Its possible for just about any application to have back doors in them, but the likely hood varies based upon who developed the application and where you downloaded it from. If you dont download it from the original authors distribution, you could be downloading an altered version of the program that could have a virus or trojan in it. An author could easily decide to add some feature that opens a back door intentially or unintentially (whether those intentions are for good or not) to their system.

There is no point in worrying over it though, if someone does create a back door, with so many eyes watching, someone will spot it, but if you still feel a little paranoid (not a bad thing), then you could always use open source clients like gnucleus or phex, and compile your own binaries just for some added security, the eyeballs watching an open source project are a lot more and its a lot easier then a closed source program. (this is I believe one of the reasons the LSB had decided that a total open source solution is the only way to go, because closed source software makes security audits from difficult to impossible to do).