Does anyone have information about such bugs/features mentioned below regarding IBM LAN-Server? I would appreciate any answer! regards Thomas Schweikle <tschweikat_private> L0pht Security Advisory Advisory released Jan. 5, 1999 Application: Windows 95/98 Network File Sharing Severity: Sniffed authentications can be used to impersonate network users Author: weldat_private http://www.l0pht.com/advisories.html Overview : Windows 95/98 network file sharing reuses the cryptographic challenges used in SMB challenge/response authentication. The reuse of the challenge enables an attacker, who has captured a legitimate network authentication, to replay the authentication and establish a connection impersonating a valid user. Description : During testing of the L0phtCrack 2.5 SMB packet capture tool to capture SMB challenge/response authentication, it became apparent to the L0phtCrack development team that Windows 95/98 issues the exact same challenge for each authentication for a period of approximately 15 minutes. During this time an attacker can connect to a network share as the user whose authentication was captured. The attacker can connect to the Win95/98 share as that user because the user name is transmitted in the clear as well as the challenge. Although the attacker does not know the user's password and therefore cannot generate the encrypted password hash from it, the attacker does not have to. She merely replays the encrypted hash that she captured. It will be correct because the challenge hasn't changed and she is impersonating that particular user. Reusing a challenge is a classic cryptographic mistake. If the challenge was simply incremented this attack would not be possible. Details : The following captures are in L0phtCrack 2.5 capture format specified as: DOMAIN\username:3:challenge:encrypted LANMAN hash:encrypted NTLM hash The following 2 captures show an NT machine connecting to another NT machine. The challenge is different, as it should be, for each authentication. DOMAIN\user:3:c21ee5e0c1a8ae89:626cc3ec9f8f1849bbd645541477be48bf261b486 9c36e7a:f9dfdb9ee9d1705a4fd45a0ed5f2c62e0c7a957860a59559 DOMAIN\user:3:ce16b6d32eee2e29:8f96e377f2b9670fa425c4e52ae4ae6ae3e23f693 d518719:d9a3180ce6e30f8a12d46703847147b70066dbaf9a5b654e The following 2 captures show an NT machine connecting to a Win98 machine. Notice that the same challenge is issued each time. DOMAIN\user:3:8f2eceae79b55000:43caa3ff5c793d04bbbe2332e8918bf80735b0100 89dc573:1c592e5dcf78cf658829d0cbe61c0e4c32b5ed7a87f5097e DOMAIN\user:3:8f2eceae79b55000:43caa3ff5c793d04bbbe2332e8918bf80735b0100 89dc573:1c592e5dcf78cf658829d0cbe61c0e4c32b5ed7a87f5097e This capture is another NT machine connecting to the same Win98 machine used above. Notice this is the same challenge as in the previous 2 authentications. DOMAIN\user:3:8f2eceae79b55000:43caa3ff5c793d04bbbe2332e8918bf80735b0100 89dc573:1c592e5dcf78cf658829d0cbe61c0e4c32b5ed7a87f5097e As you can see from the last 3 captures, if the username and challenge are the same then the encrypted hashes sent are the same. Implementation : An attacker could modify the unix Samba client to alter the way it issues encrypted password hashes. It could be modified to send a fixed encrypted password hash as entered by the attacker instead of generating it based on a password and the challenge. In this way the attacker could feed the output of an SMB packet capture into a modified Samba client to make Win95/98 file share connections from her machine. Once these connections are made, interesting files could be read from or written to the Win95/98 machines. Files that could be written include those in the Windows Startup folder which would enable programs to install themselves to automatically execute on system startup. Conclusion : This vulnerability comes at a time when many in the security community are waking up to the fact that a Win95/98/NT specific virus could spread rapidily by taking advantage of flaws in network authentication. The recent "Remote Explorer" virus did not take advantage in flaws in network authentication. It took advantage of poor Domain Administrator practice. Some day a virus will take advantage of flaws such as the aforementioned Win95/98 network impersonation or perhaps the cracking of network authentication that L0phtCrack 2.5 performs so effortlessly. Weak network security implementation and weak passwords will be the culprits. L0phtCrack is designed to help defeat the latter. ...
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