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If agent or X11 forwarding is disabled in the ssh client configuration, the client does not request these features during session setup. This is the correct behavior.
However, when the OpenSSH client receives an actual request asking for access to the ssh-agent, this client fails to check whether this feature has been negotiated during session setup. The client does not check whether the request complies with the client configuration and grants access to the ssh-agent. A similar problem exists in the X11 forwarding implementation. |
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Credit:
The information has been provided by Jacob Langseth.
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Vulnerable systems:
All versions of OpenSSH prior to 2.3.0
Impact:
Hostile servers can access your X11 display or your ssh-agent.
Workaround:
Clear both the $DISPLAY and the $SSH_AUTH_SOCK variable before connecting to untrusted hosts:
% unset SSH_AUTH_SOCK; unset DISPLAY; ssh host
Solution:
Upgrade to OpenSSH-2.3.0 or apply the attached patch. OpenSSH-2.3.0 is available from http://www.openssh.com.
Patch against openssh-2.2.0:
--- /openssh-2.2.0/clientloop.c Sun Aug 20 00:21:19 2000
+++ ssh/clientloop.c Fri Nov 10 13:54:42 2000
@@ -32,6 +32,8 @@
#include "buffer.h"
#include "bufaux.h"
+extern Options options;
+
/* Flag indicating that stdin should be redirected from /dev/null. */
extern int stdin_null_flag;
@@ -750,7 +752,6 @@
int
client_loop(int have_pty, int escape_char_arg, int ssh2_chan_id)
{
- extern Options options;
double start_time, total_time;
int len;
char buf[100];
@@ -993,7 +994,7 @@
debug("client_input_channel_open: ctype %s rchan %d win %d max %d",
ctype, rchan, rwindow, rmaxpack);
- if (strcmp(ctype, "x11") == 0) {
+ if (strcmp(ctype, "x11") == 0 && options.forward_x11) {
int sock;
char *originator;
int originator_port;
@@ -1066,11 +1067,14 @@
dispatch_set(SSH_MSG_CHANNEL_OPEN_CONFIRMATION, &channel_input_open_confirmation);
dispatch_set(SSH_MSG_CHANNEL_OPEN_FAILURE, &channel_input_open_failure);
dispatch_set(SSH_MSG_PORT_OPEN, &channel_input_port_open);
- dispatch_set(SSH_SMSG_AGENT_OPEN, &auth_input_open_request);
dispatch_set(SSH_SMSG_EXITSTATUS, &client_input_exit_status);
dispatch_set(SSH_SMSG_STDERR_DATA, &client_input_stderr_data);
dispatch_set(SSH_SMSG_STDOUT_DATA, &client_input_stdout_data);
- dispatch_set(SSH_SMSG_X11_OPEN, &x11_input_open);
+
+ dispatch_set(SSH_SMSG_AGENT_OPEN, options.forward_agent ?
+ &auth_input_open_request : NULL);
+ dispatch_set(SSH_SMSG_X11_OPEN, options.forward_x11 ?
+ &x11_input_open : NULL);
}
void
client_init_dispatch_15()
Updated Packages
Conectiva Linux has released updated packages which solve this security problem:
ftp://atualizacoes.conectiva.com.br/6.0/SRPMS/openssh-2.3.0p1-1cl.src.rpm
ftp://atualizacoes.conectiva.com.br/6.0/RPMS/openssh-2.3.0p1-1cl.i386.rpm
ftp://atualizacoes.conectiva.com.br/6.0/RPMS/openssh-askpass-2.3.0p1-1cl.i386.rpm
ftp://atualizacoes.conectiva.com.br/6.0/RPMS/openssh-askpass-gnome-2.3.0p1-1cl.i386.rpm
ftp://atualizacoes.conectiva.com.br/6.0/RPMS/openssh-clients-2.3.0p1-1cl.i386.rpm
ftp://atualizacoes.conectiva.com.br/6.0/RPMS/openssh-server-2.3.0p1-1cl.i386.rpm
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