How can i fix my wobbly table?

I have finally entered the world of DIY and built a desk based on the design of a coffee table we bought. Unfortunately it wobbles from front to back as you are sitting at it to the extent I can’t really work at it and am concerned I’ll weaken it further. Think the issue is where the top of the legs are screwed horizontally into a horizontal piece that sits beneath and across the top boards and is screwed vertically up into these (I.e. the legs are not directly attached to the top boards). Would a diagonal support at each corner from the leg into the cross board help do you Think? Any suggestions much appreciated!!