked_filepath = security.path_traversal_check(filepath, get_upload_dir())
+            if os.path.isfile(checked_filepath):
+                filepath = checked_filepath
+        except security.SuspiciousFileOperation:
+            abort(400, description="Invalid filename")
 
         return_data = io.BytesIO()
         with open(filepath, 'rb') as fo:

app/security.py

@@ -0,0 +1,14 @@
+import os
+
+class SuspiciousFileOperation(Exception):
+    pass
+
+def path_traversal_check(unsafe_path, known_safe_path):
+    known_safe_path = os.path.abspath(known_safe_path)
+    unsafe_path = os.path.abspath(unsafe_path)
+
+    if (os.path.commonprefix([known_safe_path, unsafe_path]) != known_safe_path):
+        raise SuspiciousFileOperation("{} is not safe".format(unsafe_path))
+
+    # Passes the check
+    return unsafe_path

app/templates/index.html

@@ -175,7 +175,7 @@
                         <div class="row" v-if="translationType === 'files'">
                             <div class="file-dropzone">
                                 <div v-if="inputFile === false" class="dropzone-content">
-                                    <span>Supported file format: [[ supportedFilesFormatFormatted ]]</span>
+                                    <span>Supported file formats: [[ supportedFilesFormatFormatted ]]</span>
                                     <form action="#">
                                         <div class="file-field input-field">
                                             <div class="btn">