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Figure 13-11:
Critical
information
revealed in
e-mail
headers.
Testing
Chapter 13: Communication and Messaging Systems
Figure 13-11 shows the header information revealed in a test e-mail I sent to
my free web account. As you can see, it shows off quite a bit of information
about my e-mail system:
✓ The third Received line discloses my system’s hostname, IP address,
server name, and e-mail client software version.
✓ The X-Mailer line displays the Microsoft Outlook version I used to send
this message.
Countermeasures against header disclosures
The best countermeasure to prevent information disclosures in e-mail headers
is to configure your e-mail server or e-mail firewall to rewrite your headers,
by either changing the information shown or removing it. Check your
e-mail server or firewall documentation to see whether this is an option.
If header rewriting is not available (or even allowed by your ISP), you still
might prevent the sending of some critical information, such as server software
version numbers and internal IP addresses.
Capturing traffic
E-mail traffic, including usernames and passwords, can be captured with a
network analyzer or an e-mail packet sniffer and reconstructor.
Mailsnarf is an e-mail packet sniffer and reconstructor that’s part of the dsniff
package (www.monkey.org/~dugsong/dsniff/). There’s a great commercial
(yet low-cost) program called NetResident (www.tamos.com/products/
netresident/), too. You can also use Cain & Abel (www.oxid.it/cain.
html) to highlight e-mail-in-transit weaknesses. I cover password cracking
using this tool and others in Chapter 7.
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