Page 880 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

the security vulnerabilities of email. Digital signatures can help

eliminate impersonation. The encryption of messages reduces
eavesdropping. And the use of email filters keep spamming and mail-
bombing to a minimum.

Blocking attachments at the email gateway system on your network
can ease the threats from malicious attachments. You can have a 100
percent no-attachments policy or block only attachments that are
known or suspected to be malicious, such as attachments with

extensions that are used for executable and scripting files. If
attachments are an essential part of your email communications, you’ll
need to train your users and use antivirus tools for protection.
Training users to avoid contact with suspicious or unexpected
attachments greatly reduces the risk of malicious code transference via
email. Antivirus software is generally effective against known viruses,
but it offers little protection against new or unknown viruses.

Unwanted emails can be a hassle, a security risk, and a drain on
resources. Whether spam, malicious email, or just bulk advertising,
there are several ways to reduce the impact on your infrastructure.
Blacklist services offer a subscription system to a list of known email
abuse sources. You can integrate the blacklist into your email server so
that any message originating from a known abusive domain or IP
address is automatically discarded. Another option is to use a

challenge/response filter. In these services, when an email is received
from a new/unknown origin address, an autoresponder sends a
request for a confirmation message. Spammers and auto-emailers will
not respond to these requests, but valid humans will. Once they have
confirmed that they are human and agree not to spam the destination
address, their source address is added to a whitelist for future

communications.

Unwanted email can also be managed through the use of email
repudiation filtering. Several services maintain a grading system of
email services in order to determine which are used for
standard/normal communications and which are used for spam.
These services include senderscore.org, senderbase.org,
ReputationAuthority.org, trustedsource.org, and Barracuda Central.

These and other mechanisms are used as part of several spam filtering