AES CBC 256 String Decryption only with tampering

* * * Decryption original * * *
decryptionKey (Base64): d3+KZq3lsdy5Hgd1VRVQFUhA9jU0PaBHTG2LG2PazQM=
ciphertext (Base64): 8GhkycnbzDo5WbRol5zfCQ==:gQrTOJdobbI4fuCHCfu5eqKFYCaw/AM6xMK2o+xCXB0MeAQ7rFbXAbAE3Ex+bD8c
input is (Base64) iv : (Base64) ciphertext
plaintext:  The quick brown fox jumps over the lazy dog

tampering the data without encryption key
guessedString: The quick brown 
newString:     The happy brown 

* * * Decryption tampered * * *
ciphertext (Base64): 8GhkydDP1SkrWbRol5zfCQ==:gQrTOJdobbI4fuCHCfu5eqKFYCaw/AM6xMK2o+xCXB0MeAQ7rFbXAbAE3Ex+bD8c
plaintext:  The happy brown fox jumps over the lazy dog