AUSCERT External Security Bulletin Redistribution

         ESB-2001.307 -- SSH Secure Shell 3.0.0 Security Advisory
                Secure Shell Potential Remote Root Exploit
                               24 July 2001


        AusCERT Security Bulletin Summary

Product:                SSH Secure Shell 3.0.0
Vendor:                 SSH
Operating System:       Red Hat Linux 6.1 to 7.1
                        Solaris 2.6 to 2.8
                        HP-UX 10.20
                        HP-UX 11.00
                        Caldera Linux 2.4
                        Suse Linux 6.4 to 7.0
Impact:                 Root Compromise
Access Required:        Remote

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Hash: SHA1

Dear Secure Shell Community,

A potential remote root exploit has been discovered 
in SSH Secure Shell 3.0.0, for Unix only, concerning 
accounts with password fields consisting of two or 
fewer characters. Unauthorized users could potentially 
log in to these accounts using any password, including 
an empty password.  This affects SSH Secure Shell 3.0.0
for Unix only.  This is a problem with password 
authentication to the sshd2 daemon.  The SSH Secure 
Shell client binaries (located by default in 
/usr/local/bin) are not affected.   

SSH Secure Shell 3.0.1 fixes this problem.

Please note that if using a form of authentication 
other than password, AND password authentication 
is disabled, you are NOT VULNERABLE to this issue. 

Red Hat Linux 6.1 thru 7.1 
Solaris 2.6 thru 2.8 
HP-UX 10.20 
HP-UX 11.00 
Caldera Linux 2.4 
Suse Linux 6.4 thru 7.0 

Please note that other platforms not listed here 
may also be vulnerable. 


Tru64 4.0.G, NetBSD, and OpenBSD are not vulnerable. 

Please note that other platforms not listed here 
may also be immune.


Some stock machines which have default locked accounts 
running SSH Secure Shell 3.0 are vulnerable to 
arbitrary logins.  This is a serious problem with 
Solaris, for example, which uses the sequence "NP" to 
indicate locked administrative accounts such as "lp", 
"adm", "bin" etc.  Some Linux machines which have 
accounts with !! in the etc/passwd or /etc/shadow such 
as xfs or gdm are also vulnerable. Since it is relatively 
easy to become root after gaining access to certain 
accounts, we consider this a potential root exploit.


During password authentication, if the field containing 
the encrypted password in /etc/shadow, /etc/password, 
etc. is two or less characters long, SSH 3.0.0 will 
allow anyone to access that account with ANY password.
The bug is in the code that compares the result of calling 
crypt(pw, salt) with the value of the encrypted password 
in the /etc/shadow (or /etc/password) file. SSH Secure Shell 
Server 3.0.0 does a bounded string compare bounded to the 
length of the value stored in aforementioned file (2 
characters in this case) against the return value of 
crypt(). The return value of crypt() is 13 characters, 
with the first two characters being the salt value itself.  
The salt value used is the first two characters of the 
encrypted password in /etc/shadow (or /etc/password). A 
2 character string comparison between the 2 character 
encrypted password in /etc/shadow, and the 13 character 
crypt() return value, whose first two characters ARE the 
2 characters from the password in /etc/shadow. The strings 
match, and the 3.0.0 daemon then accepts the password, no 
matter what is input. 



Immediately upgrade to SSH Secure Shell 3.0.1 
which will be available on our e-commerce site 
http://commerce.ssh.com shortly, and is available 
on the ftp site now - ftp://ftp.ssh.com/pub/ssh
A patch for 3.0.0 source code is also available there.

Alternative work-arounds 
Disable password authentication to the SSH Secure Shell 
daemon (sshd2) in the /etc/ssh2/sshd2_config and use 
another form of authentication such as public key, 
SecurID, Kerberos, certificates, Smart Cards, or 
hostbased to authenticate your users.  These alternative 
authentication methods are NOT VULNERABLE.  Please see 
our SSH Secure Shell support web pages for more 
information on how to enable these authentication methods. 


If you cannot disable password authentication fully, 
limit access to the sshd2 daemon to allow only users 
with entries in the /etc/passwd and /etc/shadow which 
exceed two characters.  This can be done using the 
AllowUsers, AllowGroups, DenyUsers, and DenyGroups 
keywords in the /etc/ssh2/sshd2_config file.  See 
our SSH Secure Shell support web pages 
http://www.ssh.com/support/ssh and man sshd2_config 
for more information. 


Assign a valid password for each account.  Because 
it is possible that assigning a password to some 
system accounts could cause problems on some operating 
systems, this work-around is limited and is provided 
only as a last-resort alternative.


Use the following patch in the source code:

File /lib/sshsession/sshunixuser.c
Function ssh_user_validate_local_password
Location near line 953, before 
/*Authentication is accepted if the encrypted 
passwords are identical. */

Add lines

if (strlen(correct_passwd) < 13)
return FALSE;


We apologize for any inconvenience this may cause. 
SSH Communications Security takes security issues very 
seriously, and a CERT advisory and submission to Bugtraq 
regarding this issue have been submitted.  Please make 
every effort to ensure that your systems are protected 
using one of the above methods as quickly as possible.  
As this information becomes widely known, your systems could 
be at even greater risk if appropriate measures are not taken. 

SSH is fully committed to serving and supporting our users 
and products. While we may not be able to address each request
for information on this issue individually, we will 
make publicly available any relevant information possible 
which addresses your questions and concerns.


This vulnerability was found and reported by an 
anonymous system administrator at the Helsinki University 
of Technology and by Andrew Newman of Yale University.

Version: PGP 7.0.1


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