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Re: security hole in mget (in ftp client)
From: jim () CALIFORNIA SANDIA GOV (Jim Hutchins)
Date: Tue, 12 Aug 1997 09:17:04 -0700


der Mouse wrote:

On most Unix platforms, when an ftp client processes an mget command,
it does not check [...for evilness like:]  In particular, a malicious
ftp server's NLST response might include lines such as "../.forward",

Perhaps the easiest solution is to fix the ftp client to ignore lines
in an NLST response that include a '/' character.

I rather dislike this.  It's too useful to "mget */*.??" and the like.

I'd rather see it refuse, or at least confirm, paths beginning with
"../" or including "/../".  One could argue the client should accept a
leading ../ when the user specified a leading ../, but that's probably
getting a little too frilly.  (Of course, this should all be
configurable off, but it also should default on.)

The problem is a bit worse than just including files in the NLST with
a leading '..' or '/'.  If the server sends a list which includes a
filename that starts with the pipe symbol, the UNIX client will happily
start the specified program and execute it, feeding the "data" to the
program as stdin.  How about a file, imbedded in a large directory with
a lot of small files, called "|sh"?  And there are one or two other special
characters to FTP, so it looks like even more filename checking is
necessary.

Jim Hutchins
Sandia National Labs, California



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