diff --git a/lib/CMTD/Main.pm b/lib/CMTD/Main.pm
index 5ffd21e..21fbe3c 100644
--- a/lib/CMTD/Main.pm
+++ b/lib/CMTD/Main.pm
@@ -13,6 +13,23 @@ sub index {
 }
 
 sub captcha {
+  my ($self) = @_;
+
+  eval {
+    my $table = $c->app->config->{captcha}->{table};
+    my @sort = qw(shown tries);
+    $self->app->db->begin;
+    my ($cid, $data) = $self->app->db->select($table, ['id', 'data'], undef, \@sort)->list;
+    $self->app->db->update($table, {shown => time()}, {id => $cid});
+    $self->app->db->commit;
+    $self->app->log->debug("showing captcha #$cid");
+    $self->render(json => {id => $cid, data => $data}); 1;
+  } or do {
+    chomp $@;
+    $self->app->log->error("error when showing captcha: $@");
+    $self->res->code(500);
+    $self->render(text => 'internal error');
+  };
 }
 
 sub c_list {