Re: access(2)--a security hole?

[karl () bagpuss demon co uk (Karl Strickland)]

> In your message of Thu, 20 Oct 1994 21:41:48 BST, you say:
>
> > the FreeBSD man page for access(2) includes a section titled "CAVEAT" 
> > which says that "Access() is a potential security hole and should never 
> > be used."
>
> hmmm..... access(2) uses the REAL uid, not the EFFECTIVE uid when
> testing permissions. The idea is that, when you write a setuid program,
> you can check to see if the user has permission to access a file while
> you have euid==root. Eg.
>
> real uid = user
> eff uid = root
>
> The problem I found with using access is that it's actually easier
> just to flip real and effective uids, test writability, open the file, etc.
> then flip the uids back. If you use normal code, you only have to
> worry about the effective uid; however, if you use access(), you have
> to worry about both real and eff uid.
>
> Actually, that's not really a security hole, just additional complexity.
> Maybe access() on FreeBSD is a different matter. At least, the access
> manpages on sunos and solaris don't mention any holes...

The problem its referring to is a race condition; and its due to the way
access(2) is normally used, rather than it being a hole in access(2) itself.
As you say, access(2) is often used by a privileged process to check whether
the current real-uid has access to a given file.  If access(2) says the user
can access the file, it is then manipulated some time later by *privileged*
code.  There is no guarantee that the file manipulated by the privileged code
is the same file that access(2) verified access to -- the file could have been
changed in between, using links or whatever.  Seems that to be secure, we'd
need a faccess(2) type of call, although I realise that wouldnt be useful
or useable in all situations.

------------------------------------------+-----------------------------------
Mailed using ELM on FreeBSD               |                    Karl Strickland
PGP 2.3a Public Key Available.            | Internet: karl () bagpuss demon co uk
                                          |