The block cipher modes ECB, CBC, OFB, CFB, CTR, and XTS provide confidentiality, but they do not protect against accidental modification or malicious tampering. Modification or tampering can be detected with a separate message authentication code such as CBC-MAC, or a digital signature.

Why is CBC mode insecure?

The problem with CBC mode is that the decryption of blocks is dependant on the previous ciphertext block. This means attackers can manipulate the decryption of a block by tampering with the previous block using the commutative property of XOR. then the application will assume the request is authenticated.

Why is CBC secure?

CBC has some nice properties. The ciphertext produced by a block cipher is encrypted, so it (hopefully) looks random. In CBC, you're mixing this random looking encrypted data into the plaintext, making it very unlikely that there will be patterns in the output.