Hi folks, about three weeks ago, I discovered a hole in IBM's websphere 4.0 session ID generation. Over a week ago, IBM made a fix for this available, so here is the information about the vulnerability: (everybody who don't want to read about this vulnerability and just want to know the patch info: install the eFix PQ47663V302) INTRO websphere can generate sessionids which are put into cookies for users, to be able to supply user tracking, e.g. user authenticates with userid and password, and access to data is checked by checking if the sessionid is authenticated or not. THE BUG during a security assessment for a bank, I collected several sessionids and they did not look that random to me ... SessionID TIME TWGYLZIAAACVDQ3UUSZQV2I 10:27:12 TWGY0WYAAACVFQ3UUSZQV2I 10:27:13 TWGZNZAAAACVHQ3UUSZQV2I 10:27:14 TWG0BUYAAACVJQ3UUSZQV2I 10:27:15 TWG0VIAAAACVLQ3UUSZQV2I 10:27:16 TWG1ICIAAACVNQ3UUSZQV2I 10:27:17 TWG111YAAACVPQ3UUSZQV2I 10:27:18 xxxx y You can see that for seven requests, only 5 characters changed, and: * the characters A-Z and 0-9 are used, hence 36 combinations possible per char * the sessionid is based on two counters which are counted up, the rest of the string seems to be fixed. * the first counter (xxxx) seems to count milliseconds (TWGxxxx), but counts a bit too slow (see seconds 15 and 16, where both 1st rows of the counters start with a 0). well, to cut a long story short, it is really a counter which increases 65536 times per second and is then encoded to the A-Z0-9 format. * If you collect many sessionids (I collected 1000, it's obvious then), you'll see that the the least signifcant char of the first counter are 95% of the time showing a Y, I, A or Q. The reason for that is (my guess) that the clock of the machine only can increase 7.500-10.000 times instead of 65536 because it's not a realtime clock and the server type is not a cray :-) * The second counter (y) is increasing by two every second. The counters are in fact longer than 4, but this is the visible changes in the example above. Then the guess was that the fixed strings may be based on the IP of the client. So I checked with different IP addresses, but no difference in the fixed strings of the sessionID. THE RISK If someone knows the time of the connect to the server (even with SSL encrypted) an attacker can issue requests with changing sessionids until it's the correct one. If an attacker just wants to have any user data, he can constantly try some guessing. As the first counter only has 7.500-10.000 values per second, and the seconds counters just increases approx. once per second (or perhaps per request), the sessionid can have 7.750 to 10.500 different values. If a user is normaly connected for 15 minutes after authentication to an eCommerce system (and does not forget to logout, otherwise the time is extended by the session timeout). As an attacker is likely to succeed after 50% of the keyspace, he needs 3.875 to 5.250 attempts, so 4 to 5 requests per second are enough. Two customers were using the sessionids for the security of their eCommerce system ... we are not talking about some weird feature nobody uses. Short: it is an easy and likely attack. THE FIX install eFix PQ47663V302 and feel better THANKS to the IBM websphere team, which fixed the bug pretty fast for the customer. Greets, Marc -- E@mail: marcat_private Function: Security Research and Advisory PGP: "lynx -source http://www.suse.de/~marc/marc.pgp | pgp -fka" Key fingerprint = B5 07 B6 4E 9C EF 27 EE 16 D9 70 D4 87 B5 63 6C Private: http://www.suse.de/~marc SuSE: http://www.suse.de/security
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