E-mail spoofing

E-mail spoofing is a term used to describe fraudulent email activity in which the sender address and other parts of the email header are altered to appear as though the email originated from a different source. E-mail spoofing is a technique commonly used for spam e-mail and phishing to hide the origin of an e-mail message. By changing certain properties of the e-mail, such as the From, Return-Path and Reply-To fields (which can be found in the message header), ill-intentioned users can make the e-mail appear to be from someone other than the actual sender. It is often associated with website spoofing which mimic an actual, well-known website but are run by another party either with fraudulent intentions or as a means of criticism of the organisation's activities.

Methods
As many spammers now use special software to create random sender addresses, even if the user finds the origin of the e-mail it is unlikely that the e-mail address will be active.

The technique is now used ubiquitously by mass-mailing worms as a means of concealing the origin of the propagation. On infection, worms such as ILOVEYOU, Klez and Sober will often try to perform searches for e-mail addresses within the address book of a mail client, and use those addresses in the From field of e-mails that they send, so that these e-mails appear to have been sent by the third party. For example:

User1 is sent an infected e-mail and then the e-mail is opened, triggering propagation
The worm finds the addresses of User2 and User3 within the address book of User1
From the computer of User1, the worm sends an infected e-mail to User2, but the e-mail appears to have been sent from User3

This can be particularly problematic in a corporate setting, where e-mail is sent to organisations with content filtering gateways in place. These gateways are often configured with default rules that send reply notices for messages that get blocked, so the example is often followed by:

User2 doesn't receive the message, but instead gets a message telling him that a virus sent to them has been blocked. User3 receives a message telling him that a virus sent by them has been blocked. This creates confusion for both User2 and User3, while User1 remains unaware of the actual infection.

Newer variants of these worms have built on this technique by randomising all or part of the e-mail address. A worm can employ various methods to achieve this, including:

• Random letter generation
• Built-in wordlists
• Amalgamating addresses found in address books, for example:

• User1 triggers an e-mail address spoofing worm, and the worm finds the addresses user2@efgh.com, user3@ijkl.com and user4@mnop.com within the users Outlook address book
• The worm sends an infected message to user2@efgh.com, but the e-mail appears to have been sent from user3@mnop.com

Protection against spoofing
There are a couple preventative measures that can be used to protect spoofing from occurring.

Router filtering
Putting a filter on your router is the first preventive step. By using an Access Control List, you can block private IP addresses.

Encryption and authentication
By using encryption and authentication, you can reduce spoofing attacks. Ensuring the right authentication procedures are in place with a secure network will make it much more difficult for an attack to take place.

http://en.wikipedia.org/wiki/E-mail_spoofing