Can I safely mount a wall-mount networking rack on one stud?
I'd say it all hinges on whether 2x4s are mounted to the wall securely, and you have no simple way to verify that. In any case, since they do not extend all the way to the floor, they are not structural, and were not designed to handle any kind of a serious load.

I'd say the most simple way to to address this would be this:


Go to a home improvement center (home depot, etc.), get 4 sleeve anchors, 1/2" x 3" at least, which are rated for cinder blocks. If your 2x4 boards are attached to the wall with their narrow edge, you'll need way longer bolts, you need at least several inches in the cinder blocks.
Drill 4 holes, 2 into each of these 2x4 members, staggering the holes and keeping well away from the edges, into the upper half of these boards, using wood drill bit. 
Drill into the cinder blocks using concrete drill bit, for the length and diameter appropriate for the fasteners that you bought. 
Install the anchors, do not over-tighten.


There are many ways to attach a board to a cinder block wall (tapcons, concrete anchors, etc.), they all will work well. Basically what I'm saying is that if you can't be sure these boards are attached securely enough, just add more fasteners that will definitely support the load. 

Here, I presume that your rack is securely mounted to the wood (real lag bolts, not some drywall screws), you'll want to inspect that as well.