diff --git a/data/Dockerfiles/dovecot/imapsync_runner.py b/data/Dockerfiles/dovecot/imapsync_runner.py
index 09fa1eda6..53dca5ea6 100755
--- a/data/Dockerfiles/dovecot/imapsync_runner.py
+++ b/data/Dockerfiles/dovecot/imapsync_runner.py
@@ -1,10 +1,12 @@
 #!/usr/bin/python3
 
 import fcntl
+import ipaddress
 import json
 import os
 import re
 import signal
+import socket
 import subprocess
 import sys
 import tempfile
@@ -18,6 +20,15 @@ max_output = 16 * 1024 * 1024
 lock_file = run_dir + '/imapsync_busy.lock'
 # An OAuth run needs a token valid this long at start (half the lifetime for short-lived tokens)
 token_min_valid = 900
+# Custom params never passed, also when stored before (same list as CustomParams::BLOCKED in the web UI)
+blocked_params = {'abort', 'delete2foldersbutnot', 'delete2foldersonly', 'logdir', 'logfile', 'maxlinelengthcmd',
+  'oauthrefreshcmd1', 'oauthrefreshcmd2', 'passfile1', 'passfile2', 'pidfile', 'pipemess', 'regexflag', 'regexmess',
+  'regextrans2', 'showpasswords', 'tests', 'testsdebug', 'testslive', 'testslive6', 'testsunit', 'tmpdir'}
+# imapsync only gets these variables, not the secrets of the container environment
+job_env_keys = ('PATH', 'LANG', 'LC_ALL', 'TZ')
+# A source must not point to mailcow itself (internal Docker network), host1 is checked before each run
+internal_nets = [ipaddress.ip_network(os.environ.get('IPV4_NETWORK', '172.22.1') + '.0/24', strict=False),
+  ipaddress.ip_network(os.environ.get('IPV6_NETWORK', 'fd4d:6169:6c63:6f77::/64'), strict=False)]
 # Refreshed tokens are written to the token files of running jobs this often, imapsync re-reads them on reconnect
 token_sync_interval = 60
 
@@ -106,6 +117,20 @@ def defer_job(job_id, text, exit_status):
     (text, exit_status, job_id))
   deferred.add(job_id)
 
+# True when host resolves to loopback, link-local or the internal network; unknown hosts are left to imapsync
+def internal_host(host):
+  try:
+    addrs = {a[4][0] for a in socket.getaddrinfo(host, None)}
+  except (socket.gaierror, UnicodeError):
+    return False
+  for addr in addrs:
+    ip = ipaddress.ip_address(addr.split('%')[0])
+    if ip.version == 6 and ip.ipv4_mapped:
+      ip = ip.ipv4_mapped
+    if ip.is_loopback or ip.is_unspecified or ip.is_link_local or any(ip in net for net in internal_nets):
+      return True
+  return False
+
 def oauth_token(j):
   if j['oauth_flow'] == 'authorization_code':
     return query(user_token_sql, (j['source_id'], j['user2'], j['user1'])).fetchone() or {}
@@ -150,6 +175,11 @@ def prepare_job(j, global_bw, files):
       f.write(content)
     return path
 
+  if internal_host(j['host1']):
+    finish_job(j['id'], 'Host1 %s is an address of mailcow itself, the sync job does not connect to it\n' % j['host1'], 0,
+      'EXIT_CONNECTION_FAILURE_HOST1')
+    return None
+
   auth_type = j['auth_type'] or 'PLAIN'
   oauth = None
   if auth_type == 'XOAUTH2':
@@ -174,7 +204,11 @@ def prepare_job(j, global_bw, files):
   except ValueError:
     pairs = []
   for p in pairs if isinstance(pairs, list) else []:
-    if not isinstance(p, dict) or not isinstance(p.get('o'), str) or not re.fullmatch(r'[a-z0-9]+', p['o']):
+    if not isinstance(p, dict) or not isinstance(p.get('o'), str) or not re.fullmatch(r'[a-z0-9]+', p['o']) \
+        or p['o'] in blocked_params:
+      continue
+    # The certificate check protects the OAuth token of the source, a job must not turn it off
+    if oauth and p['o'] == 'sslargs1':
       continue
     value = '' if p.get('v') is None else str(p['v'])
     custom_params.append('--' + p['o'] if value == '' else '--' + p['o'] + '=' + value)
@@ -205,7 +239,8 @@ def prepare_job(j, global_bw, files):
       cmd.append(opt)
   enc1 = {'TLS': '--tls1', 'SSL': '--ssl1'}.get(j['enc1'])
   if enc1:
-    cmd.append(enc1)
+    # Check the certificate of the source (imapsync does not by default), custom sslargs1 can turn it off
+    cmd += [enc1, '--sslargs1', 'SSL_verify_mode=1']
   cmd += ['--host1', j['host1'], '--user1', j['user1']] + auth_args + ['--port1', str(j['port1'])]
   cmd += ['--host2', 'localhost', '--user2', j['user2'] + '*' + master_user, '--passfile2', tmpfile(master_pass + '\n')]
   if j['dry'] == 1:
@@ -216,8 +251,10 @@ def prepare_job(j, global_bw, files):
 
 # Worker: imapsync with output to a file, no inherited fds (DB socket, lock)
 def start_job(job):
+  env = {k: os.environ[k] for k in job_env_keys if k in os.environ}
+  env['HOME'] = run_dir
   with open(job['outfile'], 'wb') as out:
-    return subprocess.Popen(job['cmd'], stdin=subprocess.DEVNULL, stdout=out, stderr=subprocess.STDOUT)
+    return subprocess.Popen(job['cmd'], stdin=subprocess.DEVNULL, stdout=out, stderr=subprocess.STDOUT, env=env)
 
 # Store the result of a finished worker and remove its temp files
 def reap_job(job):
diff --git a/data/web/inc/lib/mailcow/Imapsync/Model/CustomParams.php b/data/web/inc/lib/mailcow/Imapsync/Model/CustomParams.php
index 4a900e855..7f31d67d7 100644
--- a/data/web/inc/lib/mailcow/Imapsync/Model/CustomParams.php
+++ b/data/web/inc/lib/mailcow/Imapsync/Model/CustomParams.php
@@ -19,6 +19,14 @@ class CustomParams
 {
     private const MAX_PAIRS = 100;
     private const MAX_VALUE_LENGTH = 1000;
+    /**
+     * Never allowed, even when configured: code or command execution, local files, the environment
+     * of the container (tests*), passwords in the output and stopping other runs (abort).
+     * The runner (dovecot/imapsync_runner.py) skips the same options.
+     */
+    public const BLOCKED = ['abort', 'delete2foldersbutnot', 'delete2foldersonly', 'logdir', 'logfile', 'maxlinelengthcmd',
+        'oauthrefreshcmd1', 'oauthrefreshcmd2', 'passfile1', 'passfile2', 'pidfile', 'pipemess', 'regexflag', 'regexmess',
+        'regextrans2', 'showpasswords', 'tests', 'testsdebug', 'testslive', 'testslive6', 'testsunit', 'tmpdir'];
 
     private static bool $warnedOldFormat = false;
 
@@ -57,7 +65,7 @@ class CustomParams
 
     /**
      * The effective allowlist: $IMAPSYNC_OPTIONS (after vars.local.inc.php) without the old
-     * 'whitelist' key and malformed entries; the defaults when nothing valid is left.
+     * 'whitelist' key, malformed entries and BLOCKED options; the defaults when nothing valid is left.
      */
     public static function allowlist(array $configured, array $defaults): array
     {
@@ -73,7 +81,7 @@ class CustomParams
             self::$warnedOldFormat = true;
             error_log("mailcow: \$IMAPSYNC_OPTIONS['whitelist'] (old format) is ignored, use 'option' => true|false|'regex' (see data/web/inc/vars.inc.php)");
         }
-        return empty($allowlist) ? $defaults : $allowlist;
+        return array_diff_key(empty($allowlist) ? $defaults : $allowlist, array_flip(self::BLOCKED));
     }
 
     /** The value to store for one option; throws when the allowlist rule is violated */
diff --git a/data/web/inc/vars.inc.php b/data/web/inc/vars.inc.php
index 451704565..3892ae315 100644
--- a/data/web/inc/vars.inc.php
+++ b/data/web/inc/vars.inc.php
@@ -296,8 +296,8 @@ $RSPAMD_MAPS = array(
 // The old format $IMAPSYNC_OPTIONS['whitelist'] (list of names) is no longer supported and ignored (logged).
 // NEVER add (RCE): 'delete2foldersonly', 'delete2foldersbutnot', 'regexflag', 'regexmess', 'pipemess', 'regextrans2', 'maxlinelengthcmd', 'oauthrefreshcmd1', 'oauthrefreshcmd2'
 // NEVER add options taking local file paths (e.g. 'logfile', 'logdir', 'pidfile', 'passfile1', 'sslargs1' with SSL_ca_file)
+// These, the tests* options (print the environment), 'showpasswords' and 'abort' (stops other runs) are also blocked in code
 $IMAPSYNC_OPTIONS_DEFAULT = array(
-  'abort'                  => false,
   'authmd51'               => false,
   'authuser1'              => true,
   'debug'                  => false,
@@ -373,9 +373,6 @@ $IMAPSYNC_OPTIONS_DEFAULT = array(
   'syncduplicates'         => false,
   'syncinternaldates'      => false,
   'synclabels'             => false,
-  'tests'                  => false,
-  'testslive'              => false,
-  'testslive6'             => false,
   'truncmess'              => true,
   'usecache'               => false,
   'useheader'              => true,
diff --git a/docker-compose.yml b/docker-compose.yml
index c77c7baaa..81bdd7468 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -259,7 +259,7 @@ services:
             - sogo
 
     dovecot-mailcow:
-      image: ghcr.io/mailcow/dovecot:2.3.21.1-3
+      image: ghcr.io/mailcow/dovecot:2.3.21.1-4
       depends_on:
         - mysql-mailcow
         - netfilter-mailcow
@@ -295,6 +295,7 @@ services:
         - MAILCOW_HOSTNAME=${MAILCOW_HOSTNAME}
         - MAILCOW_PASS_SCHEME=${MAILCOW_PASS_SCHEME:-BLF-CRYPT}
         - IPV4_NETWORK=${IPV4_NETWORK:-172.22.1}
+        - IPV6_NETWORK=${IPV6_NETWORK:-fd4d:6169:6c63:6f77::/64}
         - ALLOW_ADMIN_EMAIL_LOGIN=${ALLOW_ADMIN_EMAIL_LOGIN:-n}
         - MAILDIR_GC_TIME=${MAILDIR_GC_TIME:-7200}
         - ACL_ANYONE=${ACL_ANYONE:-disallow}
diff --git a/tests/php/Imapsync/Model/CustomParamsTest.php b/tests/php/Imapsync/Model/CustomParamsTest.php
index 924db327b..9ca43eace 100644
--- a/tests/php/Imapsync/Model/CustomParamsTest.php
+++ b/tests/php/Imapsync/Model/CustomParamsTest.php
@@ -132,6 +132,16 @@ class CustomParamsTest extends TestCase
         $this->assertSame(['default' => true], CustomParams::allowlist(['bad name' => true], ['default' => true]));
     }
 
+    public function testBlockedOptionsAreNeverAllowed(): void
+    {
+        $allowlist = CustomParams::allowlist(['tests' => false, 'pipemess' => true, 'folder' => true], ['abort' => false]);
+        $this->assertSame(['folder' => true], $allowlist);
+        $this->assertSame([], CustomParams::allowlist([], ['abort' => false, 'showpasswords' => false]));
+
+        $this->expectException(UserError::class);
+        CustomParams::toJson([['o' => '--tests', 'v' => '']], $allowlist);
+    }
+
     public function testOldWhitelistFormatIsIgnored(): void
     {
         $errorLog = ini_set('error_log', '/dev/null');