CBC without integrity is not secure

[technion]

Utilising encryption in CBC mode must always be accompanied by a MAC, or it is vulnerable to several attacks.
An example bitflipping attack which exploits this was recently written here:

https://github.com/technion/matasano_challenge/blob/master/set2/chal16/chal16.rb

[madrobby]

So what to do about it to make it better?

[technion]

The safest thing would be automatically append a HMAC, which is verified before decrypting.
The sanest HMAC would be SHA256-HMAC, keyed with a different key to the encryption key.

[ahill-shopatron]

It might be worth considering if you really need AES. If no, you are probably better off letting RbNaCl do the actual cryptography. Your use case seems to correspond to SimpleBox operating in secret-key mode.