chmod 666: readable and writable by every user

What chmod 666 means

In 666 the first digit is the owner, the second the group and the third everyone else: the owner gets read and write, the group gets read and write, and others get read and write.

ls -l shows -rw-rw-rw-. Each 6 is 4 + 2, read plus write. There is no execute anywhere, which is why 666 is the starting point the kernel uses for new regular files before the umask is applied: under umask 022 a new file becomes 644, under 002 it becomes 664.

Where you actually see 666

Device nodes such as /dev/null, /dev/zero and /dev/tty are crw-rw-rw-, mode 666, because every process must be able to write to them. That is legitimate for a device; for a regular file it means any local account can change or truncate its contents.

You also meet it when a program creates a file with an explicit 0666 and the process umask is 0, common in some containers. If files in a volume appear as 666, check the umask of the process that wrote them before reaching for chmod.

Risks and better choices

A world-writable log, config or data file lets any local user plant content another program will trust. Prefer 644 when only the owner writes, 664 when a group shares writing, and 600 when the content is private.

Note that deleting or renaming a file is controlled by its directory's write permission, not by the file's mode, so 666 does not by itself let others delete the file. It does let them empty it, which is usually just as bad.

Related values

666 is the no-execute counterpart of 777, and shares its main flaw. 664 removes write from others, which is the right choice for files a group edits together. 644 removes group write as well, leaving the owner as the only writer. 600 removes everyone but the owner, and 444 removes write from all three classes when the goal is a read-only file.