[Samba] Wide links and insecure wide links

Jeremy Allison jra at samba.org
Wed Feb 28 20:41:45 UTC 2018

On Wed, Feb 28, 2018 at 08:36:14PM +0000, Stilez wrote:
> Thank you.
> So, If I understand correctly, "ordinary" "wide links = yes", means Samba
> *will* traverse an existing symlink that points outside the root of the
> share, if permissions allow. However because it *also* disables SMB1 Unix
> extensions, it *also* prevents the user from creating or modifying symlinks
> on the share, so in wffect it inherently prevents this being exploited
> unless an insecure symlink already exists or is created by some *other*
> route. And thus, that enabling "insecure" wide links simply  removes that
> safeguard.


> If that's right, my clarification questions are
> 1) does this mean that configs containing "ordinary" wide link = yes might
> become a risk, when SMB2 style functionality eventually lands, or will it
> presumably be mitigated or remain unchanged from a security perspective at
> that time, as far as is known?

My plan for SMB2 unix extensions is to prevent a client from
creating *any* server-followed symlinks. So wide links = yes
will remain the same as it is now, in that it will allow existing
links to be followed outside the share path, but no new ones
created. If you have existing wide links that point to /etc/passwd
etc. then you're already unsafe. If you don't, then you're

> 2) when you say in your reply, that "ordinary" wide links enabled  "means
> the server will follow symlinks **on the file system*" that point outside
> the root of the share definition", do you in fact mean that it is also
> barred from crossing a device boundary onto another device (similar to
> "ls|rm|find -x", using stat to determine same file system), or something
> else (in which case what?)

No, it's nothing to do with underlying filesystem boundaries.
What it means is that for a share definition of:

path = /foo/bar

that any link pointing to a file object that does *not* start
with /foo/bar is disallowed by the server if "wide links = no",
(e.g. /tmp or /foo/bibble) and all link paths are allowed if
"wide links = yes".

> Thanks for the help!
> Last thing - how can this helpful info added to smb.conf doc/man pages where
> it might help others?

Add to the Samba wiki ?

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