The Web Application Hacker’s Handbook Discovering and Exploiting Security Flaws

Chapter 7 

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Attacking Session Management

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session token prediction attack. The server used the Java API 

java.util

.Random

to generate session tokens. This implements a “linear congruential

generator,” which generates the next number in the sequence as follows:

synchronized protected int next(int bits) {

seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);

return (int)(seed >>> (48 - bits));

}

This algorithm in effect takes the last number generated, multiplies it by one

constant, and adds another constant, to obtain the next number. The number is

truncated to 48 bits, and the algorithm shifts the result to return the specific

number of bits requested by the caller.

Knowing this algorithm and a single number generated by it, we can easily

derive the sequence of numbers that the algorithm will generate next, and also

(with a little number theory) derive the sequence that it generated previously.

This means that an attacker who obtains a single session token from the server

can obtain the tokens of all current and future sessions.

N OT E