[Samba] Claimed Zero Day exploit in Samba.

[Jeremy Allison]

Claimed Zero Day exploit in Samba.

A user named "kcopedarookie" posted what they claim to
be a video of a zero-day exploit in Samba on youtube
yesterday here:

http://www.youtube.com/watch?v=NN50RtZ2N74&aia=true

The video shows modifications to smbclient allowing
/etc/passwd to be downloaded from a remote server.

The issue is actually a default insecure configuration
in Samba.

Quick FAQ: What do I do !
-------------------------

Set:

wide links = no

in the [global] section of your smb.conf and restart
smbd to eliminate this problem.

Longer FAQ: The real issue
--------------------------

The problem comes from a combination of two features in
Samba, each of which on their own are useful to Administrators,
but in combination allow users to access any file on the system
that their logged in username has permissions to read (this is
not a privilege escalation problem).

By default Samba ships with the parameter "wide links = yes",
which allows Administrators to locally (on the server) add
a symbolic link inside an exported share which SMB/CIFS clients
will follow.

As an example, given a share definition:

[tmp]
	path = /tmp
	read only = no
	guest ok = yes

The administrator could add a symlink:

$ ln -s /etc/passwd /tmp/passwd

and SMB/CIFS clients would then see a file called "passwd"
within the [tmp] share that could be read and would allow
clients to read /etc/passwd.

If the "wide links" parameter is set to "no", any attempt
to read this file will fail with an "access denied" error.

The problem occurs as Samba allows clients using the UNIX
extensions (which are also turned on by default) to create
symlinks on remotely mounted shares on which they have write
access that point to any path on the file system.

This is by design, as applications running on UNIX clients
may have good reasons to create symlinks anywhere on the
filesystem they have write access that point to local files
(such as /etc/passwd).

UNIX clients will resolve these links locally, but Windows
clients will resolve them on the server. It is this combination
that causes the problem.

All future versions of Samba will have the parameter
"wide links" set to "no" by default, and the manual
pages will be updated to explain this issue.