This page is devoted to the Users and Groups module, which allows you to create and manage Unix user accounts and Unix groups.

Introduction to Unix users and groups

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Password cracking is the art of recovering stored or transmitted passwords. Password strength is determined by the length, complexity, and unpredictability of a password value. Common password techniques include dictionary attacks, brute force, rainbow tables, spidering and cracking.

On Linux and other Unix operating systems, a user is an account who can login to the system via SSH, telnet, FTP or at the console. Users can also receive email and own files on the server's local filesystems. Each user has a login name, a password, and a home directory {text:alphabetical-index-mark} in which all its files are stored. Users also have several additional attributes such as a real name, shell (the program that is run when the user logs in) and expiry date.

Each user is a member of at least one group, called their primary group. In addition, a user can be a member of an unlimited number of secondary groups. Group membership can be used to control which files a user can read and edit. For example, if two users were working on the same project you might put them in the same group, so that they could both edit a particular file which other users could not.

Every system will have several standard user accounts like root and nobody that are created when the system is installed, although most of these (except for root) cannot be used to login. If your server is to be used by more than one person, you will need to create an additional user account for each person so that they can keep their files and email separate. Even if you are the only person who uses your machine, it is a good idea to create a user account for yourself that you use to login with, instead of using the root account.

Depending on your operating system, user and group information will be stored in different files in the /etc directory. On modern versions of Linux, /etc/passwd and /etc/shadow are used to store user details, and /etc/group for group details. The Users and Groups module works by directly editing those files, not by calling any external programs or functions. This means that if you are using NIS or storing users in an LDAP server, this module is not for you.

The Users and Groups module

The Webmin module Users and Groups which is found under the System category can be used to create, edit and delete all the Unix users and groups on your system. You should always be careful when using this module to edit existing system users like root and daemon, as changing or deleting them could stop your system from working. Some users have their home directory set to / (the root directory), so deleting such as user would cause all the files on your system to be deleted!

In addition to managing the Unix user accounts on your system, this module can also effect user settings in other modules. For example, Samba has its own list of users and passwords that should be kept in sync with the Unix password list. Webmin can handle this for you automatically using the other modules option that appears on the user creation, editing and deletion forms. However, you must enable this in each other module that you want automatically updated.

Once you enter the module, the main page lists all the users that currently exist on your system in one table, and all the groups in another, both shown below. If there are too many users or groups to sensibly display in a table, then a small form allowing you to search for a user or group will be displayed instead.

List of existing users

List of existing groups

Creating a new user

To create a new Unix user, complete the following steps :

1. Click on the Create a new user link above or below the table of existing users. A form for entering the details of the new user will appear, as shown in the screenshot below.
2. The User ID field should generally be left unchanged, as it is worked out for you by Webmin. If you set it to the same user ID as some other user, they will be able to access each other's files. This is generally not a good idea.
3. In the Real name field, you should enter the user's full name, like_ Jamie Cameron_.
4. Every user has a home directory, in which he stores his personal documents and preference files. In the Home directory field, you should enter a directory that does not exist yet, like /home/jcameron. When the user is created, this directory will be created and its ownership granted to the new user. If Webmin on your system offers an Automatic option for the home directory, it is generally best to stick with that.
5. The user's shell is a program that is run when he makes a text mode login of some kind (such as via SSH), or opens a shell prompt after logging in graphically at the console. The shell is responsible for running the commands that you type (such as ls and cat), running scripts on login and logout and providing an interface for command editing. Shells like bash and tcsh are easier for users to use, because they allow the up and down arrows to be used to scroll through previous commands, and the tab key to auto-complete commands and filenames. In some cases, you might not want a user to be able to make a shell login at all--for example, if the user is only meant to be able to read and send email. In that case, his shell should be set to /bin/false , which is a program that does nothing and exits immediately. You should select whatever shell you want the user to have from the list in the Shell field, or if your choice is not on the list, select the Other option and enter the path to the shell in the field below.
6. For the Password field, you have four choices : *No password required* The user can login without needing to enter any password No login allowed The user can never login *Normal password* You get to enter the user's password *Pre-encrypted password* You must enter a password that is already encrypted, such as one taken from the /etc/shadow file on another system. Generally you will want to use the Normal password option. Note that on many operating systems, only the first 8 characters of the password are actually used.
7. On most systems, a set of inputs under the heading *Password options* will be available. The first of these is the *Expiry date*--if you want the user to be unable to login after a particular date, fill in this field.
8. The Minimum days field is the number of days since the user was created or the password last changed that the user must wait before changing it again. Leave it blank to allow changing as soon as the user wants.
9. The Maximum days field is the number of days after the user was created or the password last changed that the password expires and must be changed. Effectively, a user with this option set will be forced to change his password periodically, which is good for system security. Leave it blank to prevent the password from ever expiring.
10. The Warning days field is the number of days before the password expiry date that the user will be warned at login that his password is about to expire. If left blank, the user will not know that his account has expired until he tried to login and is forced to choose a new password.
11. The Inactive days is the number of days after the password expires that the entire account will be disabled, if the user has not chosen a new password. If left empty, the account will never expire.
12. For the Primary group, either select an existing group or enter the name of a new one, which Webmin will create for you.
13. If you want the user to be a member of more than one group, select some of the groups from the Secondary group list.
14. If you want the user's home directory to be created, select the Create home directory? option. If the directory does not already exist, you should select this as well as *Copy files to home directory?* so that the user gets a basic set of preference files like .profile and Desktop.
15. To have the user created in other modules that you have configured to do so, select Create user in other modules? It is possible to set up the Samba module to automatically create a user in its user list, and the MySQL module to create a new database user, among others.
16. To create the user, click the Create button. After a short delay, you will be returned to the list of existing users, which should include your newly created user.

The user creation form

Once the Create button has been clicked, the new user will be able to login via SSH, telnet and whatever other services you have set up

Editing an existing user

You can change any of the details of any user that already exists on your system by following these steps :

1. Click on the user you want to edit from the list of existing users. A form containing all the details of the user will appear, as showing in {text:bookmark-ref} .
2. Change any of the details that you want to modify, including the username. The fields have the same meanings as described in Creating a new user above.
3. If you have modified the User ID or changed the Primary group, files owned by the user may need to be updated to use the new IDs. The options at the bottom of the page labeled *Change user ID on files?* and Change group ID on files? control which directories will be searched for files with the old IDs.
4. If you have changed the user's home directory, you can have Webmin rename it to the new path. However, this may not always be what you want if the new home directory already exists. The Move home directory if changed? option determines if is moved or not.
5. To have the user updated in other modules where this has been set up, select Modify user in other modules? This will also rename the user's Sendmail mail file and Cron jobs if you are changing the username.
6. Click the Save button to have Webmin update the user. Once it is complete, you will be returned to the lists of users and groups.

The user editing form

Deleting a user.

You should always be careful when deleting a user, as important files in the user's home directory may be lost. It is generally never a good idea to delete any of the users that are created when your system is first installed - especially root! Even normal users that you have created can be disabled by editing the user and setting the Password option to No login allowed. If you still want to go ahead and delete a user, follow these steps :

1. Click on the user you want to edit from the list of existing users. A form containing all the details of the user will appear, as shown above.
2. Click the Delete button at the bottom of the page. This will bring up a form asking you to confirm the deletion, with buttons to delete just the user or his home directory as well. The amount of disk space used by the user's home directory will be shown.
3. Select the Delete user in other modules? option if you want the user to be deleted from other modules in which deletion has been set up. Any Cron jobs belonging to the user will be deleted, as will his Sendmail mail file.
4. Click either the Delete User or Delete User and Home Directory button to delete the user. A page showing the progress of the deletion will be displayed while it is taking place.

Creating a new group

A new Unix group can be added by following these steps:

1. Click on the Create a new group link at the top or bottom of the list of existing groups. A form for entering the details of the group will appear, as shown below.
2. Choose a name for the new group, and enter it into the *Group name* field. The name must not be used by any other group, and should be short and contain no spaces.
3. The Group ID field should be left alone, as it is automatically determined by Webmin. If you change it, make sure that it is not the same as any existing group's ID.
4. The Password field can be ignored, as group passwords are never used.
5. In the members field, enter the names of any existing users that you want to be members of this group. You can use the button to the left of the field to pop up a window that allows you to select from a list of all existing users.
6. Click the Create button to have Webmin create the new group. Once it is complete, you will be returned to the lists of users and groups.

The group creation form

Once the new group has been created, you can edit users to make it their primary group or one of their secondary groups.

Editing an existing group

You do not often need to edit an existing group, as users can be added to or removed from it by editing them directly. However, if you do want to edit a group, follow these steps:

1. Click on the name of the group that you want to edit from the list of existing groups. This will bring up the group editing form as shown in the image below.
2. Change any of the details, such as the group ID or member list. It is not possible to change the name of an existing group.
3. If you are changing the group ID, files owned by the group may need to be updated to use the new ID. Use the *Change group ID on files?* option to control which directories will be searched for files that need updating.
4. Click on the Save button to make the changes active. Once they are complete, you will be returned to the lists of users and groups.

The group editing form

Deleting a group

You can safely delete a group at any time, but Webmin will only let you do so if there are no users who have it as their primary group. To delete, follow these steps:

1. Click on the name of the group you want to delete from the list of existing groups. This will bring up the group editing form as shown in the screenshot above.
2. Click the Delete button at the bottom of the page. A page asking if you really want to delete the group will appear.
3. Click the Delete Group button to confirm the deletion. A page showing the progress of the deletion will be displayed.

Viewing recent and current logins

All Unix systems keep track of recent logins by users made using SSH, telnet or at the console. Some also track FTP logins as well. You can display recent logins by users that include the date, time and source address by following these steps:

1. Below the lists of users and groups, enter the username of the user you want to track into the Display logins by field, and click the button. Of, if you want to see logins by all users just leave the field blank.
2. A page listing recent logins by the user or users will be displayed. The list may not cover all logins since your system was first installed, as many operating systems automatically truncate the log file periodically in order to save disk space.

It is also possible to display a list of users who are logged in right now by clicking the Logged In Users below the lists of users and groups. If a user is logged in graphically at the console, he may be listed multiple times--once for each shell window he has open.

Reading users' email

When editing a user, you can view mail in the user's mailbox by clicking on the Read Email button at the bottom of the page. This will take you directly to the mailbox viewing page of either the Sendmail or Qmail module, depending on what you have chosen for the Display user email from option in the module configuration. For more documentation on using the mail interface, see chapter 37.

Creating users from batch files

Sometimes you want to create a large number of users at once, without having to go through the process of filling out the user creation form over and over again. Often you will have the details of these users in a text file of some kind, containing their usernames, passwords and real names. Fortunately, Webmin has a feature that automates this task for you.

If you click on the 'Create, modify and delete users from batch file' link above or below the list of existing users, a form will appear that allows you to upload a file containing the details of users to create, as shown in Figure 4-8.Your file must contain one line of text for each user that you want to create, and the format of each line must match the format shown on the batch file page.

The batch file execution form

The exact file format depends on what information your system stores about each user, but on most systems each line must follow the format:

create:_username_:_passwd_:_uid_:_gid_:_realname_:_homedir_:_shell_:_min_:_max_:_warn_:_inactive_:_expire_

An example line to create a user with the user ID automatically assigned by Webmin would be :

create:jcameron:mysecret::3001:Jamie Cameron:/home/jcameron:/bin/bash:::::

As you can see, the line is made up of a series of fields, each separated by a colon : . When creating a user, the first field must be the text create. The meanings of the other fields are:

Once you have created a file containing the details of users to create, select it using either the Upload batch file or Local batch file fields, and click the Execute batch button. A page displaying each user created and any errors encountered will be displayed. The most common error is a missing field in one of the lines - each must have exactly the right number of fields, and even if a field is blank the colon separator next to it must still be included.

Configuring the Users and Groups module

Like other Webmin modules, Users and Groups has several options that can be configured by clicking on the Module Config link above the lists of users and groups, as shown in Figure 4-9. The options that you can safely change and their meanings are:

The other options on the page under the System configuration heading control which files Webmin reads and writes user and group information from. Because they are set automatically based on your operating system type, they should not be changed unless you know what you are doing.

Before and after commands

As the section on 'Configuring the Users and Groups module' explains, you can specify shell commands to be run before and after any action is taken in the module. Because these commands are called for every addition, modification or deletion of a user or group, they need some way of telling exactly what action is being performed. They can do this using environment variables which are set before the command is run. The available environment variables are:

So for example, if you wanted to send out email when a user is created, you could set the Command to run after making changes option to:

Module access control

It is possible to grant a Webmin user or group access to only a subset of features in the Users and Groups module. This is most commonly used to allow a sub-administrator the right to edit only selected users and groups on the system, and to change their attributes in only limited ways. For example, in a virtual hosting environment you may want to give a Webmin user the ability to create and edit up to 10 users with UIDs in a limited range, and home directories under a fixed directory. These privileges give the user no way to gain root access to effect users that do not belong to him:

1. In the Webmin Users module, click on Users and Groups next to the name of the user that you want to edit. This will take you to the access control form covered in WebminUsers.
2. Change the Can edit module configuration? field to No.
3. The Unix users who can be edited field controls which users can be changed by this Webmin user. Typically, you would set it to Users with UIDs in range and enter maximum and minimum UIDs into the fields next to it, such as 5000 and 5010.
4. To allow the addition of new Unix users, set the *Can create new users?* field to Yes.
5. Set the Can view batch file form? option to No. This will prevent the Webmin user from creating and editing users from a batch script, which is not normally necessary. Allowing it does not grant the user any additional privileges and is not a security risk though.
6. For the UIDs for new and modified users fields, enter the same UIDs as in step 4.
7. De-select the More than one user can have the same UID option, but leave UIDs of existing users can be changed selected. An un-trusted sub-administrator should not normally be allowed to create multiple users with the same UID due to the problems that this can cause. When UID clashes are prevented, the Webmin user will not be able to create any more Unix users than fit in his allowed UID range.
8. In the Allowed groups for new or modified users field, you would typically select the Only groups option and enter the names of any groups that new users can be primary or secondary members of. Normally you would just enter a single group like users. Leaving this field set to All groups is a very bad idea, because it would allow the creation of users who are members of the root or bin groups, and who can thus edit important system files and executables. The Groups with GIDs in range option can be useful if this Webmin user is allowed to create multiple groups of his own within the same GID range.
9. To restrict the shells that a new user can be assigned, set the Allowed shells for new or modifed users to Listed and enter their paths into the text box below. This can be useful to allow the creation of only mail-only users who always have the shell /bin/false.
10. Set the Home directories must be under field to a directory that will only be used for accounts created by this Webmin user. Setting it to /home is a bad idea, because this would allow the sub-administrator to rename or delete directories belonging to other users that are under /home. Instead, enter something like /home/_subadmin_. To force every user's home directory to be based on his username (such as /home/subadmin/_username_), check the Home directory is always same as username box.
11. To stop the Webmin user de-selecting some of the options at the bottom of the user creation, editing and deletion forms, de-select the matching Allowed on save options. Any that are not chosen will be effectively always turned on.
12. Assuming you just want the Webmin user to create and edit Unix users, set the U*nix groups who can be edited* field to *No groups*.
13. If you want to restrict the user from viewing recent logins, change the Can display logins by field. Any user who can login with telnet or SSH can run the last command anyway to display logins, so setting this option to No users does not usually make your system any more secure.
14. Finally, click Save. You will be returned to the module's main page and the new access control restrictions will be immediately applied to the Webmin user.

Be careful when granting a Webmin user access to certain Unix users, as a mistake may allow him to edit the root user or create a new user who is equivalent to root. There are also many other users like bin, uucp and httpd that own important system files or are used for running server and daemon processes. Someone who can edit or login as one of these users could gain root privileges on your system or access files that he is not supposed to.

Often the access control in the Disk Quotas and Scheduled Cron Jobs module is set up to allow editing of the quotas and Cron jobs of the same Unix users as those that can be edited and created in this module. All modules support the UID range and primary group access control options, which can be set in the same way.

It is also possible to use the Uses and Groups access control form to allow a user to edit or create selected Unix groups, though this is not generally as useful. Granting an un-trusted user the rights to edit all groups on the system is a bad idea, as he would make himself a member of the root or bin group and so be able to read or write critical files.

Other operating systems

Different operating systems store different information about users than Linux does. This is due to the different files and file formats used for storing user information. Some for example do not have an /etc/shadow file, meaning that information about password change and expiry times does not exist. This section explains the major differences between other supported operating systems and Linux :

• FreeBSD, OpenBSD and NetBSD
  All these operating systems use the /etc/master.passwd file for storing user information, which combines /etc/passwd with some fields from /etc/shadow. When editing or creating a user, you can enter a Password change time which is the date and time after which the password must be next changed, and an Account expiry time after which account can no longer be used. Each user can also have a Login class, which is used in conjunction with the /etc/login.conf file to determine memory, CPU and other limits.
• Sun Solaris and SCO UnixWare
  Both these operating systems use the same files and formats as Linux, and so have all the same options.
• HP/UX, SGI Irix and Compaq Tru64/OSF1
  Because none of these systems use an /etc/shadow file by default, none of the options related to password and account expiration are available when editing or creating a user.
• Apple MacOS X
  OSX does not store user and group information in files at all - instead, it uses a network database called NetInfo which Webmin manipulates using the nidump and niutil commands. However, this database stores the same information as the BSD master.passwd file, so when editing or creating a user the same fields are available as for FreeBSD.
• IBM AIX
  AIX uses the files /etc/passwd and /etc/security/passwd for storing user information, and so when editing or creating users on AIX there are some options that do not existing on other operating systems. The Expiry date field can be used to set the date and time after which the account cannot be used. The *Minimum weeks* and Maximum weeks fields are very similar to the *Maximum days* and Minimum days fields on Linux, but are in weeks instead of days. The Warning days field is has exactly the same meaning as on Linux, and is in days not weeks. The unique Account flags field sets special options, whose meanings are explained on the form.
• SCO OpenServer
  OpenServer uses /etc/passwd and /etc/shadow files, but the shadow file stores slightly different information than on Linux. This means that when editing a user, the *Expiry date* field is replaced with an option to control whether the user is prompted for a password at their next login, and the *Warning days* and Inactive days fields are not available.

Those few operating systems that are not listed above cannot use the Users and Groups module, as their file formats are not currently known to Webmin.

What is Password Cracking?

Password cracking is the process of attempting to gain Unauthorized access to restricted systems using common passwords or algorithms that guess passwords. In other words, it’s an art of obtaining the correct password that gives access to a system protected by an authentication method.

Password cracking employs a number of techniques to achieve its goals. The cracking process can involve either comparing stored passwords against word list or use algorithms to generate passwords that match

In this Tutorial, we will introduce you to the common password cracking techniques and the countermeasures you can implement to protect systems against such attacks.

Topics covered in this tutorial

What is password strength?

Password strength is the measure of a password’s efficiency to resist password cracking attacks. The strength of a password is determined by;

• Length: the number of characters the password contains.
• Complexity: does it use a combination of letters, numbers, and symbol?
• Unpredictability: is it something that can be guessed easily by an attacker?

Let’s now look at a practical example. We will use three passwords namely

1. password

How To Crack Webmin Password Manager

2. password1

3. #password1$

For this example, we will use the password strength indicator of Cpanel when creating passwords. The images below show the password strengths of each of the above-listed passwords.

Note: the password used is password the strength is 1, and it’s very weak.

Note: the password used is password1 the strength is 28, and it’s still weak.

Note: The password used is #password1$ the strength is 60 and it’s strong.

The higher the strength number, better the password.

Let’s suppose that we have to store our above passwords using md5 encryption. We will use an online md5 hash generator to convert our passwords into md5 hashes.

We will now use http://www.md5this.com/ to crack the above hashes. The images below show the password cracking results for the above passwords.

As you can see from the above results, we managed to crack the first and second passwords that had lower strength numbers. We didn’t manage to crack the third password which was longer, complex and unpredictable. It had a higher strength number.

Password cracking techniques

There are a number of techniques that can be used to crack passwords. We will describe the most commonly used ones below;

• Dictionary attack– This method involves the use of a wordlist to compare against user passwords.
• Brute force attack– This method is similar to the dictionary attack. Brute force attacks use algorithms that combine alpha-numeric characters and symbols to come up with passwords for the attack. For example, a password of the value “password” can also be tried as p@$$word using the brute force attack.
• Rainbow table attack– This method uses pre-computed hashes. Let’s assume that we have a database which stores passwords as md5 hashes. We can create another database that has md5 hashes of commonly used passwords. We can then compare the password hash we have against the stored hashes in the database. If a match is found, then we have the password.
• Guess– As the name suggests, this method involves guessing. Passwords such as qwerty, password, admin, etc. are commonly used or set as default passwords. If they have not been changed or if the user is careless when selecting passwords, then they can be easily compromised.
• Spidering– Most organizations use passwords that contain company information. This information can be found on company websites, social media such as facebook, twitter, etc. Spidering gathers information from these sources to come up with word lists. The word list is then used to perform dictionary and brute force attacks.

Spidering sample dictionary attack wordlist

Password cracking tool

These are software programs that are used to crack user passwords. We already looked at a similar tool in the above example on password strengths. The website www.md5this.com uses a rainbow table to crack passwords. We will now look at some of the commonly used tools

John the Ripper

John the Ripper uses the command prompt to crack passwords. This makes it suitable for advanced users who are comfortable working with commands. It uses to wordlist to crack passwords. The program is free, but the word list has to be bought. It has free alternative word lists that you can use. Visit the product website https://www.openwall.com/john/ for more information and how to use it.

Cain & Abel

Cain & Abel runs on windows. It is used to recover passwords for user accounts, recovery of Microsoft Access passwords; networking sniffing, etc. Unlike John the Ripper, Cain & Abel uses a graphic user interface. It is very common among newbies and script kiddies because of its simplicity of use. Visit the product website https://www.softpedia.com/get/Security/Decrypting-Decoding/Cain-and-Abel.shtml for more information and how to use it.

Ophcrack

Ophcrack is a cross-platform Windows password cracker that uses rainbow tables to crack passwords. It runs on Windows, Linux and Mac OS. It also has a module for brute force attacks among other features. Visit the product website https://ophcrack.sourceforge.io/ for more information and how to use it.

Password Cracking Counter Measures

• An organization can use the following methods to reduce the chances of the passwords been cracked
• Avoid short and easily predicable passwords
• Avoid using passwords with predictable patterns such as 11552266.
• Passwords stored in the database must always be encrypted. For md5 encryptions, its better to salt the password hashes before storing them. Salting involves adding some word to the provided password before creating the hash.
• Most registration systems have password strength indicators, organizations must adopt policies that favor high password strength numbers.

Hacking Activity: Hack Now!

In this practical scenario, we are going to crack Windows account with a simple password. Windows uses NTLM hashes to encrypt passwords. We will use the NTLM cracker tool in Cain and Abel to do that.

Cain and Abel cracker can be used to crack passwords using;

How To Crack Webmin Password Key

• Dictionary attack
• Brute force
• Cryptanalysis

We will use the dictionary attack in this example. You will need to download the dictionary attack wordlist here 10k-Most-Common.zip

For this demonstration, we have created an account called Accounts with the password qwerty on Windows 7.

Password cracking steps

• Open Cain and Abel, you will get the following main screen

• Make sure the cracker tab is selected as shown above
• Click on the Add button on the toolbar.

• The following dialog window will appear

• The local user accounts will be displayed as follows. Note the results shown will be of the user accounts on your local machine.

• Right click on the account you want to crack. For this tutorial, we will use Accounts as the user account.

• The following screen will appear

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• Right click on the dictionary section and select Add to list menu as shown above
• Browse to the 10k most common.txt file that you just downloaded

• Click on start button
• If the user used a simple password like qwerty, then you should be able to get the following results.

• Note: the time taken to crack the password depends on the password strength, complexity and processing power of your machine.
• If the password is not cracked using a dictionary attack, you can try brute force or cryptanalysis attacks.

How To Crack Webmin Password On Mac

Summary

How To Crack Webmin Password Recovery Tool

• Password cracking is the art of recovering stored or transmitted passwords.
• Password strength is determined by the length, complexity, and unpredictability of a password value.
• Common password techniques include dictionary attacks, brute force, rainbow tables, spidering and cracking.
• Password cracking tools simplify the process of cracking passwords.