From the MamboForge site:
Patches are available for Mambo Version 4.5.0-1.0.9 and Version 4.5.1a at http://mamboforge.net/frs/?group_id=5 to counter a vulnerability within Mambo. All administrators of Mambo sites are encouraged to upgrade at their earlist convenience.
New installations can use the 4.5.1b release, which incorporates these security fixes.