sses-sshauth.txt

`Sword & Shield Enterprise Security, Inc. - Security Advisory  
www.sses.net, Copyright (c) 2000  
  
Advisory: Secure Shell Authentication Vulnerability  
Release Date: May 10, 2000  
Application: sshd  
Severity: High - A user (local or remote) can log into any account  
with a valid login shell.  
Status: Affected systems should install alternative version.  
Archive: The advisory sses-002-auth-vul.txt  
is available at ftp://ftp.sses.net/pub/security/advisories  
  
SUMMARY  
-------  
A vulnerable secure shell distribution is available from the popular  
Zedz Consultants FTP site (formally known as replay.com). The  
RedHat Linux RPM ssh-1.2.27-8i.src.rpm contains a PAM patch which  
contains faulty logic allowing users to essentially pass through  
the username/password authentication step and gain shell access.  
  
It should be stressed that the ssh distribution 1.2.27-7us.rpm  
and 1.2.27-7i.rpm available from the web site do not contain this  
vulnerability. The vulnerable distribution (1.2.27-8i.src.rpm) is  
located in the "incoming" directory at ftp://ftp.zedz.net/pub/  
cryptoI/incoming (formerly pub/crypto/incoming). It is also  
possible that the distribution could have been obtained from  
the pub/crypto/redhat/unsorted directory.  
  
Due to the obscure location of the vulnerable distribution  
it is not known at this time how wide spread the impact of  
this vulnerability is.  
  
The vulnerable distribution has been removed from the site and  
should no longer be accessible.  
  
  
DESCRIPTION  
-----------  
The vulnerable ssh distribution is patched with defective logic  
related to PAM authentication. The offending code from the patch  
file ssh-1.2.27-pam.patch is:  
  
+#ifdef HAVE_PAM  
+ {  
+ retval = origretval;  
+ pampasswd = xstrdup(password);  
+ if (retval == PAM_SUCCESS)  
+ retval = pam_authenticate ((pam_handle_t *)pamh, 0);  
+ if (retval == PAM_SUCCESS || retval == PAM_AUTH_ERR)  
+ retval = pam_acct_mgmt ((pam_handle_t *)pamh, 0);  
+ xfree(pampasswd);  
+ }  
+#else /* HAVE_PAM */  
  
Note the last 'if' statement - in essence whether the pam_authenticate()  
call is successful or not, the pam_acct_mgmt() call is made overwriting  
the contents of retval. Assuming the pam_acct_mgmt() call is  
successful, and it tends to be, then the remaining patch code dealing  
with PAM authentication opens a session with:  
  
+#ifdef HAVE_PAM  
+ {  
+ if (retval == PAM_SUCCESS)  
+ retval = pam_open_session ((pam_handle_t *)pamh, 0);  
+ return (retval == PAM_SUCCESS);  
+ }  
+#endif /* HAVE_PAM */  
  
  
By running the patch command patch -p0 -b < ssh-1.2.27-pam.patch we get,  
  
patching file `ssh-1.2.27/acconfig.h'  
patching file `ssh-1.2.27/auth-passwd.c'  
patching file `ssh-1.2.27/config.h.in'  
patching file `ssh-1.2.27/configure.in'  
patching file `ssh-1.2.27/sshd.c'  
  
The faulty PAM authentication logic is then inserted into the  
auth_password() function in the auth-passwd.c file at lines  
745-755 and 879-885.  
  
  
IMPACT  
------  
The impact of this bug can be quite severe.  
  
On systems where the /etc/ssh/sshd_config file contains  
'PermitRootLogin=yes' the effect is that any remote or  
local user can obtain root access by specifying a root  
login and entering a non-null password.  
  
Example, if a system (pigpen) configured with the vulnerable ssh server  
package has a valid user account (joe) then the command:  
  
% ssh -l joe pigpen  
joe@pigpen's password: 123 <--- sshd prompts for password, enter '123'.  
... <--- faulty authentication check performed  
[joe@pigpen]$ <--- user shell accessed  
  
Checking the syslogs also reveals signs of the problem:  
  
1- May 8 13:12:50 pigpen sshd[13422]: connect from 10.10.10.10  
2- May 8 13:12:50 pigpen sshd[13422]: log: Connection from 10.10.10.10  
port 1209  
3- May 8 13:12:52 pigpen PAM_pwdb[13422]: authentication failure;  
(uid=0) -> joe for ssh service  
4- May 8 13:12:53 pigpen PAM_pwdb[13422]: (ssh) session opened for user joe  
by (uid=0)  
5- May 8 13:12:53 pigpen sshd[13422]: log: Password authentication for joe  
accepted.  
  
  
Note the authentication failure recorded by PAM on line 3, but lines 4-5  
show the session being opened.  
  
It is important to note a few things here:  
  
1) Even if an account is password locked, if it contains a valid shell  
the account can be accessed.  
  
2) Site specific user accounts are not necessary for searching out vulnerable  
systems when standard Linux distribution/package accounts like 'gdm',  
'postgres', or 'mysql' will do.  
  
  
This vulnerable SSH RPM has been available since March 1, 2000.  
  
  
RESOLUTION  
----------  
  
De-install: If your ssh installation is vulnerable, you should remove  
the vulnerable version and install version 1.2.27-7us.  
  
Use OpenSSH: Another alternative to consider is switching over to openssh  
available from http://www.openssh.com.  
  
  
AFFECTED VERSIONS and SYSTEMS  
-----------------------------  
  
RedHat Linux, RPM ssh-1.2.27-8i.src.rpm.  
  
ACKNOWLEDGEMENTS  
----------------  
The bug discovery, test, demonstration, vendor coordination,  
and advisory generation are the results of SSES, Inc. security engineers  
John McNeely and Dennis Edmonds.  
  
Thanks to Alex De Joode at Zedz Consultants for a quick response and  
removal of the vulnerable distribution.  
  
DISCLAIMER  
----------  
Although SSES, Inc. intends to provide accurate information, this  
advisory does not claim to be complete or usable for any purpose.  
  
NO WARRANTY  
-----------  
This advisory is provided on an "as is" basis. SSES, Inc. makes no  
warranties of any kind, either expressed or implied as to any matter  
including, but not limited to, warranty of fitness for a particular  
purpose or merchantability, exclusivity or results obtained from use  
of the material. SSES, Inc. does not make any warranty of any kind  
with respect to freedom from patent, trademark, or copyright infringement.  
  
The supplied advisory is not to be used for malicious purposes and  
should be used for informational purposes only.  
`