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409: 名無しさん＠お腹いっぱい。 [sage] 2015/03/10(火) 12:10:48 ID:W0fAy0Xd0

Similar to the last example, permitting any type of outbound VPN session establishment can lead to data leaks. Whi
le I will focus on Secure Shell (SSH) in this example, this problem is just as applicable to permitting outbound S
SL or IPSec transmissions. All of these VPN solutions can typically be tunn
eled through any TCP po　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　rt. This can lead to additional access being provided thro
ugh a network perimeter without the knowledge of the local IT group.

SSH is a multi-platform VPN solution. While it is typically used as a secure replac
ement for clear text tools such as Telnet and FTP, for many years it has also had the abilit

y to tunnel any TCP base　　　　　　　　　　　d application. As of the beginning of 2006, support for tunneling UDP
, ICMP as well as other IP transports was added in as well.
nfigured to create a forward tunnel to the proxy server. Once they logon via SSH, its now just a simply matter of confi
guring the browser　　　　　　　　　　 to use a proxy server located at the loopback address. When the user browses the Web, the connection req
uests are sent through the SSH session to the HTTP proxy located on the Internet. As content passes the corporate perimeter, it is e
ncrypted as part of the SSH session. While you can attempt to thwart this activity by blocking outbound access to SSH's well known
port (TCP/22), the user can easily configure SSH to run over any TCP port. Again, TCP/443 is usually a good choice, as this port is usually not scrutinized.

SSH is not an evil tool p　　　　　　　　　　　　er se'. In the hands of a system or security administrator it can be an invaluable tool t
hat helps to augment security as well as simplify many daily tasks. The problem 　　　　　with SSH is that in the hands o
f a malicious user it can easily be used for breech corporate policy. This can include circumventing content checking as w
ell as exposing intern　　　　al services to outside atta　　　　　　　ck. The problems revolve around SSH's ability to tunnel other I
P applications. These can be forward tunnels (used to forward application informati　　　on up to the server) or rever
such a way that 　　　　　　　　　it will go undetected. Figure 3 shows a possible use for the forward tunnel capability of SSH which woul
d permit this user to circumvent your content checks.To start　　　　, the use　r needs access to an external system running both an SSH server as
well as an HTTP proxy server. Both of these services can easily be depl　　　　　　　　top with it co
nfigured to create a forward tunnel to the proxy server. Once they logon via SSH, its now just a simply matter of confi
guring the browser　　　　　　　　　　 to use a proxy server located at the loopback address. When the user browses the Web, the connection req
uests are sent through the SSH session to the HT　TP proxy located on the Internet. As content passes the corporate perimeter, it is e
ncrypted as part of the SSH session. While you can attempt to thwart this activity by blocking outbound access to SSH's well known
port (TCP/22), the user can easily configure SSH to run over any TCP port. Ag　　　ain, TCP/443 is 　
SSH's reverse tunnel capa　　　　　　　　　　　bility can　　　　　 be even more dangerous. This is shown in Figure 4. In this example when the user runs the SS
H client on the corporate desktop they request a reverse tunnel and specify which port the SSH server should open up. Any connecti
on requests sent to the S　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　SH on that port will be forwarded to the corporate desktop. The user then tells the SSH cl
ient which internal system should receive these data requests. and specify which port the SSH server should open up. Any connecti
on requests sent to the S　　　　　　　　　　　　　　　　　　 and specify

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