If the cookie is only the username (or the first part of the cookie is the username) and you want to impersonate the username "admin". Then, you can create the username "bdmin" and bruteforce the first byte of the cookie.
In cryptography, a cipher block chaining message authentication code (CBC-MAC) is a technique for constructing a message authentication code from a block cipher. The message is encrypted with some block cipher algorithm in CBC mode to create a chain of blocks such that each block depends on the proper encryption of the previous block. This interdependence ensures that a change to any of the plaintext bits will cause the final encrypted block to change in a way that cannot be predicted or counteracted without knowing the key to the block cipher.
To calculate the CBC-MAC of message m, one encrypts m in CBC mode with zero initialization vector and keeps the last block. The following figure sketches the computation of the CBC-MAC of a message comprising blocks using a secret key k and a block cipher E:
With CBC-MAC usually the IV used is 0.
This is a problem because 2 known messages (
m2) independently will generate 2 signatures (
E(m1 XOR 0) = s1
E(m2 XOR 0) = s2
Then a message composed by m1 and m2 concatenated (m3) will generate 2 signatures (s31 and s32):
E(m1 XOR 0) = s31 = s1
E(m2 XOR s1) = s32
Which is possible to calculate without knowing the key of the encryption.
Imagine you are encrypting the name Administrator in 8bytes blocks:
You can create a username called Administ (m1) and retrieve the key (s1).
Then, you can create a username called the result of
rator\00\00\00 XOR s1. This will generate
E(m2 XOR s1 XOR 0) which is s32.
now, knowing s1 and s32 you can put them together an generate the encryption of the full name Administrator.
Get the signature of username Administ (m1) which is s1
Get the signature of username rator\x00\x00\x00 XOR s1 XOR 0 is s32.
Set the cookie to s1 followed by s32 and it will be a valid cookie for the user Administrator.
If you can control the used IV the attack could be very easy. If the cookies is just the username encrypted, to impersonate the user "administrator" you can create the user "Administrator" and you will get it's cookie. Now, if you can control the IV, you can change the first Byte of the IV so IV XOR "A" == IV' XOR "a" and regenerate the cookie for the user Administrator. This cookie will be valid to impersonate the user administrator with the initial IV.
More information in https://en.wikipedia.org/wiki/CBC-MAC