JeremyAllison Samba Team

samba@samba.org

19 Apr 1999 With the development of LanManager and Windows NT compatible password encryption for Samba, it is now able to validate user connections in exactly the same way as a LanManager or Windows NT server. This document describes how the SMB password encryption algorithm works and what issues there are in choosing whether you want to use it. You should read it carefully, especially the part about security and the "PROS and CONS" section. LanManager encryption is somewhat similar to UNIX password encryption. The server uses a file containing a hashed value of a user's password. This is created by taking the user's plaintext password, capitalising it, and either truncating to 14 bytes or padding to 14 bytes with null bytes. This 14 byte value is used as two 56 bit DES keys to encrypt a 'magic' eight byte value, forming a 16 byte value which is stored by the server and client. Let this value be known as the "hashed password". Windows NT encryption is a higher quality mechanism, consisting of doing an MD4 hash on a Unicode version of the user's password. This also produces a 16 byte hash value that is non-reversible. When a client (LanManager, Windows for WorkGroups, Windows 95 or Windows NT) wishes to mount a Samba drive (or use a Samba resource), it first requests a connection and negotiates the protocol that the client and server will use. In the reply to this request the Samba server generates and appends an 8 byte, random value - this is stored in the Samba server after the reply is sent and is known as the "challenge". The challenge is different for every client connection. The client then uses the hashed password (16 byte values described above), appended with 5 null bytes, as three 56 bit DES keys, each of which is used to encrypt the challenge 8 byte value, forming a 24 byte value known as the "response". In the SMB call SMBsessionsetupX (when user level security is selected) or the call SMBtconX (when share level security is selected), the 24 byte response is returned by the client to the Samba server. For Windows NT protocol levels the above calculation is done on both hashes of the user's password and both responses are returned in the SMB call, giving two 24 byte values. The Samba server then reproduces the above calculation, using its own stored value of the 16 byte hashed password (read from the smbpasswd file - described later) and the challenge value that it kept from the negotiate protocol reply. It then checks to see if the 24 byte value it calculates matches the 24 byte value returned to it from the client. If these values match exactly, then the client knew the correct password (or the 16 byte hashed value - see security note below) and is thus allowed access. If not, then the client did not know the correct password and is denied access. Note that the Samba server never knows or stores the cleartext of the user's password - just the 16 byte hashed values derived from it. Also note that the cleartext password or 16 byte hashed values are never transmitted over the network - thus increasing security. The unix and SMB password encryption techniques seem similar on the surface. This similarity is, however, only skin deep. The unix scheme typically sends clear text passwords over the network when logging in. This is bad. The SMB encryption scheme never sends the cleartext password over the network but it does store the 16 byte hashed values on disk. This is also bad. Why? Because the 16 byte hashed values are a "password equivalent". You cannot derive the user's password from them, but they could potentially be used in a modified client to gain access to a server. This would require considerable technical knowledge on behalf of the attacker but is perfectly possible. You should thus treat the smbpasswd file as though it contained the cleartext passwords of all your users. Its contents must be kept secret, and the file should be protected accordingly. Ideally we would like a password scheme which neither requires plain text passwords on the net or on disk. Unfortunately this is not available as Samba is stuck with being compatible with other SMB systems (WinNT, WfWg, Win95 etc). Note that Windows NT 4.0 Service pack 3 changed the default for permissible authentication so that plaintext passwords are never sent over the wire. The solution to this is either to switch to encrypted passwords with Samba or edit the Windows NT registry to re-enable plaintext passwords. See the document WinNT.txt for details on how to do this. Other Microsoft operating systems which also exhibit this behavior includes MS DOS Network client 3.0 with the basic network redirector installed Windows 95 with the network redirector update installed Windows 98 [se] Windows 2000 Note :All current release of Microsoft SMB/CIFS clients support authentication via the SMB Challenge/Response mechanism described here. Enabling clear text authentication does not disable the ability of the client to participate in encrypted authentication. plain text passwords are not passed across the network. Someone using a network sniffer cannot just record passwords going to the SMB server. WinNT doesn't like talking to a server that isn't using SMB encrypted passwords. It will refuse to browse the server if the server is also in user level security mode. It will insist on prompting the user for the password on each connection, which is very annoying. The only things you can do to stop this is to use SMB encryption. plain text passwords are not kept on disk. uses same password file as other unix services such as login and ftp you are probably already using other services (such as telnet and ftp) which send plain text passwords over the net, so sending them for SMB isn't such a big deal. In order for Samba to participate in the above protocol it must be able to look up the 16 byte hashed values given a user name. Unfortunately, as the UNIX password value is also a one way hash function (ie. it is impossible to retrieve the cleartext of the user's password given the UNIX hash of it), a separate password file containing this 16 byte value must be kept. To minimise problems with these two password files, getting out of sync, the UNIX /etc/passwd and the smbpasswd file, a utility, mksmbpasswd.sh, is provided to generate a smbpasswd file from a UNIX /etc/passwd file. To generate the smbpasswd file from your /etc/passwd file use the following command : $ cat /etc/passwd | mksmbpasswd.sh > /usr/local/samba/private/smbpasswd If you are running on a system that uses NIS, use $ ypcat passwd | mksmbpasswd.sh > /usr/local/samba/private/smbpasswd The mksmbpasswd.sh program is found in the Samba source directory. By default, the smbpasswd file is stored in : /usr/local/samba/private/smbpasswd The owner of the /usr/local/samba/private/ directory should be set to root, and the permissions on it should be set to 0500 (chmod 500 /usr/local/samba/private). Likewise, the smbpasswd file inside the private directory should be owned by root and the permissions on is should be set to 0600 (chmod 600 smbpasswd). The format of the smbpasswd file is (The line has been wrapped here. It should appear as one entry per line in your smbpasswd file.) username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX: [Account type]:LCT-<last-change-time>:Long name Although only the username, uid, XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX, [Account type] and last-change-time sections are significant and are looked at in the Samba code. It is VITALLY important that there by 32 'X' characters between the two ':' characters in the XXX sections - the smbpasswd and Samba code will fail to validate any entries that do not have 32 characters between ':' characters. The first XXX section is for the Lanman password hash, the second is for the Windows NT version. When the password file is created all users have password entries consisting of 32 'X' characters. By default this disallows any access as this user. When a user has a password set, the 'X' characters change to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii representation of the 16 byte hashed value of a user's password. To set a user to have no password (not recommended), edit the file using vi, and replace the first 11 characters with the ascii text "NO PASSWORD" (minus the quotes). For example, to clear the password for user bob, his smbpasswd file entry would look like : bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:[U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell If you are allowing users to use the smbpasswd command to set their own passwords, you may want to give users NO PASSWORD initially so they do not have to enter a previous password when changing to their new password (not recommended). In order for you to allow this the smbpasswd program must be able to connect to the smbd daemon as that user with no password. Enable this by adding the line : null passwords = yes to the [global] section of the smb.conf file (this is why the above scenario is not recommended). Preferably, allocate your users a default password to begin with, so you do not have to enable this on your server. Note : This file should be protected very carefully. Anyone with access to this file can (with enough knowledge of the protocols) gain access to your SMB server. The file is thus more sensitive than a normal unix /etc/passwd file. The smbpasswd command maintains the two 32 byte password fields in the smbpasswd file. If you wish to make it similar to the unix passwd or yppasswd programs, install it in /usr/local/samba/bin/ (or your main Samba binary directory). Note that as of Samba 1.9.18p4 this program MUST NOT BE INSTALLED setuid root (the new smbpasswd code enforces this restriction so it cannot be run this way by accident). smbpasswd now works in a client-server mode where it contacts the local smbd to change the user's password on its behalf. This has enormous benefits - as follows. smbpasswd no longer has to be setuid root - an enormous range of potential security problems is eliminated. smbpasswd now has the capability to change passwords on Windows NT servers (this only works when the request is sent to the NT Primary Domain Controller if you are changing an NT Domain user's password). To run smbpasswd as a normal user just type : $ smbpasswd Old SMB password: <type old value here - or hit return if there was no old password> New SMB Password: <type new value> Repeat New SMB Password: <re-type new value If the old value does not match the current value stored for that user, or the two new values do not match each other, then the password will not be changed. If invoked by an ordinary user it will only allow the user to change his or her own Samba password. If run by the root user smbpasswd may take an optional argument, specifying the user name whose SMB password you wish to change. Note that when run as root smbpasswd does not prompt for or check the old password value, thus allowing root to set passwords for users who have forgotten their passwords. smbpasswd is designed to work in the same way and be familiar to UNIX users who use the passwd or yppasswd commands. For more details on using smbpasswd refer to the man page which will always be the definitive reference. This is a very brief description on how to setup samba to support password encryption. compile and install samba as usual enable encrypted passwords in smb.conf by adding the line encrypt passwords = yes in the [global] section create the initial smbpasswd password file in the place you specified in the Makefile (--prefix=<dir>). See the notes under the The smbpasswd File section earlier in the document for details. Note that you can test things using smbclient.