In use of AES CBC, can a cipher text signature be replaced with appending a known string to the plain text? Would appending a known string to the end of plain text then validating the string after decrypting adequately verify the authenticity of the cipher text?
Would appending a known string to the end of plain text then validating the string after decrypting adequately verify the authenticity of the cipher text?


No.

Here's what CBC-mode decryption looks like



Consider what happens if the attacker modifies the the first ciphertext block shown (as marked in red).

Then, the decryption of that block produces something unpredictable; that block turns into an unpredictable (by the attacker) value; the corresponding plaintext block is unpredictable.

For the second plaintext block, the modified first ciphertext block is xor'ed in with the unmodified decrypted second ciphertext block, the result is original plaintext block, except that the bits the attacker flipped in the ciphertext block will be flipped.

As for the rest of the decrypted plaintext, there are totally unchanged.

Hence, if you were to append a known string at the end, that part would decrypt properly, and hence your check would not detect the modifications done earlier.