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
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.
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.
The following captures are in L0phtCrack 2.5 capture format specified
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
The following 2 captures show an NT machine connecting to a Win98
machine. Notice that the same challenge is issued each time.
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.
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.
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
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
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
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