diff --git a/acl/acl-lib.pl b/acl/acl-lib.pl
index d56c1b11a..7a03bdfa0 100755
--- a/acl/acl-lib.pl
+++ b/acl/acl-lib.pl
@@ -362,6 +362,7 @@ sub create_user
 my ($user, $clone) = @_;
 my %miniserv;
 my @mods = &list_modules();
+$user->{'name'} eq "webmin" && &error("Invalid username webmin for new user");
 
 &get_miniserv_config(\%miniserv);
 
@@ -530,6 +531,8 @@ sub modify_user
 my ($username, $user) = @_;
 my (%miniserv, @pwfile, @acl, @mods, $m);
 &get_miniserv_config(\%miniserv);
+$user->{'name'} eq "webmin" &&
+	&error("Invalid username webmin for modified user");
 
 if ($user->{'proto'}) {
 	# In users and groups DB
diff --git a/web-lib-funcs.pl b/web-lib-funcs.pl
index 06bbd06aa..682a7e73b 100755
--- a/web-lib-funcs.pl
+++ b/web-lib-funcs.pl
@@ -4134,6 +4134,7 @@ sub save_module_acl
 {
 my $u = defined($_[1]) ? $_[1] : $base_remote_user;
 my $m = defined($_[2]) ? $_[2] : &get_module_name();
+$u eq "webmin" && &error("Invalid username webmin for ACL");
 if (!$_[3] && &foreign_check("acl")) {
 	# Check if this user is a member of a group, and if he gets the
 	# module from a group. If so, update its ACL as well