If I have a deployed Solana Program which creates some Solana accounts for storing data in it. Can these Solana accounts be accessed by any other Solana program using the account addresses? I am confused about the security features for the data stored in the account since accounts are mutable and if any other program can access it then there's a risk of data getting changed right?
Any program can read data from and credit SOL to any Solana account.
Where permission to write to an account is concerned, there are restrictions. All Solana accounts have a field of metadata on them named
owner. In that field you will find the address of a program. The Solana runtime will only grant a program write access to an account if the program's address matches the address stored in the
owner field of the account being written to.
By default, newly created accounts are owned by the System Program. Writes to these accounts are thereby mediated by the System Program. As an example: let's say that you wanted to debit an account (ie. transfer some SOL tokens out of it). Since the System Program is the only program that can perform such a mutation, you will have to convince it to do so on your behalf. The canonical way to do this is to call its
Transfer instruction. In order for that instruction to succeed, the System Program will require that you produce a cryptographic signature as proof that you own the account being debited.
 The address of the System Program is