- djm@cvs.openbsd.org 2008/06/10 22:15:23

     [PROTOCOL ssh.c serverloop.c]
     Add a no-more-sessions@openssh.com global request extension that the
     client sends when it knows that it will never request another session
     (i.e. when session multiplexing is disabled). This allows a server to
     disallow further session requests and terminate the session.
     Why would a non-multiplexing client ever issue additional session
     requests? It could have been attacked with something like SSH'jack:
     http://www.storm.net.nz/projects/7
     feedback & ok markus

1 files changed, 28 insertions, 5 deletions

diff --git a/PROTOCOL b/PROTOCOL
index 4d7a447b0..76e6adb4b 100644
--- a/PROTOCOL
+++ b/PROTOCOL
@@ -61,7 +61,30 @@ remain open after a "eow@openssh.com" has been sent and more data may
 still be sent in the other direction. This message does not consume
 window space and may be sent even if no window space is available.
 
-4. sftp: Reversal of arguments to SSH_FXP_SYMLINK
+4. connection: disallow additional sessions extension
+   "no-more-sessions@openssh.com"
+
+Most SSH connections will only ever request a single session, but a
+attacker may abuse a running ssh client to surreptitiously open
+additional sessions under their control. OpenSSH provides a global
+request "no-more-sessions@openssh.com" to mitigate this attack.
+
+When an OpenSSH client expects that it will never open another session
+(i.e. it has been started with connection multiplexing disabled), it
+will send the following global request:
+
+        byte            SSH_MSG_GLOBAL_REQUEST
+        string          "no-more-sessions@openssh.com"
+        char            want-reply
+
+On receipt of such a message, an OpenSSH server will refuse to open
+future channels of type "session" and instead immediately abort the
+connection.
+
+Note that this is not a general defence against compromised clients
+(that is impossible), but it thwarts a simple attack.
+
+5. sftp: Reversal of arguments to SSH_FXP_SYMLINK
 
 When OpenSSH's sftp-server was implemented, the order of the arguments
 to the SSH_FXP_SYMLINK method was inadvertendly reversed. Unfortunately,
@@ -74,7 +97,7 @@ SSH_FXP_SYMLINK as follows:
         string          targetpath
         string          linkpath
 
-5. sftp: Server extension announcement in SSH_FXP_VERSION
+6. sftp: Server extension announcement in SSH_FXP_VERSION
 
 OpenSSH's sftp-server lists the extensions it supports using the
 standard extension announcement mechanism in the SSH_FXP_VERSION server
@@ -95,7 +118,7 @@ ever changed in an incompatible way. The server MAY advertise the same
 extension with multiple versions (though this is unlikely). Clients MUST
 check the version number before attemping to use the extension.
 
-6. sftp: Extension request "posix-rename@openssh.com"
+7. sftp: Extension request "posix-rename@openssh.com"
 
 This operation provides a rename operation with POSIX semantics, which
 are different to those provided by the standard SSH_FXP_RENAME in
@@ -112,7 +135,7 @@ rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
 This extension is advertised in the SSH_FXP_VERSION hello with version
 "1".
 
-7. sftp: Extension requests "statvfs@openssh.com" and
+8. sftp: Extension requests "statvfs@openssh.com" and
          "fstatvfs@openssh.com"
 
 These requests correspond to the statvfs and fstatvfs POSIX system
@@ -153,5 +176,5 @@ The values of the f_flag bitmask are as follows:
 This extension is advertised in the SSH_FXP_VERSION hello with version
 "2".
 
-$OpenBSD: PROTOCOL,v 1.5 2008/06/09 13:38:46 dtucker Exp $
+$OpenBSD: PROTOCOL,v 1.6 2008/06/10 22:15:23 djm Exp $