[Bug 241657] Re: Multiple vulnerabilities in Ruby may lead to a denial of service (DoS) condition or allow execution of arbitrary code.

You can use Ruby 1.8.6 patch 111 in Ubuntu 8.10 See the article: http://railsgeek.com/2008/11/27/ubuntu-8-10-downgrade-ruby-1-8-7-to-1-8-6 -- Multiple vulnerabilities in Ruby may lead to a denial of service (DoS) condition or allow execution of arbitrary code.

[Bug 282302] Re: Ruby 1.8 package breaks the Metasploit Framework (short-named constants)

I have found many advices howto downgrade Ruby 1.8.7 to 1.8.6 as recommended by Rails core team. You can found the article http://railsgeek.com/2008/11/27/ubuntu-8-10-downgrade-ruby-1-8-7-to-1-8-6 -- Ruby 1.8 package breaks the Metasploit Framework (short-named constants)