diff --git a/core/admin/mailu/__init__.py b/core/admin/mailu/__init__.py
index 76c53f4b..8ab8ed0e 100644
--- a/core/admin/mailu/__init__.py
+++ b/core/admin/mailu/__init__.py
@@ -1,8 +1,8 @@
+""" Mailu admin app
+"""
+
 import flask
 import flask_bootstrap
-import redis
-from flask_kvsession import KVSessionExtension
-from simplekv.memory.redisstore import RedisStore
 
 from mailu import utils, debug, models, manage, configuration
 
@@ -21,7 +21,7 @@ def create_app_from_config(config):
     # Initialize application extensions
     config.init_app(app)
     models.db.init_app(app)
-    KVSessionExtension(RedisStore(redis.StrictRedis().from_url('redis://{0}/3'.format(config['REDIS_ADDRESS']))), app).cleanup_sessions(app)
+    utils.session.init_app(app)
     utils.limiter.init_app(app)
     utils.babel.init_app(app)
     utils.login.init_app(app)
@@ -56,8 +56,7 @@ def create_app_from_config(config):
 
 
 def create_app():
-    """ Create a new application based on the config module 
+    """ Create a new application based on the config module
     """
     config = configuration.ConfigManager()
     return create_app_from_config(config)
-
diff --git a/core/admin/mailu/configuration.py b/core/admin/mailu/configuration.py
index 6f65d17d..3d1b4fb5 100644
--- a/core/admin/mailu/configuration.py
+++ b/core/admin/mailu/configuration.py
@@ -14,6 +14,7 @@ DEFAULT_CONFIG = {
     'DEBUG': False,
     'DOMAIN_REGISTRATION': False,
     'TEMPLATES_AUTO_RELOAD': True,
+    'MEMORY_SESSIONS': False,
     # Database settings
     'DB_FLAVOR': None,
     'DB_USER': 'mailu',
@@ -55,6 +56,7 @@ DEFAULT_CONFIG = {
     'RECAPTCHA_PRIVATE_KEY': '',
     # Advanced settings
     'LOG_LEVEL': 'WARNING',
+    'SESSION_KEY_BITS': 128,
     'SESSION_LIFETIME': 24,
     'SESSION_COOKIE_SECURE': True,
     'CREDENTIAL_ROUNDS': 12,
@@ -65,7 +67,6 @@ DEFAULT_CONFIG = {
     'HOST_SMTP': 'smtp',
     'HOST_AUTHSMTP': 'smtp',
     'HOST_ADMIN': 'admin',
-    'WEBMAIL': 'none',
     'HOST_WEBMAIL': 'webmail',
     'HOST_WEBDAV': 'webdav:5232',
     'HOST_REDIS': 'redis',
@@ -136,9 +137,9 @@ class ConfigManager(dict):
 
         self.config['RATELIMIT_STORAGE_URL'] = 'redis://{0}/2'.format(self.config['REDIS_ADDRESS'])
         self.config['QUOTA_STORAGE_URL'] = 'redis://{0}/1'.format(self.config['REDIS_ADDRESS'])
+        self.config['SESSION_STORAGE_URL'] = 'redis://{0}/3'.format(self.config['REDIS_ADDRESS'])
         self.config['SESSION_COOKIE_SAMESITE'] = 'Strict'
         self.config['SESSION_COOKIE_HTTPONLY'] = True
-        self.config['SESSION_KEY_BITS'] = 128
         self.config['PERMANENT_SESSION_LIFETIME'] = timedelta(hours=int(self.config['SESSION_LIFETIME']))
         # update the app config itself
         app.config = self
diff --git a/core/admin/mailu/utils.py b/core/admin/mailu/utils.py
index ce12a09a..02150754 100644
--- a/core/admin/mailu/utils.py
+++ b/core/admin/mailu/utils.py
@@ -1,11 +1,28 @@
-from mailu import models, limiter
+""" Mailu admin app utilities
+"""
+
+try:
+    import cPickle as pickle
+except ImportError:
+    import pickle
+
+import hmac
+import secrets
+import time
+
+from multiprocessing import Value
+
+from mailu import limiter
 
 import flask
 import flask_login
-import flask_script
 import flask_migrate
 import flask_babel
+import redis
 
+from flask.sessions import SessionMixin, SessionInterface
+from itsdangerous.encoding import want_bytes
+from werkzeug.datastructures import CallbackDict
 from werkzeug.contrib import fixers
 
 
@@ -15,6 +32,7 @@ login.login_view = "ui.login"
 
 @login.unauthorized_handler
 def handle_needs_login():
+    """ redirect unauthorized requests to login page """
     return flask.redirect(
         flask.url_for('ui.login', next=flask.request.endpoint)
     )
@@ -27,12 +45,17 @@ babel = flask_babel.Babel()
 
 @babel.localeselector
 def get_locale():
-    translations = list(map(str, babel.list_translations()))
+    """ selects locale for translation """
+    translations = [str(translation) for translation in babel.list_translations()]
     return flask.request.accept_languages.best_match(translations)
 
 
 # Proxy fixer
 class PrefixMiddleware(object):
+    """ fix proxy headers """
+    def __init__(self):
+        self.app = None
+
     def __call__(self, environ, start_response):
         prefix = environ.get('HTTP_X_FORWARDED_PREFIX', '')
         if prefix:
@@ -48,3 +71,384 @@ proxy = PrefixMiddleware()
 
 # Data migrate
 migrate = flask_migrate.Migrate()
+
+
+# session store (inspired by https://github.com/mbr/flask-kvsession)
+class RedisStore:
+    """ Stores session data in a redis db. """
+
+    has_ttl = True
+
+    def __init__(self, redisstore):
+        self.redis = redisstore
+
+    def get(self, key):
+        """ load item from store. """
+        value = self.redis.get(key)
+        if value is None:
+            raise KeyError(key)
+        return value
+
+    def put(self, key, value, ttl=None):
+        """ save item to store. """
+        if ttl:
+            self.redis.setex(key, int(ttl), value)
+        else:
+            self.redis.set(key, value)
+
+    def delete(self, key):
+        """ delete item from store. """
+        self.redis.delete(key)
+
+    def list(self, prefix=None):
+        """ return list of keys starting with prefix """
+        if prefix:
+            prefix += b'*'
+        return list(self.redis.scan_iter(match=prefix))
+
+class DictStore:
+    """ Stores session data in a python dict. """
+
+    has_ttl = False
+
+    def __init__(self):
+        self.dict = {}
+
+    def get(self, key):
+        """ load item from store. """
+        return self.dict[key]
+
+    def put(self, key, value, ttl_secs=None):
+        """ save item to store. """
+        self.dict[key] = value
+
+    def delete(self, key):
+        """ delete item from store. """
+        try:
+            del self.dict[key]
+        except KeyError:
+            pass
+
+    def list(self, prefix=None):
+        """ return list of keys starting with prefix """
+        if prefix is None:
+            return list(self.dict.keys())
+        return [key for key in self.dict if key.startswith(prefix)]
+
+class MailuSession(CallbackDict, SessionMixin):
+    """ Custom flask session storage. """
+
+    # default modified to false
+    modified = False
+
+    def __init__(self, key=None, app=None):
+
+        self.app = app or flask.current_app
+
+        initial = None
+
+        key = want_bytes(key)
+        if parsed := self.app.session_config.parse_key(key, self.app):
+            try:
+                initial = pickle.loads(app.session_store.get(key))
+            except (KeyError, EOFError, pickle.UnpicklingError):
+                # either the cookie was manipulated or we did not find the
+                # session in the backend or the pickled data is invalid.
+                # => start new session
+                pass
+            else:
+                (self._uid, self._sid, self._created) = parsed
+                self._key = key
+
+        if initial is None:
+            # start new session
+            self.new = True
+            self._uid = None
+            self._sid = None
+            self._created = self.app.session_config.gen_created()
+            self._key = None
+
+        def _on_update(obj):
+            obj.modified = True
+
+        CallbackDict.__init__(self, initial, _on_update)
+
+    @property
+    def saved(self):
+        """ this reflects if the session was saved. """
+        return self._key is not None
+
+    @property
+    def sid(self):
+        """ this reflects the session's id. """
+        if self._sid is None or self._uid is None or self._created is None:
+            return None
+        return b''.join([self._uid, self._sid, self._created])
+
+    def destroy(self):
+        """ destroy session for security reasons. """
+
+        self.delete()
+
+        self._uid = None
+        self._sid = None
+        self._created = None
+
+        self.clear()
+
+        self.modified = True
+        self.new = False
+
+    def regenerate(self):
+        """ generate new id for session to avoid `session fixation`. """
+
+        self.delete()
+
+        self._sid = None
+        self._created = self.app.session_config.gen_created()
+
+        self.modified = True
+
+    def delete(self):
+        """ Delete stored session. """
+        if self.saved:
+            self.app.session_store.delete(self._key)
+            self._key = None
+
+    def save(self):
+        """ Save session to store. """
+
+        set_cookie = False
+
+        # set uid from dict data
+        if self._uid is None:
+            self._uid = self.app.session_config.gen_uid(self.get('user_id', ''))
+
+        # create new session id for new or regenerated sessions and force setting the cookie
+        if self._sid is None:
+            self._sid = self.app.session_config.gen_sid()
+            set_cookie = True
+
+        # get new session key
+        key = self.sid
+
+        # delete old session if key has changed
+        if key != self._key:
+            self.delete()
+
+        # remember time to refresh
+        self['_refresh'] = int(time.time()) + self.app.permanent_session_lifetime.total_seconds()/2
+
+        # save session
+        self.app.session_store.put(
+            key,
+            pickle.dumps(dict(self)),
+            self.app.permanent_session_lifetime.total_seconds()
+        )
+
+        self._key = key
+
+        self.new = False
+        self.modified = False
+
+        return set_cookie
+
+    def needs_refresh(self):
+        """ Checks if server side session needs to be refreshed. """
+
+        return int(time.time()) > self.get('_refresh', 0)
+
+class MailuSessionConfig:
+    """ Stores sessions crypto config """
+
+    # default size of session key parts
+    uid_bits = 64 # default if SESSION_KEY_BITS is not set in config
+    sid_bits = 128 # for now. must be multiple of 8!
+    time_bits = 32 # for now. must be multiple of 8!
+
+    def __init__(self, app=None):
+
+        if app is None:
+            app = flask.current_app
+
+        bits = app.config.get('SESSION_KEY_BITS', self.uid_bits)
+        if not 64 <= bits <= 256:
+            raise ValueError('SESSION_KEY_BITS must be between 64 and 256!')
+
+        uid_bytes = bits//8 + (bits%8>0)
+        sid_bytes = self.sid_bits//8
+
+        key = want_bytes(app.secret_key)
+
+        self._hmac    = hmac.new(hmac.digest(key, b'SESSION_UID_HASH', digest='sha256'), digestmod='sha256')
+        self._uid_len = uid_bytes
+        self._uid_b64 = len(self._encode(bytes(uid_bytes)))
+        self._sid_len = sid_bytes
+        self._sid_b64 = len(self._encode(bytes(sid_bytes)))
+        self._key_min = self._uid_b64 + self._sid_b64
+        self._key_max = self._key_min + len(self._encode(bytes(self.time_bits//8)))
+
+    def gen_sid(self):
+        """ Generate random session id. """
+        return self._encode(secrets.token_bytes(self._sid_len))
+
+    def gen_uid(self, uid):
+        """ Generate hashed user id part of session key. """
+        _hmac = self._hmac.copy()
+        _hmac.update(want_bytes(uid))
+        return self._encode(_hmac.digest()[:self._uid_len])
+
+    def gen_created(self, now=None):
+        """ Generate base64 representation of creation time. """
+        return self._encode(int(now or time.time()).to_bytes(8, byteorder='big').lstrip(b'\0'))
+
+    def parse_key(self, key, app=None, validate=False, now=None):
+        """ Split key into sid, uid and creation time. """
+
+        if not (isinstance(key, bytes) and self._key_min <= len(key) <= self._key_max):
+            return None
+
+        uid = key[:self._uid_b64]
+        sid = key[self._uid_b64:self._key_min]
+        crt = key[self._key_min:]
+
+        # validate if parts are decodeable
+        created = self._decode(crt)
+        if created is None or self._decode(uid) is None or self._decode(sid) is None:
+            return None
+
+        # validate creation time when requested or store does not support ttl
+        if validate or not app.session_store.has_ttl:
+            if now is None:
+                now = int(time.time())
+            created = int.from_bytes(created, byteorder='big')
+            if not created < now < created + app.permanent_session_lifetime.total_seconds():
+                return None
+
+        return (uid, sid, crt)
+
+    def _encode(self, value):
+        return secrets.base64.urlsafe_b64encode(value).rstrip(b'=')
+
+    def _decode(self, value):
+        try:
+            return secrets.base64.urlsafe_b64decode(value + b'='*(4-len(value)%4))
+        except secrets.binascii.Error:
+            return None
+
+class MailuSessionInterface(SessionInterface):
+    """ Custom flask session interface. """
+
+    def open_session(self, app, request):
+        """ Load or create session. """
+        return MailuSession(request.cookies.get(app.config['SESSION_COOKIE_NAME'], None), app)
+
+    def save_session(self, app, session, response):
+        """ Save modified session. """
+
+        # If the session is modified to be empty, remove the cookie.
+        # If the session is empty, return without setting the cookie.
+        if not session:
+            if session.modified:
+                session.delete()
+                response.delete_cookie(
+                    app.session_cookie_name,
+                    domain=self.get_cookie_domain(app),
+                    path=self.get_cookie_path(app),
+                )
+            return
+
+        # Add a "Vary: Cookie" header if the session was accessed
+        if session.accessed:
+            response.vary.add('Cookie')
+
+        set_cookie = session.permanent and app.config['SESSION_REFRESH_EACH_REQUEST']
+        need_refresh = session.needs_refresh()
+
+        # save modified session or refresh unmodified session
+        if session.modified or need_refresh:
+            set_cookie |= session.save()
+
+        # set cookie on refreshed permanent sessions
+        if need_refresh and session.permanent:
+            set_cookie = True
+
+        # set or update cookie if necessary
+        if set_cookie:
+            response.set_cookie(
+                app.session_cookie_name,
+                session.sid,
+                expires=self.get_expiration_time(app, session),
+                httponly=self.get_cookie_httponly(app),
+                domain=self.get_cookie_domain(app),
+                path=self.get_cookie_path(app),
+                secure=self.get_cookie_secure(app),
+                samesite=self.get_cookie_samesite(app)
+            )
+
+class MailuSessionExtension:
+    """ Server side session handling """
+
+    @staticmethod
+    def cleanup_sessions(app=None):
+        """ Remove invalid or expired sessions. """
+
+        app = app or flask.current_app
+        now = int(time.time())
+
+        count = 0
+        for key in app.session_store.list():
+            if not app.session_config.parse_key(key, app, validate=True, now=now):
+                app.session_store.delete(key)
+                count += 1
+
+        return count
+
+    @staticmethod
+    def prune_sessions(uid=None, keep=None, app=None):
+        """ Remove sessions
+            uid: remove all sessions (NONE) or sessions belonging to a specific user
+            keep: keep listed sessions
+        """
+
+        keep = keep or set()
+        app = app or flask.current_app
+
+        prefix = None if uid is None else app.session_config.gen_uid(uid)
+
+        count = 0
+        for key in app.session_store.list(prefix):
+            if key not in keep:
+                app.session_store.delete(key)
+                count += 1
+
+        return count
+
+    def init_app(self, app):
+        """ Replace session management of application. """
+
+        if app.config.get('MEMORY_SESSIONS'):
+            # in-memory session store for use in development
+            app.session_store = DictStore()
+
+        else:
+            # redis-based session store for use in production
+            app.session_store = RedisStore(
+                redis.StrictRedis().from_url(app.config['SESSION_STORAGE_URL'])
+            )
+
+            # clean expired sessions oonce on first use in case lifetime was changed
+            def cleaner():
+                with cleaned.get_lock():
+                    if not cleaned.value:
+                        cleaned.value = True
+                        flask.current_app.logger.error('cleaning')
+                        MailuSessionExtension.cleanup_sessions(app)
+
+            app.before_first_request(cleaner)
+
+        app.session_config = MailuSessionConfig(app)
+        app.session_interface = MailuSessionInterface()
+
+cleaned = Value('i', False)
+session = MailuSessionExtension()
diff --git a/core/admin/requirements-prod.txt b/core/admin/requirements-prod.txt
index 5c291e24..c4663557 100644
--- a/core/admin/requirements-prod.txt
+++ b/core/admin/requirements-prod.txt
@@ -13,7 +13,6 @@ Flask==1.0.2
 Flask-Babel==0.12.2
 Flask-Bootstrap==3.3.7.1
 Flask-DebugToolbar==0.10.1
-Flask-KVSession==0.6.2
 Flask-Limiter==1.0.1
 Flask-Login==0.4.1
 flask-marshmallow==0.14.0
diff --git a/core/admin/requirements.txt b/core/admin/requirements.txt
index 46500295..e1de3b01 100644
--- a/core/admin/requirements.txt
+++ b/core/admin/requirements.txt
@@ -3,7 +3,6 @@ Flask-Login
 Flask-SQLAlchemy
 Flask-bootstrap
 Flask-Babel
-Flask-KVSession
 Flask-migrate
 Flask-script
 Flask-wtf