From 101c4a7bc96556d22ccf4c2095086353e4e61ca2 Mon Sep 17 00:00:00 2001
From: Damien Miller <djm@mindrot.org>
Date: Thu, 19 Sep 2002 11:51:21 +1000
Subject:    - stevesk@cvs.openbsd.org 2002/09/16 20:12:11      [sshd_config.5]
      more details on X11Forwarding security issues and threats; ok markus@

---
 ChangeLog     |  5 ++++-
 sshd_config.5 | 31 ++++++++++++++++++++++++++++---
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index aaadccdb..63bfc9f5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,9 @@
    - stevesk@cvs.openbsd.org 2002/09/16 19:55:33
      [session.c]
      log when _PATH_NOLOGIN exists; ok markus@
+   - stevesk@cvs.openbsd.org 2002/09/16 20:12:11
+     [sshd_config.5]
+     more details on X11Forwarding security issues and threats; ok markus@
 
 20020912
  - (djm) Made GNOME askpass programs return non-zero if cancel button is 
@@ -663,4 +666,4 @@
      save auth method before monitor_reset_key_state(); bugzilla bug #284;
      ok provos@
 
-$Id: ChangeLog,v 1.2467 2002/09/19 01:50:48 djm Exp $
+$Id: ChangeLog,v 1.2468 2002/09/19 01:51:21 djm Exp $
diff --git a/sshd_config.5 b/sshd_config.5
index 8d90785f..0944ba07 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -34,7 +34,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: sshd_config.5,v 1.12 2002/09/04 18:52:42 stevesk Exp $
+.\" $OpenBSD: sshd_config.5,v 1.13 2002/09/16 20:12:11 stevesk Exp $
 .Dd September 25, 1999
 .Dt SSHD_CONFIG 5
 .Os
@@ -630,10 +630,35 @@ from interfering with real X11 servers.
 The default is 10.
 .It Cm X11Forwarding
 Specifies whether X11 forwarding is permitted.
+The argument must be
+.Dq yes
+or
+.Dq no .
 The default is
 .Dq no .
-Note that disabling X11 forwarding does not improve security in any
-way, as users can always install their own forwarders.
+.Pp
+When X11 forwarding is enabled, there may be additional exposure to
+the server and to client displays if the
+.Nm sshd
+proxy display is configured to listen on the wildcard address (see
+.Cm X11UseLocalhost
+below), however this is not the default.
+Additionally, the authentication spoofing and authentication data
+verification and substitution occur on the client side.
+The security risk of using X11 forwarding is that the client's X11
+display server may be exposed to attack when the ssh client requests
+forwarding (see the warnings for
+.Cm ForwardX11
+in
+.Xr ssh_config 5 ).
+A system administrator may have a stance in which they want to
+protect clients that may expose themselves to attack by unwittingly
+requesting X11 forwarding, which can warrant a
+.Dq no
+setting.
+.Pp
+Note that disabling X11 forwarding does not prevent users from
+forwarding X11 traffic, as users can always install their own forwarders.
 X11 forwarding is automatically disabled if
 .Cm UseLogin
 is enabled.
-- 
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