the-web-application-hackers-handbook

224 Chapter 7 n Attacking Session Management
However, in some situations, depending on the encryption algorithm used and
the manner in which the application processes the tokens, it may nonetheless be
possible for users to tamper with the tokens’ meaningful contents without actually
decrypting them. Bizarre as it may sound, these are actually viable attacks
that are sometimes easy to deliver, and numerous real-world applications have
proven vulnerable to them. The kinds of attacks that are applicable depend on
the exact cryptographic algorithm that is being used.
ECB Ciphers
Applications that employ encrypted tokens use a symmetric encryption algorithm
so that tokens received from users can be decrypted to recover their meaningful
contents. Some symmetric encryption algorithms use an “electronic codebook”
(ECB) cipher. This type of cipher divides plaintext into equal-sized blocks (such
as 8 bytes each) and encrypts each block using the secret key. During decryption,
each block of ciphertext is decrypted using the same key to recover the
original block of plaintext. One feature of this method is that patterns within the
plaintext can result in patterns within the ciphertext, because identical blocks of
plaintext will be encrypted into identical blocks of ciphertext. For some types
of data, such as bitmap images, this means that meaningful information from
the plaintext can be discerned within the ciphertext, as illustrated in Figure 7-4.
Figure 7-4: Patterns within plaintext that
is encrypted using an ECB cipher may be
visible within the resulting ciphertext.
In spite of this shortcoming with ECB, these ciphers are often used for encrypting
information within web applications. Even in situations where the problem
of patterns within plaintext does not arise, vulnerabilities can still exist. This
is because of the cipher’s behavior of encrypting identical plaintext blocks into
identical ciphertext blocks.
Consider an application whose tokens contain several different meaningful
components, including a numeric user identifier:
rnd=2458992;app=iTradeEUR_1;uid=218;username=dafydd;time=634430423694715
000;