Linux kernel 2.6.21.4 patches three vulnerabilities. A null pointer dereference in netfilter can crash the kernel when handling certain connections; this bug can be triggered remotely. Plus, a flaw in the cpuset_tasks_read function lets logged-in users peek at chunks of kernel memory, potentially exposing sensitive info to attackers. Lastly, there’s a glitch in how seeds for random number generation are handled when the system lacks an entropy source, which could weaken the security of apps relying on random numbers.
~SeeJay
SeeJay Online! 
Oh! so that was what the recent Kubuntu kernel update was all about 🙂 For anyone reading this post and thinks you need to upgrade to 2.6.21.4 don’t worry!.. Chances are that this bug fix has already been back ported the older kernels so doing an apt-get update/upgrade, yum update or what ever should fix this for you.
If you compiled your kernel from scratch then you’d probably wanna upgrade to this exact version