The Three Levels of Permissions for Files and Directories Are Read, Write, and Delete.

File Permission

Resistance Strategies

Timothy J. Shimeall , Jonathan K. Spring , in Introduction to Data Security, 2014

File Permissions

File permissions control what user is permitted to perform which deportment on a file. File permissions form a crucial part of a resistance strategy. On public systems, only part of the system is public. The system files, at least, need to be protected from wanton modification by attackers. Furthermore, on internal systems, file permissions assist support the best practices of least privilege and least access, and therefore reduce damage from attacks by insiders.

File permissions are a construct developed on multi-user systems, namely Multics and all the *nix operating systems. Microsoft'southward Windows did not have a concept of file permissions until Windows NT, more than xx years later the Unix method was determined. The two methods for describing file permissions are the traditional Unix method and ACLs. Either method is a class of DAC—users are permitted to change file permissions, at least on files they ain. In the traditional method, files have attributes describing the possessor of the file and the group the file is in, as well every bit permissions for the owner, group, and everyone else.

On a *nix system, every object is treated as a file (including directories and network devices), and and so every object has file permissions. At that place are iii possible permissions, which tin be granted in any combination. These are read (r), write (w), and execute (x) [36]. These tin can exist granted independently to each of three mutually disjoint sets of users: the owner (u), the group (g), and other (o), which means anyone on the system [36]. Additionally, there is a single special-purpose 1-bit flag that can exist on or off chosen the sticky bit, or restricted deletion flag.

The meaning of the permissions changes slightly depending on the type of object. For files, the permissions have their common English meanings. If a user has read permission, he or she can read but not modify the file. A user needs write permissions to alter the file. To run the file as a compiled program, the user needs execute permissions. Nonetheless, with the proliferation of powerful scripting languages similar Python and Perl, to name just a couple, it is important to note that scripts simply demand to exist read to exist run by the interpreter, and so do not need execute permissions themselves. The pasty bit promotes beliefs that helps the file load more apace (to stick in retentivity) [36].

For directories, read, write, and execute have slightly different meanings. Read allows the indicated users to view the names of files in the directory. Write permission is needed to add or remove files from the directory. Notwithstanding, unless the sticky bit is set, properly called restricted deletion flag on directories, a user with write admission to a directory tin can delete any file in that directory, regardless of whether he or she owns it. If the restricted delete flag is set, a user with write access in a directory can but delete files that he or she owns. Execute permissions for a directory permit the user to piece of work from that directory.

*nix systems have a variety of users and groups past default. Many users are human users of the system, but many are besides software agents such equally the web server, DNS server, or the process that controls writing to the network interface. By making these specific processes owners of the files they need, merely no more, file permissions can help resist an attacker who compromises the spider web server process in the same way as an attacker who compromises a user account. The end goal for an attacker is normally superuser access, sometimes called the root user, because that user can read and modify all files on the system, including the ones that maintain file permissions and grouping admission.

The system maintains a file of what users are in what groups (often /etc/group), which allows for a very rudimentary role-based kind of access. There are lots of default groups for various purposes. This is also configurable. For instance, if the administrator puts all the human users who are full-time employees in 1 grouping, then all of them can be given admission to certain resources without worrying near exactly who has been hired recently. A user can then as well share results with colleagues, hands, but without giving the interns access to data that perhaps they should non have.

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File Arrangement

Josh Shaul , Aaron Ingram , in Practical Oracle Security, 2007

Managing Change

Verifying file permissions is an excellent first pace in locking down the file organisation. However, this must exist scheduled forth with checking for new files, removing one-time ones, and verifying that existing ones have not changed. As you might imagine, in an agile database organisation, many files are going to be added, removed, and changed on a regular basis. When creating your lockdown procedures for your file system, include not just a list of files and their permissions, merely too a method that dynamically updates that list and a method to ensure that static components have not been modified. When you notice new files or detect missing ones, verify that this is okay. For case, new datafiles and redo files may appear, or backups may have been copied to record then removed from deejay.

Software and configuration files should change only when authorized, and then this should exist a relatively infrequent occurrence. A cryptographic hash is a large, unique number that acts as a fingerprint for a file. Summate cryptographic hashes of these files during your beginning laissez passer of locking down your database and and so the next time you review your system, you tin can summate them once again to determine if annihilation has changed. Message Assimilate 5 (MD5) and Secure Hashing Algorithm 1 (SHA-one) are common hash algorithms, simply in recent years flaws were found in both. SHA-256 and RI PEMD-160 are better options. Be sure to store your hashes offline in case the host is compromised.

It is important to assess file permissions on a regular ground to ensure they remain at their proper settings. This is especially truthful after an upgrade or even after applying a Critical Patch Update (CPU). These can alter your Oracle installation in any number of means. It is possible for someone (e.g., an operating system administrator or a database administrator) to change permissions to be more than permissive accidentally … or intentionally. In the section of your lockdown programme that deals with security tasks to be performed on a schedule, include these details.

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Securing Linux

Graham Speake , in Eleventh Hour Linux+, 2010

UMASK

umask sets the default file permissions that a file gets when it is first created and uses a list of octal values to signal what rights to remove. A typical umask is 0022, with the two's significant new files will have the write privilege removed for members of group and other. You can view your umask by simply typing umask or change information technology by using umask newmask.

You can likewise use umask with the same letter syntax as chmod, by using the -S parameter which tells the system which bits to fix, equally opposed to which bits not to set for the number representation. To make a change permanent, you can add together the command to your crush startup script, so it gets run every fourth dimension you start a shell.

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Understanding the Terrain

In Host Integrity Monitoring Using Osiris and Samhain, 2005

Windows Access Tokens

Runtime privileges, like file permissions, are more complicated on Windows. Every running process on a Windows system contains an access token. Access tokens provide data about the identity and privileges associated with a user business relationship. Windows runtime security involves many elements such as SIDs, security descriptors (ACLs), security principles, and generic access permissions. All of these are managed using admission tokens.

Upon login or authentication, a primary access token is created. This admission token includes the user's SID, the group SIDs for all groups that the user is a member of, default access control information, impersonation level (explained later in this chapter), and other privilege specifications. After hallmark, any process launched on behalf of the user maintains a copy of this access token. Modifications to user accounts on Windows or whatever of the groups or privileges associated with the user do non take effect until the next time that user authenticates.

Whenever a procedure requests access to the system or attempts to perform a privileged performance, the arrangement consults that process's access token to determine if the operation should or should not exist immune. In the same style that UNIX and Linux systems have effective and existent UID values, Windows processes and threads accept a similar concept with access tokens. In the Windows world, there are primary access tokens and impersonation access tokens. The principal access token is associated with the user who is responsible for the process, or thread, whereas the impersonation access token is a difference from the primary token, though not quite the same in purpose as the effective UID on UNIX.

The chief goal of impersonation access tokens is to allow services to presume a user's privileges when providing admission to a resources; these are usually client/server interactions. When a request is made of a service, the client provides an impersonation level that designates to what degree the service can impersonate the client. The service then assumes the identity of the client for the elapsing of the request by using an impersonation access token.

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Stiff Access Controls

Anton Chuvakin , in PCI Compliance (Third Edition), 2012

POSIX (UNIX/Linux Systems) Access Control

UNIX-based systems such equally Linux used POSIX-style admission control lists. This means files have three permission modes: read (r), write (due west), and execute (x). These modes can exist assigned either using the letters just listed or they also have equivalent numbers. Read is 4, write is 2, and execute is 1. If file permissions are beingness set using letters, it volition be a string of letters or dashes (e.thou. a file with read-only permission would show r– and a file with read, write, and execute would testify rwx). When using numbers, they are added to announce permissions. Read permission would simply be a 4, and read and write permission would be 6 (4 plus two). When using POSIX-style access controls, at that place are 3 groups or users y'all fix permissions for. The first set is for that specific user who owns the file. The second set is for the group who owns the file. The tertiary is for all other users who do not accept whatsoever ownership over the file, similar to the Everybody grouping in Windows. So, a file that allows the owner to read and write, and everyone else only read access would expect like this –rw–r–r– or in numeric format it would exist 644.

Linux has corking command-line tools for irresolute file permissions and file buying. Although exploring all that these commands can practise is beyond the scope of this volume, we will discuss some basics here. In Linux, to list file permissions, the ls command tin be used. The syntax to list the file permission and the group and user who own the file is as follows:

ls–lg [filename]

To change file permissions in Linux, you unremarkably utilise the chmod control. You can run the chmod command using numbers. The post-obit instance uses POSIX permission number format to set a file to allow the user who owns it to read, write, and execute the file, and everyone else to read and execute only non write to information technology, similar to a standard executable file:

chmod 755 filename

Or you could employ letters and specify if you are going to add together them or delete them from users (u), groups (grand), others (o), or all (a). For example, to allow the user who owns the file to read from information technology and write to it, you lot would practise the following:

chmod u = rw filename

To take away permissions employ a—in front of the permissions parameter. To deny read, write, and execute permission to the group that owns the file and to all users other than the one that owns the file, you would do the post-obit:

chmod go-rwx filename

To change the file ownership, use the chown command. To alter the user and grouping that owns a file, practice the following:

chown newuser:newgroup filename

In POSIX-style systems, at that place are iii additional attributes that affect how files are executed are accessed. These are set user ID (SUID), the set group ID (SGID), and sticky. These settings work differently when they're practical to files or directories. The SUID scrap tin be configured to tell the file what user information technology should run under when the file is executed. Many times this is used to permit a nonroot user to run a file as the root user. This is used if a user needs to run a file that requires root access, and yous don't want to give their account root access or the root password. SGID for a file works the same way as SUID, but information technology specifies what grouping the file should execute as. The sticky has no effect on private files. The SUID flake has no effect on directories. If the SGID bit is assail a directory, whatsoever new files created in that directory will be endemic past the grouping specified using the SGID instead of the grouping of the user who created the file. This is sometimes used in directories where many users will share files. When the sticky bit is set on a directory, only the user owner of the file or root can delete or rename a file (the grouping owner cannot). This is sometimes used in shared directories where you don't want users other than the owner or root to delete or rename a file.

In Linux, in that location are also several mandatory access command systems. Most of them are somewhat express to protecting only a subset of files on the system (normally merely critical organisation files). SE Linux is an example of this. SE Linux was developed by the National Security Agency (NSA) and has been incorporated into the 2.vi series Linux kernel. SE Linux uses targets to specify what files it will control and how it will control them. Other mandatory admission control systems that are currently being used in Linux include Suse's AppArmor, Rule Set up Based Access Control (RSBAC).

Linux Enforce Password Complexity Requirements

Almost Linux distributions support countersign complication enforcement using Pluggable Authentication Modules (PAM). This is ordinarily set in /etc/pam.d/system-auth. To comply with PCI requirements, a password must be vii characters long and incorporate capital letter, lowercase, and numeric characters. pam_cracklib has parameters to aid yous meet these requirements. The minlen parameter is used to specify the minimum length of a password. The dcredit parameter is used to requite digits, the ucredit is used to require uppercase letters, and the lcredit parameter is used to crave lowercase letters. The retry parameter is used to specify how many attempts a user gets before the password program exits. Permit's put all these together to show the entry in /etc/pam.d/organisation-auth:

password required /lib/security/pam_cracklib.so minlen=7 dcredit=ane ucredit=i lcredit=1 retry=5

Depending on your implementation, y'all may see different names for the PAM configuration files where this information is placed (e.g. in Debian, you would find this information in the /etc/pam.d/common-password configuration file).

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Access Controls

Lauren Collins , in Cyber Security and Information technology Infrastructure Protection, 2014

Discretionary Admission Control

Discretionary access control (DAC), as well known as file permissions, is the admission control in Unix and Linux systems. Whenever you lot accept seen the syntax drwxr-xs-x, it is the ugo abbreviation for owner, group, and other permissions in the directory list. Ugo is the abridgement for user access, group access, and other system user's access, respectively. These file permissions are set to permit or deny access to members of their own group, or any other groups. Modification of file, directory, and devices are achieved using the chmod command. Tables eleven.1 and 11.2 illustrate the syntax to assign or remove permissions. Permissions can be assigned using the grapheme format:

Tabular array 11.1. Note to Add, Remove Access, and how to Explicitly Assign Access.

+	add access
−	remove access
=	admission explicitly assigned

Table xi.2. Notation for File Permissions.

r	Permission to read file
	Permission to read a directory (also requires 'x')
westward	Permission to delete or modify a file
	Permission to delete or modify files in a directory
10	Permission to execute a file/script
	Permission to read a directory (also requires 'r')
southward	Fix user or grouping ID on execution
u	Permissions granted to the user who owns the file
t	Prepare glutinous fleck. Execute file/script as a user root for regular user

Chmod [ugoa] [+−=] [rwxXst] fileORdirectoryName

In DAC, usually the resource owner will command who access resources. Anybody has administered a system in which they decide to give total rights to anybody so that it is less to manage. The issue with this approach is that users are allowed not just to read, write, and execute files, merely also to delete whatever files they take admission to. This writer has so often seen system files deleted in fault past users, or only by the user'south lack of knowledge. This is an case where DAC could exist seen as a disadvantage, or less advantageous.

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Domain 3: Security Engineering (Engineering and Management of Security)

Eric Conrad , ... Joshua Feldman , in CISSP Written report Guide (Third Edition), 2016

Users and File Permissions

File permissions, such as read, write, and execute, control access to files. The types of permissions available depend on the file organisation being used.

Linux and UNIX permissions

Most Linux and UNIX file systems back up the following file permissions:

•

Read ("r")

•

Write ("due west")

•

Execute ("x")

Each of those permissions may be fix separately to the owner, group, or world. Figure 4.eleven shows the output of a Linux "ls –la /etc" (listing all files in the /etc directory, long output) command.

Figure 4.11. Linux "ls -la" Command

The output in Effigy 4.xi shows permissions, owner, group, size, date, and filename. Permissions beginning with "d" (such every bit "acpi") are directories. Permissions beginning with "-" (such every bit at.deny) describe files. Figure 4.12 zooms in on files in /etc. highlighting the owner, group, and world permissions.

Figure 4.12. Linux /etc Permissions, Highlighting Possessor, Grouping and Earth

The adduser.conf file in Effigy 4.12 is endemic by root and has "-rw-r--r--" permissions. This means adduser.conf is a file (permissions begin with "-"), has read and write (rw-) permissions for the owner (root), read (r--) for the group (also root), and read permissions (r--) for the world.

Microsoft NTFS Permissions

Microsoft NTFS (New Technology File System) has the following basic file permissions:

•

Read

•

Write

•

Read and execute

•

Modify

•

Total control (read, write, execute, modify, and in addition the ability to change the permissions.)

NTFS has more types of permissions than virtually UNIX or Linux file systems. The NTFS file is controlled past the owner, who may grant permissions to other users. Figure 4.13 shows the permissions of a sample photo at C:\Users\Public\Pictures\Sample Pictures\Penguins.jpg.

Figure 4.13. NTFS Permissions

To run into these permissions, correct-click an NTFS file, choose "properties," and and then "security."

Privileged Programs

On UNIX and Linux systems, a regular user cannot edit the password file (/etc/passwd) and shadow file (/etc/shadow), which store account information and encrypted passwords, respectively. But users need to be able to alter their passwords (and thus those files). How can they change their passwords if they cannot (directly) change those files?

The answer is setuid (set user ID) programs. Setuid is a Linux and UNIX file permission that makes an executable run with the permissions of the file'southward owner, and not every bit the running user. Setgid (set group ID) programs run with the permissions of the file'due south group.

Effigy 4.xiv shows the permissions of the Linux control /usr/bin/passwd, used to set and modify passwords. It is setuid root (the file is owned by the root user, and the owner's execute bit is set to "s," for setuid), pregnant it runs with root (super user) permissions, regardless of the running user.

Effigy 4.14. Linux Setuid Root Program /usr/bin/passwd

The "passwd" program runs as root, allowing any user to change their password, and thus the contents of /etc/passwd and /etc/shadow. Setuid programs must be advisedly scrutinized for security holes: attackers may attempt to play a joke on the passwd command to alter other files. The integrity of all setuid and setgid programs on a system should exist closely monitored.

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File Direction Revisited

Philip Bourne , ... Joseph McMullen , in UNIX for OpenVMS Users (3rd Edition), 2003

8.iv.i Modify File Permission: chmod

In OpenVMS, one has file protections, but in UNIX one has file permissions. No affair the name, they serve the aforementioned purpose. The UNIX command chmod, like the OpenVMS Prepare PROTECTION command, changes the permissions assigned to a file or directory. The command chmod provides ii methods for specifying a change in file permission, as shown in the post-obit examples.

OpenVMS	UNIX
Grade:
$ Ready PROTECTION = -	% chmod v file(due south)
( Classification-. LEVEL) file-spec[, . . . ]
Example:
$ SET PROTECT=(O:RWED,G:RE,W:RE) A.DAT	% chmod 755 a.dat
	% chmod +   x a.dat
Instance:
$ Set up PR0TECT=(O:RWED,K,Westward) A.DAT	% chmod chiliad-w, o-w a.dat
	% chmod u=rwx, g   =, o   = a.dat

The starting time example illustrates the accented class for specifying a file's permissions. A level of protection is specified using an octal representation for each of the three types of user—owner, group, and world (in that order): where

user (owner)	group (group)	other (world)
rwx	Rwx	rwx
421	421	421

Hence,

7 = iv + 2 + 1	Read, write, and execute
six = 4 + 2	Read and write
5 = 4 + 1	Read and execute
4 = iv	Read only
three =   2+1	Write and execute
two = 2	Write but
1 =   1	Execute only

Note that specifying the levels of file permission with chmod is inverse to the umask command, where 7 implies no admission, one implies read and write access, but non execute access, so on. The command chmod 755 a.dat changes the permissions of the file a. dat to requite the possessor read, write, and execute access, group members read and execute access, and the globe read and execute access.

The second and 3rd examples introduce the symbolic grade of the chmod command. The command chmod +   x a. dat adds execute access to all types of users, that is, to the user owning the file, group members, and all others. The control chmod g-westward, o-w a.dat removes write access from group members and all others. Note that the symbolic forms of chmod employ   +   (plus) and - (minus) to add and decrease levels of permission, but do non change the permission for classes of users or permission levels not specifically addressed. That is, the 2nd case gives execute access to the owner, group, and all others, but does not change previously established read and write levels of permission. The equals sign (=) assigns permissions absolutely. The last case, chmod u=rwx a.dat, illustrates absolute permission assignment by giving the user owning the file read, write, and execute access to a. dat and removing all access from the group and all others.

You should also note from the table a critical difference in nomenclature between OpenVMS and UNIX. OpenVMS deals with a file's owner, the owners group, and the residual of the globe (ignoring for the moment the arrangement protection). UNIX deals with the file user, the user's grouping, and all others. It's unfortunate that the OpenVMS abbreviation for owner, o, is the same as the UNIX abbreviation for all others. Be careful how you abbreviate when using the symbolic form of chmod!

For all uses of the chmod command, you must own the file for which y'all request a change in permission. Just the superuser may change the protection of files owned by other users.

eight.4.1.one Change Group Ownership: chgrp

The control chgrp changes the group buying of a file. Information technology corresponds to irresolute the ACE for an OpenVMS file, except that an OpenVMS file can have multiple ACEs, whereas a UNIX file can belong to merely one group. To change group ownership, the UNIX user requesting the alter must be the possessor of the file and must be a member of the group beingness assigned to the file.

OpenVMS

Grade:

$ Bear witness SECURITY [ / QUALIFIER (S) ] file-spec

Example:

$ Testify SECURITY FILE1

DUA3:[PARNIGONI]FILE1.;1 object of class FILE

Possessor: [PROJECTA, PARNIGONI]

Protection: (System: RWED, Owner: RWED, Group: RE, Globe)

Access Command List:

(IDENTIFIER=PROJECTA, Admission=READ+WRITE+DELETE+Control)

Form:

$ Gear up SECURITY [ / QUALIFIER (S) ] object-proper noun

Instance:

$ Gear up SECURITY /ACL=(IDENTIFIER=[ADMIN], - Admission=READ+WRITE+DELETE+CONTROL) DUA3:[PARNIGONI]FILE1

UNIX

Grade:

% groups [username]

% chgrp [-f] group file(s)

Example:

% groups; ls- 1   m file1; chgrp hmwrk file1; ls -1   g file1 hmwrk projecta

-rw-rw-r-- two melvin projecta 15 Feb 25 13:58 file1

-rw-rw-r-- 2 melvin hmwrk 15 February 25 thirteen:58 file1

In the above example, the groups control indicates that the user (in this case, melvin) belongs to the groups hmwrk and projecta. The control ls -1   g file1 indicates that user melvin owns file1 and is therefore entitled to change the group from projecta to hmwrk. The command line chgrp hmwrk file1 changes the grouping ownership of the file file1 from projecta to hmwrk, which is verified past again issuing the command ls -ane   g file1.

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