Vulnerability demonstration on Ubuntu 9.04

The 9.04 release was supported until October 23 2010. The vulnerability you found was reported in August 2009. It seems reasonable that, since the release was still current and supported at the time, the ISO was patched and what you downloaded was a version that is no longer vulnerable.

Furthermore, you seem to have demonstrated quite nicely that it isn't vulnerable. After all, you tried the exploit and it looks like it failed.

Why don't you try a newer exploit? Something like CVE-2013-2094 which should also affect Ubuntu, for example.