diff --git a/README.md b/README.md
index 91fac27..4218c8a 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,9 @@ Create a file (e.g., called `run.sh`) with the following:
 #!/bin/bash
 export FLASK_SERVE_DIR="/home/YOURUSER/workspace/somesite/build/html"
 export FLASK_MOTHERSHIP="http://localhost:8888/api/staticshield"
+# Optional path to 403.html, 404.html to show on those errors; leave empty to use default messages
+export FLASK_ERROR_PAGES_DIR=""
+#export FLASK_ERROR_PAGES_DIR="/home/YOURUSER/workspace/errorpages/"
 export FLASK_SESSION_COOKIE_NAME="staticshield"
 export FLASK_PERMANENT_SESSION_LIFETIME=7200
 
@@ -55,6 +58,9 @@ WorkingDirectory=/srv/staticshield
 #StandardOutput=file:/srv/logs/staticshield.log
 Environment=FLASK_SERVE_DIR="/srv/some_static_website/html"
 Environment=FLASK_MOTHERSHIP="https://api.example.com/api/staticshield"
+# Optional path to 403.html, 404.html to show on those errors; leave empty to use default messages
+Environment=FLASK_ERROR_PAGES_DIR=""
+#Environment=FLASK_ERROR_PAGES_DIR="/srv/shared/errorpages/"
 Environment=FLASK_SESSION_COOKIE_NAME="staticshield"
 # Max session length of 2h
 Environment=FLASK_PERMANENT_SESSION_LIFETIME=7200
diff --git a/staticshield.py b/staticshield.py
index ddb63a2..8981f17 100644
--- a/staticshield.py
+++ b/staticshield.py
@@ -33,7 +33,7 @@ Session(app)
 # Verify the required configuration
 # SERVE_DIR: Base dir of the files we want to serve; Flask will take care not to escape this dir
 # MOTHERSHIP: Mothership server and login-url, which will redirect here with a sessionstart/SEKRIT
-config_vars = ['SERVE_DIR', 'MOTHERSHIP']
+config_vars = ['SERVE_DIR', 'MOTHERSHIP', 'ERROR_PAGES_DIR']
 for config_var in config_vars:
     if config_var not in app.config:
         print(f'FLASK_{config_var} env var not set')
@@ -42,6 +42,22 @@ for config_var in config_vars:
         app.logger.info('Config env %s with value "%s"', config_var, app.config[config_var])
 
 
+def handle_error_code(http_code):
+    """Handle 403/404/whatever HTTP response; either with a simple default, or with custom file.
+
+    :param int http_code: HTTP status code to handle and return
+    """
+    path = f'{http_code}.html'
+    if app.config['ERROR_PAGES_DIR']:
+        file_path = os.path.join(app.config['ERROR_PAGES_DIR'], path)
+        app.logger.debug(file_path)
+        if os.path.isfile(file_path):
+            app.logger.info('Serving file %s', str(file_path))
+            # This takes a base directory and a path, and ensures that the path is contained in the directory, which makes it safe to accept user-provided paths.
+            return send_from_directory(app.config['ERROR_PAGES_DIR'], path), http_code
+    return 'Unable to set up session', http_code
+
+
 @app.route('/<path:path>', methods=['GET', 'POST'])
 def all_routes(path):
     """Intercept all routes and proxy it through a session.
@@ -67,7 +83,7 @@ def all_routes(path):
 
         if secret == 'denied':
             # Mother says no
-            return 'Unable to set up session', 403
+            return handle_error_code(403)
 
         # Ask the mothership if the secret is known to them, to prevent someone from just making up a URL
         # Mothership will invalidate this secret token upon handling this request to prevent replay
@@ -87,16 +103,16 @@ def all_routes(path):
                     app.logger.warning('new session aborted, secret "%s" was incorrect, not redirecting to %s', secret, redirect_path)
                 except ValueError as e:
                     app.logger.error('Error while decoding challenge response: %s', str(e))
-                    return 'Unable to set up session', 403
+                    return handle_error_code(403)
         except urllib.error.URLError as e:
             app.logger.error('lolwtf, server not found: %s', str(e.reason))
-            return 'Unable to set up session', 403
+            return handle_error_code(403)
         except urllib.error.HTTPError as e:
             app.logger.error('lolnope, server says no: %s', str(e.reason))
-            return 'Unable to set up session', 403
+            return handle_error_code(403)
 
         # Fallback to disallow
-        return 'Unable to set up session', 403
+        return handle_error_code(403)
 
     # check if the users exist or not
     if not session.get('id'):
@@ -114,7 +130,7 @@ def all_routes(path):
         return send_from_directory(app.config['SERVE_DIR'], path)
     else:
         app.logger.error('File not found: %s', str(file_path))
-        return 'Sorry, 404'
+        return handle_error_code(404)
 
 
 @app.route('/')