The core development team have released an urgent upgrade called Joomla! 1.0.11 [ Sunbird ] which you can download download here. and is  Critical Security Release.

All existing Joomla! users MUST UPGRADE to this version, due to several High Level vulnerabilities that affect ALL Previous versions of Joomla! 

1.0.11 contains the following critical security fixes

• 04 High Level Security Fixes
• 04 Medium Level Security Fixes
• 18 Low Level security
  25 General bug fixes