OSEC

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Subject: OpenSSH's UseLogin option allows remote access with root privilege.
From: Markus Friedl (markus.friedlINFORMATIK.UNI-ERLANGEN.DE)
Date: Fri Jun 09 2000 - 10:06:30 CDT


OpenSSH's UseLogin option allows remote access with root privilege.

1. Systems affected:

        The default installation of OpenSSH is not vulnerable, since
        UseLogin defaults to 'no'. However, if UseLogin is enabled,
        all versions of OpenSSH prior to 2.1.1 are affected.

2. Description:

        If the UseLogin option is enabled the OpenSSH server (sshd)
        does not switch to the uid of the user logging in. Instead,
        sshd relies on login(1) to do the job. However, if the user
        specifies a command for remote execution login(1) cannot
        be used and sshd fails to set the correct user id. The
        command is run with the same privilege as sshd (usually
        with root privilege).

3. Impact:

        If the administrator enables UseLogin users can get privileged
        access to the server running sshd.

4. Short Term Solution:

        Do not enable UseLogin on your machines or disable UseLogin
        again in /etc/sshd_config:
                UseLogin no

5. Solution:

        Upgrade to OpenSSH-2.1.1 or apply the attached patch.
        OpenSSH-2.1.1 is available from www.openssh.com.

Appendix:

1. OpenSSH-1.2.2

--- sshd.c.orig Thu Jan 20 18:58:39 2000
+++ sshd.c Tue Jun 6 10:12:00 2000
-2231,6 +2231,10
         struct stat st;
         char *argv[10];

+ /* login(1) is only called if we execute the login shell */
+ if (options.use_login && command != NULL)
+ options.use_login = 0;
+
         f = fopen("/etc/nologin", "r");
         if (f) {
                 /* /etc/nologin exists. Print its contents and exit. */

2. OpenSSH-1.2.3

--- sshd.c.orig Mon Mar 6 22:11:17 2000
+++ sshd.c Tue Jun 6 10:14:07 2000
-2250,6 +2250,10
         struct stat st;
         char *argv[10];

+ /* login(1) is only called if we execute the login shell */
+ if (options.use_login && command != NULL)
+ options.use_login = 0;
+
         f = fopen("/etc/nologin", "r");
         if (f) {
                 /* /etc/nologin exists. Print its contents and exit. */

3. OpenSSH-2.1.0

--- session.c.orig Wed May 3 20:03:07 2000
+++ session.c Tue Jun 6 10:10:50 2000
-744,6 +744,10
         struct stat st;
         char *argv[10];

+ /* login(1) is only called if we execute the login shell */
+ if (options.use_login && command != NULL)
+ options.use_login = 0;
+
         f = fopen("/etc/nologin", "r");
         if (f) {
                 /* /etc/nologin exists. Print its contents and exit. */

EOF