Email Headers are lines of metadata attached to each email that contain lots of useful information for a forensic investigator. However, email headers can be easily forged, so they should never be used as the only source of information.
There is no single way to make sense of email headers. Some examiners favor reading from the bottom up, some favor reading from the top down. Because information in the headers can be put there by the user's MUA, a server in transit, or the recipient's MUA, it can be difficult to determine when a line was added.
Mail User Agents
Main article List of MUA Header Formats
Every MUA sets up the headers for a message slightly differently. The format and order of the entries can vary slightly under the RFC 2822. The examiner can use this to show that messages were forged, but not that they were legitimate. For example, if a message purports to be from Apple Mail but the order or the headers do not match the Apple Mail Header Format, the message has been forged. If the headers of the message do match that format, however, it does not guarantee that the message was sent by that program.
Servers in Transit
Mail servers can add lines onto email headers, usually in the form of "Received" lines, like this:
Received: by servername.recipeienthost.com (Postfix, from userid 506)
id 77C30808A; Sat, 24 Feb 2007 20:43:56 -0500 (EST)
Message Id Field
According to the current guidelines for email RFC 2822), every email should have a Message-ID field:
The "Message-ID:" field provides a unique message identifier that
refers to a particular version of a particular message. The
uniqueness of the message identifier is guaranteed by the host that
generates it (see below). This message identifier is intended to be
machine readable and not necessarily meaningful to humans. A message
identifier pertains to exactly one instantiation of a particular
message; subsequent revisions to the message each receive new message
identifiers.
...
The message identifier (msg-id) itself MUST be a globally unique
identifier for a message. The generator of the message identifier
MUST guarantee that the msg-id is unique. There are several
algorithms that can be used to accomplish this. Since the msg-id has
a similar syntax to angle-addr (identical except that comments and
folding white space are not allowed), a good method is to put the
domain name (or a domain literal IP address) of the host on which the
message identifier was created on the right hand side of the "@", and
put a combination of the current absolute date and time along with
some other currently unique (perhaps sequential) identifier available
on the system (for example, a process id number) on the left hand
side. Using a date on the left hand side and a domain name or domain
literal on the right hand side makes it possible to guarantee
uniqueness since no two hosts use the same domain name or IP address
at the same time. Though other algorithms will work, it is
RECOMMENDED that the right hand side contain some domain identifier
(either of the host itself or otherwise) such that the generator of
the message identifier can guarantee the uniqueness of the left hand
side within the scope of that domain.
Where known, the Message-ID algorithms for known programs are given on the separate pages for those programs.
This is an (incomplete) excerpt from an email header:
Received: from lists.securityfocus.com (lists.securityfocus.com [205.206.231.19])
by outgoing2.securityfocus.com (Postfix) with QMQP
id 7E9971460C9; Mon, 9 Jan 2006 08:01:36 -0700 (MST)
Mailing-List: contact forensics-help@securityfocus.com; run by ezmlm
Precedence: bulk
List-Id: <forensics.list-id.securityfocus.com>
List-Post: <mailto:forensics@securityfocus.com>
List-Help: <mailto:forensics-help@securityfocus.com>
List-Unsubscribe: <mailto:forensics-unsubscribe@securityfocus.com>
List-Subscribe: <mailto:forensics-subscribe@securityfocus.com>
Delivered-To: mailing list forensics@securityfocus.com
Delivered-To: moderator for forensics@securityfocus.com
Received: (qmail 20564 invoked from network); 5 Jan 2006 16:11:57 -0000
From: YJesus <yjesus@security-projects.com>
To: forensics@securityfocus.com
Subject: New Tool : Unhide
User-Agent: KMail/1.9
MIME-Version: 1.0
Content-Disposition: inline
Date: Thu, 5 Jan 2006 16:41:30 +0100
Content-Type: text/plain;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Message-Id: <200601051641.31830.yjesus@security-projects.com>
X-HE-Spam-Level: /
X-HE-Spam-Score: 0.0
X-HE-Virus-Scanned: yes
Status: RO
Content-Length: 586
Lines: 26
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