Substitution ciphers amended with cipher block chaining: susceptible to frequency analysis?
CBC in this context means that each letter of the ciphertext is used to help encrypt the next letter.
Let's say, making up some numbers, that a certain ciphertext letter is C. The next plaintext letter is T. So to encrypt it, we will add C (or 2) to T, giving V. Then we do the substitution step, which could be anything and happens to be J.
The key observation here is that every time there is a C in the ciphertext, the next letter goes through the same thing. Plus two, substitute. And every time you find a C in the ciphertext, the next letter all Ts go to Js.
The break, then, is to go through the ciphertext looking for Cs. Then you write down all the letters that immediately follow. You get 200 Rs, 160 Js, whatever it is. So you can conclude that after a C, E is probably mapping to R and T is probably mapping to J.
Do the same thing with other letters: everything following Ds, everything following Es, and so on. You'll have 26 different sets of letters to do frequency analysis on. And once you have done them, you'll know the substitution several times over.