Cipher Block Chaining Ciphertext Alteration I am new to cryptography. My professor's slides say that "Disadvantage: altered ciphertext only influences two blocks" for the CBC. However, since we xor the ciphertext from the previous block and the present plaintext, before encryption; it seems to me like if I alter a ciphertext block, every ciphertext block coming afterwards will change. I do not understand why would this chain reaction only influence two blocks.
One way is simply to look at the usual illustration of CBC decryption and follow the arrows.
Here is the figure from Wikipedia:



Imagine you alter the first block of ciphertext. Follow the arrows to see what will be altered in the plaintext: only two blocks.

We can also do this with math formulas:
CBC mode encryption can be described as follow,
where $ P_n $  and $ C_n $ are the $n$th plaintext and ciphertext blocks, respectively:

$$ C_n = Enc(P_n \oplus C_{n-1}) $$

(With the IV playing the role of $ C_0 $)

Thus 

$$ P_n = Dec(C_n) \oplus C_{n-1} $$

As a result, if you alter $ C_n $ it is clear that
$ P_n $ and $ P_{n+1} $ will be affected but not $ P_{n+2} $ and the next blocks.

If you want to see this in practice, you may want to do the cryptopals challenges up to challenge 17.