CWE 6

If attackers use a botnet with hundreds or thousands of drone computers, it is reasonable to assume that they could attempt tens of thousands of guesses per second. If the web site in question is large and popular, a high volume of guessing might go unnoticed for some time.

Session identifiers should be at least 128 bits long to prevent brute-force session guessing. A shorter session identifier leaves the application open to brute-force session guessing attacks.

Phase: Implementation

A lower bound on the number of valid session identifiers that are available to be guessed is the number of users that are active on a site at any given moment. However, any users that abandon their sessions without logging out will increase this number. (This is one of many good reasons to have a short inactive session timeout.) With a 64 bit session identifier, assume 32 bits of entropy. For a large web site, assume that the attacker can try 1,000 guesses per second and that there are 10,000 valid session identifiers at any given moment. Given these assumptions, the expected time for an attacker to successfully guess a valid session identifier is less than 4 minutes. Now assume a 128 bit session identifier that provides 64 bits of entropy. With a very large web site, an attacker might try 10,000 guesses per second with 100,000 valid session identifiers available to be guessed. Given these assumptions, the expected time for an attacker to successfully guess a valid session identifier is greater than 292 years.

Session ID's can be used to identify communicating parties in a web environment.

The expected number of seconds required to guess a valid session identifier is given by the equation: (2^B+1)/(2*A*S) Where: - B is the number of bits of entropy in the session identifier. - A is the number of guesses an attacker can try each second. - S is the number of valid session identifiers that are valid and available to be guessed at any given time. The number of bits of entropy in the session identifier is always less than the total number of bits in the session identifier. For example, if session identifiers were provided in ascending order, there would be close to zero bits of entropy in the session identifier no matter the identifier's length. Assuming that the session identifiers are being generated using a good source of random numbers, we will estimate the number of bits of entropy in a session identifier to be half the total number of bits in the session identifier. For realistic identifier lengths this is possible, though perhaps optimistic.