Lets say that I have a server which encrypts information at the server side using ECB mode with the blocksize of 16. If the server appends data that I control with a secret and sends back the ciphertext, it is possible for me to figure out what the secret is, by bruteforcing one character at a time.
Now lets consider a scenario where there is some secret data(say A) being prepended to my data(whose length I'm unaware of). There is also some some other secret data(say B, whose length I'm also unaware of) appended to my data. This is then encrypted and sent back over to me. Is there any way I can figure out what "B" is?