As always I'm never a fan of security being open sourced, because it opens up the possibility for people to find work arounds.

Ex. say you block Virus X through a specific pattern and then open sourced the algorithm to find the pattern.

Now the developer of Virus X can just modify Virus X to not be detected by the algorithm that detected it before.

Note: This is strictly a personal opinion about security and I'm sure there pros/cons to both.