AUTH-9328 - Default umask

(Authentication)

This information is provided as part of the Lynis community project. It is related to Lynis control AUTH-9328 and should be considered as-is and without guarantees. Any advice and commands should be tested before implementing them in production environments.

Description

The umask defines what default file permissions will be applied on a file or directory. Usually servers can have a more strict umask like 027, where desktops may be less strict (022).

How to solve

Files and directories are created with a default set of permissions. These depend on the parent directory and the umask value. This umask value contains three or four numbers and gives the system a hint on how to create new objects on the file system. For example using the umask 027 will be translated into 750 for directories or 640 for files. This means that the owner can read and write, with additional execution rights for directories. This latter part means that you are allowed to traverse the directory. The group will get read permissions for files, and again execution rights for directories. The other won't get any access to the file.

Using the right umask helps with limiting who can access created files. This is especially important for systems with multiple users. This is also the case when loose file permissions can result in unauthorized information disclosure, like a web server.

Additional resources

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Lynis is a technical security auditing tool for Unix flavors like Linux, macOS, AIX, Solaris, and *BSD. It is open source software and free to use. Typical usage include system hardening, compliance testing, and vulnerability scanning. The project has an active community, including development via GitHub.

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