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Next Oct 7 Previous

666: the number of the beast

When you use tempnam() to create a file with a unique filename the documentation states that the file permissions will be set to 0600. What does this mean?

A: Only the current script can write to the file
B: All users of the group to which the Web server system user (typically www-data) belongs can read and write to the file.
C: Only actions on behalf of the Web server system user can read and manipulate the file.
D: 0600 is the highest file permission for read/writable files: everybody/-thing on the system can read or write to the file.

Answer

I suppose this is a no-brainer for those working with UNIX (like) systems. In those systems permissions are set in some bit-like fashion. Permissions are set on the user, group and other level and on each level you can mark a file readable, writable and executable. Those are 9 (3*3) possibilities and these can be represented as a 9 bit number or more commonly by 3 octal numbers (an octal number takes 3 bits when it is converted to a binary number).

In each of the three octal numbers the bit at position 3 (most significant or most left) stands for readable, the one at position 2 stands for writable and the one at position 1 stands for executable.

In an octal number group the octal number at position 3 (most significant or most left) stands for user, the one at position 2 stands for group and the one at position 1 stands for other.

In the number 0600 the first is the octal indicator (in many computer languages preceding a number with a will make it an octal number). So we' ll split it up as follows

6: binary 110 → user can read, can write and can't execute. : binary 000 → group users can' t read, can't write and can't execute. : binary 000 → others can' t read, can't write and can't execute.

So the created file can only be read or written to by the Web server process, therefore answer C is correct.

Now this is on UNIX like systems, but what if you're running Windows? Honestly I don' t know. I suppose something equivalent, but watch out for IIS 7 and up. If you run that thing in integrated Windows authentication mode you're in for some surprises.