What are file and directory permissions?

    October 8, 2019

    Different options available for setting the permissions of files and directories in different hosting environments.

    Permission basics

    Setting permissions is one of the most basic elements of web security. Assigning the correct permissions to the files and directories helps prevent data theft and malicious intrusions. Permissions specify who and what can read, write, modify, and access content on your site.

    There are two different methods to express permissions in Linux. Permissions may be expressed numerically or alphabetically. At Nexcess, we prefer to numeric permissions because they are easier to read.

    Each file and directory will have three permission categories for all users: owner, group, and other. The owner group identifies the owner of the file or directory. The group identifies entities assigned to the file or directory. The category of other lists all entities who do not fall into the other two categories.

    There are three permission types: read, write, and execute.

    • Read: The capability to read contents. This is expressed as either the number 4 or letter r.
    • Write: The capability to write or modify. This is expressed as either the number 2 or letter w.
    • Execute: The capability to execute. This is expressed as either the number 1 or letter x.

    It is critical to know the permissions of your files and directories. To list the contents of a directory and see the permissions, run this command"

    ls -l

    The output of this command would show a similar output to the following:

    -rw-r--r--  1 user user   418 Oct 20 23:59 index.php

    The columns in a directory listing like the one above are: permissions, number of links, owner, group, size, timestamp, and file or directory name. 

    Permissions

    Number of Links

    Owner

    Group

    Size

    Timestamp (last modified time)

    File or Directory Name

    -rw-r--r--

    1

    user

    user

    418

    Oct 20 23:59

    index.php

    In this example, the directory permissions, -rw-r--r--,  can be divided into the three permission categories.

    Owner

    Group

    Other

    6

    4

    4

    r + w

    r

    r

    4 + 2 + 0

    4 + 0 + 0

    4 + 0 + 0

    The permissions -rw-r--r-- translate into the numeric value 644. The read and write permissions’ numeric value is added to provide both read and write permissions to the owner category. Only the read permission is added to the group and other categories.

    This is a breakdown of the combinations possible and the permissions they apply.

    7 = 4 + 2 + 1 (read/write/execute)
    6 = 4 + 2 (read/write)
    5 = 4 + 1 (read/execute)
    4 = 4 (read)
    3 = 2 + 1 (write/execute)
    2 = 2 (write)
    1 = 1 (execute)

    Restricting permissions

    When setting permissions on your site, only provide the files and directories with as much access as they need. Open permissions like 777 give files and directories the unlimited capacity to modify and execute code, leaving your site vulnerable to attack. Files holding sensitive information should not be openly accessible, otherwise you risk compromising your site’s data and your visitor’s data.  


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