How to upgrade only core with drush? There's a huge security leak in &lt;7.32. So I want to upgrade all my Drupal sites asap without too much worrying about breaking stuff.

But...

$ drush dl drupal-7.32
It's forbidden to download drupal core into an existing core.


This one works:

$ drush up


But that's not what I want now.

How can I only upgrade core to latest version?

drush up drupal is not sufficient on its own:
(Sorry about the Dutch. You'll get the gist.)

$ drush up drupal
Controle of updategegevens beschikbaar zijn ...                                                            [ok]
Update information last refreshed: di, 10/14/2014 - 20:57

Update status information on all installed and enabled Drupal projects:
 Naam                               Geïnstalleer  Proposed     Status
                                    de versie     version
 Administration menu (admin_menu)   7.x-3.0-rc4   7.x-3.0-rc4  Actueel
 Drupal                             7.31          7.31         Actueel
 Chaos tools (ctools)               7.x-1.4       7.x-1.4      Actueel
 Service links (service_links)      7.x-2.2       7.x-2.2      Actueel
 Entity API (entity)                7.x-1.5       7.x-1.5      Actueel
 Google Analytics                   7.x-2.0       7.x-2.0      Actueel
 (google_analytics)
 Localization update (l10n_update)  7.x-1.0       7.x-1.0      Actueel
 Module filter (module_filter)      7.x-1.8       7.x-1.8      Actueel
 Pathauto (pathauto)                7.x-1.2       7.x-1.2      Actueel
 Redirect (redirect)                7.x-1.0-rc1   7.x-1.0-rc1  Actueel
 Token (token)                      7.x-1.5       7.x-1.5      Actueel
 Views (views)                      7.x-3.8       7.x-3.8      Actueel
 Webform (webform)                  7.x-3.20      7.x-3.21     Nieuwe versie beschikbaar



No code updates available.
In this case, it looks like you need to do

drush rf    # pm-refresh


before you do 

drush up drupal


This will refresh the list of available updates so Drush knows there is a new release for Drupal.