hi bugtraqers, Background ========== i sent the following advisory to Microsoft there is about 1 month of that, and since i did not get any reply. The problem described below is still working on the latest MSN client version currently available. A bug in the Hotmail Messenger cryptographic system may allow the recovery of millions of hotmail mailboxes's password. Microsoft MSN messenger is a very handy little win32 application designed to keep in touch with friends, family, collaborators around the world. It offers many nice features like real time chats, hotmail mailbox access, etc... Messenger runs with its own protocol to communicate with a bunch of Microsoft dedicated servers and authenticate itself with the same password than hotmail is using (through the global passport system). The password is not sent clearly on the wire but hashed with MD5 in the following manner: Authentication Scheme ===================== while negociating a connection with a remote Microsoft server, msn clients clearly send the target user mailbox to be authenticated with (basically the username) and get back a scrambler string to be prepend to the password before hashing it and sending it. client ----- VER xx MSNP5 MSNP4 CVR0 ---------------> MSN server client <---- VER xx MSNP5 MSNP4 CVR0 --------------- MSN server client ----- INF (xx+1) ----------------------------> MSN server client <---- INF (xx+1) MD5 ------------------------ MSN server client ----- USR (xx+2) MD5 I ----------------------> MSN server client <---- USR (xx+2) MD5 S yyyyyyyyy.yyyyyyyyy -- MSN server (the scrambler string is actually made with seconds.microseconds) client ----- USR (xx+3) MD5 S xxxxx...(32 chars) ---> MSN server here it is, the password hash has been sent and may be easily broken by bruteforcing it. the hash creation process is as follow: ====================================== say user toto has a password "titan" then his client generate the string "yyyyyyyyy.yyyyyyyyytitan" and the according MD5 hash, say xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx. the client send MD5(yyyyyyyyy.yyyyyyyyytitan) on the wire. Problem ======= by sniffing the wire, a malicious user can obviously retrieve the scrambler string and the final hash. then he can start a bruteforce session trying all password combinaisons with the same scrambler prepended and comparing the resulting hash with this he previously sniffed. (an exhaustive attack) Basically, without any bug, messenger is already vulnerable because of the weak cryptographic scheme it uses. Exploit ======= I wrote, with the great help of Simeon Pilgrim, a very fast MD5 bruteforcer designed to use scrambler strings to finally retrieve an original password for a given MD5 hash. Currently it takes only 12 days to exhaust all 8 chars length passwords in the charset [a-z0-9] with an average speed of 2 600 000 hashes sec with only one 1 Ghz athlon processor. (i considere to be able to test up to 4 000 000 hashes/sec with the next release and one 1.3 Ghz processor) Win32/Unix versions are freely available at http://mdcrack.multimania.com Problem 2 ========= Another important point is that, if nowadays users can't actually choose a new password lesser than 8 chars length, all old and weak passwords (from 1 to 7 chars) are still in use and just works perfectly with MSN. How many users are currently at risk ? too much for my fingers i bet. Problem 3 ========= The last point is a nasty bug in the client implementation that allow a malicious user, spoofing the MSN server, to send a (NULL) scrambler string In such a case and intead of simply closing the connection, the client send the mere password hash making things even faster for a further bruteforce attack. client <---- USR (xx+2) MD5 S ---------------------- fake MSN server client ----- USR (xx+3) MD5 S xxxxx...(32 chars) ---> fake MSN server where xxxx...(32 chars) is actually MD5(password). Note that if this technic is still stealthy, it may need, in some network topologies, the use of icmp redirect/ arp spoofing to redirect all the flow to the attacker machine inside a given network. But this kind of attacks are well known by networks crackers. Note that all communications between clients and servers are in a clear form, and by the way, many other identity robbery attacks remain available for instance, when our victim is asking messenger to open his mailbox, the malicious user may send another URL to the client like a spoofing site with a false hotmail relogging page. Conclusion ========== Because hotmail and MSN are using the same authentication system called passport, compromising users MSN account is finally the same trick than compromising hotmail users mailbox. A malicious user with a freshly hacked MSN password can use it either with messenger or with www.hotmail.com Because a tremendous number of people are using these services without taking too much care about their password strongness, the number of potential victims is really great. The attack described above can be released from any place in the path between the victim and MSN servers or simply in the same network, this is the mere prerequisite. Choose a quite strong password (at least 9 chars length with a good charset) and change it as regularly as possible. Finally, never never trust hotmail and any other web based free accounts for you very own mails. ======== Gregory Duchemin Security Consultant NEUROCOM CANADA 1001 Bd Maisonneuve Ouest, Suite 200 Montreal Quebec H3A 3C8 Canada phone: 514 908 6800 Email: gdnat_private _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.
This archive was generated by hypermail 2b30 : Mon Jul 09 2001 - 12:11:21 PDT