How phishers forge email headers

By Lance James
23 Mar 2006 | SearchExchange.com

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You are reading tip #4 from "10 tips in 10 minutes: Phishing exposed," excerpted from Chapter 3 of the book Phishing Exposed, published by Syngress Publishing.

Forging headers is trivial, but the more appropriate question is, how is it possible? The MTA that we contact via Telnet can demonstrate how easy it is to forge headers. We will be adding Header-1: xxx and Header-2: yyy, which do not indicate anything special but make a great example:

$ telnet mail.sendingemail.com 25
Trying 127.0.0.1...
Connected to mail.sendingemail.com.
Escape character is '^]'.
220 mail.sendingemail.com ESMTP Postfix
HELO hostname
250 mail.sendingemail.com Hello sender.sendingemail.com 
[xx.7.239.24], pleased to meet you 
MAIL FROM: madeup@spoofedemail.com
250 Ok
RCPT TO: me@sendingemail.com
250 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
Header-1: xxx
Header-2: yyy

Message body.
.
250 Ok: queued as 73F50EDD2B
QUIT
221 Bye

Now we check our email and find the following email content and header information:

Return-Path: <madeup@spoofedemail.com>
X-Original-To: me@sendingemail.com
Delivered-To: me@sendingemail.com
Received: by mail.sendingemail.com (Postfix, from userid 1999)
id D3750EDD2B; Tue,  5 Apr 2005 21:33:55 -0700 (PDT)
Received: from hostname (xx.7.239.24)
by mail.sendingemail.com (Postfix) with SMTP id 73F50EDD2B
for ; Tue,  5 Apr 2005 21:33:37 -0700 (PDT)
Header-1: xxx
Header-2: yyy
Message-Id: <20050406023337.73F50EDD2B@mail.sendingemail.com>
Date: Tue,  5 Apr 2005 21:33:37 -0700 (PDT)
From: madeup@spoofedemail.com
To: me@sendingemail.com
X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on 
mail.sendingemail.com
X-Spam-Status: No, hits=2.3 required=5.0 tests=BAYES_90,NO_REAL_NAME
autolearn=no version=2.63

Message body.

We can see that our email has come in from madeup@spoofedemail.com and was delivered. Our added headers made it into the email, and those could easily be replaced by fake Received headers, X-headers, and any other content someone wanted to place in there. The flexibility of SMTP struts its stuff when it comes to what can go into an email. At this stage it is up to the email clients to judge whether the email is valid or not.

10 tips in 10 minutes: Phishing exposed

Home: Introduction
Tip 1: Email basics for Exchange admins
Tip 2: Understanding email delivery
Tip 3: Anonymous phishing email
Tip 4: How phishers forge email headers
Tip 5: Phishers use of open relays and proxy servers
Tip 6: How phishers send anonymous email
Tip 7: Phishers techniques for email harvesting
Tip 8: Phishers, hackers and insiders
Tip 9: Sending spam; phishing tools of the trade
Tip 10: Phishing email and spam filters

This chapter excerpt from Phishing Exposed, Lance James, is printed with permission from Syngress Publishing, Copyright 2005. Click here for the chapter download.

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