diff --git a/onionshare/web.py b/onionshare/web.py
index 2ff60bd2..68b7fcb2 100644
--- a/onionshare/web.py
+++ b/onionshare/web.py
@@ -21,7 +21,7 @@ from distutils.version import StrictVersion as Version
 import queue, mimetypes, platform, os, sys, socket, logging
 from urllib.request import urlopen
 
-from flask import Flask, Response, request, render_template_string, abort
+from flask import Flask, Response, request, render_template_string, abort, make_response
 from flask import __version__ as flask_version
 
 from . import strings, common
@@ -47,6 +47,16 @@ file_info = []
 zip_filename = None
 zip_filesize = None
 
+strings.load_strings(common)
+security_headers = [
+    ('Content-Security-Policy', 'default-src \'self\'; style-src \'unsafe-inline\'; img-src \'self\' data:;'),
+    ('X-Frame-Options', 'DENY'),
+    ('X-Xss-Protection', '1; mode=block'),
+    ('X-Content-Type-Options', 'nosniff'),
+    ('Referrer-Policy', 'no-referrer'),
+    ('Server', strings._('version_string').format(common.get_version()))
+]
+
 def set_file_info(filenames, processed_size_callback=None):
     """
     Using the list of filenames being shared, fill in details that the web
@@ -175,16 +185,23 @@ def index(slug_candidate):
     global stay_open, download_in_progress
     deny_download = not stay_open and download_in_progress
     if deny_download:
-        return render_template_string(open(common.get_resource_path('html/denied.html')).read())
+        r = make_response(render_template_string(open(common.get_resource_path('html/denied.html')).read()))
+        for header,value in security_headers:
+            r.headers.set(header, value)
+        return r
 
     # If download is allowed to continue, serve download page
-    return render_template_string(
+
+    r = make_response(render_template_string(
         open(common.get_resource_path('html/index.html')).read(),
         slug=slug,
         file_info=file_info,
         filename=os.path.basename(zip_filename),
         filesize=zip_filesize,
-        filesize_human=common.human_readable_filesize(zip_filesize))
+        filesize_human=common.human_readable_filesize(zip_filesize)))
+    for header,value in security_headers:
+        r.headers.set(header, value)
+    return r
 
 # If the client closes the OnionShare window while a download is in progress,
 # it should immediately stop serving the file. The client_cancel global is
@@ -203,7 +220,10 @@ def download(slug_candidate):
     global stay_open, download_in_progress
     deny_download = not stay_open and download_in_progress
     if deny_download:
-        return render_template_string(open(common.get_resource_path('html/denied.html')).read())
+        r = make_response(render_template_string(open(common.get_resource_path('html/denied.html')).read()))
+        for header,value in security_headers:
+            r.headers.set(header, value)
+        return r
 
     global download_count
 
@@ -286,13 +306,14 @@ def download(slug_candidate):
             shutdown_func()
 
     r = Response(generate())
-    r.headers.add('Content-Length', zip_filesize)
-    r.headers.add('Content-Disposition', 'attachment', filename=basename)
-
+    r.headers.set('Content-Length', zip_filesize)
+    r.headers.set('Content-Disposition', 'attachment', filename=basename)
+    for header,value in security_headers:
+        r.headers.set(header, value)
     # guess content type
     (content_type, _) = mimetypes.guess_type(basename, strict=False)
     if content_type is not None:
-        r.headers.add('Content-Type', content_type)
+        r.headers.set('Content-Type', content_type)
     return r
 
 
@@ -311,7 +332,10 @@ def page_not_found(e):
             force_shutdown()
             print(strings._('error_rate_limit'))
 
-    return render_template_string(open(common.get_resource_path('html/404.html')).read())
+    r = make_response(render_template_string(open(common.get_resource_path('html/404.html')).read()))
+    for header,value in security_headers:
+        r.headers.set(header, value)
+    return r
 
 # shutting down the server only works within the context of flask, so the easiest way to do it is over http
 shutdown_slug = common.random_string(16)