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CBC

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.

CBC-MAC

Cipher block chaining message authentication code (CBC-MAC) is a method used in cryptography. It works by taking a message and encrypting it block by block, where each block's encryption is linked to the one before it. This process creates a chain of blocks, making sure that changing even a single bit of the original message will lead to an unpredictable change in the last block of encrypted data. To make or reverse such a change, the encryption key is required, ensuring security.

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 blockshttps://wikimedia.org/api/rest_v1/media/math/render/svg/bbafe7330a5e40a04f01cc776c9d94fe914b17f5 using a secret key k and a block cipher E:

https://upload.wikimedia.org/wikipedia/commons/thumb/b/bf/CBC-MAC_structure_(en).svg/570px-CBC-MAC_structure_(en).svg.png

Vulnerability

With CBC-MAC usually the IV used is 0.
This is a problem because 2 known messages (m1 and m2) independently will generate 2 signatures (s1 and s2). So:

  • 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:

  • Administ
  • rator\00\00\00

You can create a username called Administ (m1) and retrieve the signature (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, you can use s32 as the signature of the full name Administrator.

Summary

  1. Get the signature of username Administ (m1) which is s1
  2. Get the signature of username rator\x00\x00\x00 XOR s1 XOR 0 is s32**.**
  3. Set the cookie to s32 and it will be a valid cookie for the user Administrator.

Attack Controlling IV

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[0] XOR "A" == IV'[0] XOR "a" and regenerate the cookie for the user Administrator. This cookie will be valid to impersonate the user administrator with the initial IV.

References

More information in https://en.wikipedia.org/wiki/CBC-MAC

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