Threat Level: green Handler on Duty: Daniel Wesemann

SANS ISC: When spammers use your own e-mails SANS ISC InfoSec Forums

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When spammers use your own e-mails

Some time ago, one of our readers, Mike S, sent an e-mail with an interesting observation about how spammers used e-mails from one of his customers (this has been actually sitting in my own inbox for way too long).

The e-mails contained all "standard" elements such as spoofed headers etc, but there was a very interesting thing with the body content.

As with most e-mail spammers send, these e-mails were HTML as well. However, the interesting part was that the spammers took his clients' e-mails and modified the HTML a bit to include their own message.

The spammers added the link they wanted to spam at the top and then opened a <TITLE> HTML tag. After the TITLE tag came the full original e-mail, but the tag was never actually closed. This resulted in Outlook displaying only the spammed link, but not showing the original e-mail content.

The raw e-mail looked like this:

Content-Type: text/html; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit

<html><center><FONT SIZE="5" COLOR="#10566D">Spammers message</font><br><br><A HREF="http://spammers link">http://spammers link</A>
<title><body leftmargin=5 topmargin=5 marginwidth=0 marginheight=0>
<table width=100% cellpadding=0 cellspacing=0 bgcolor=white align=center border=0>
<tr><td style='{font-family: Verdana, sans-serif; color=#7a929f;font-weight:700;font-size: 11px;text-transform : capitalize;}'>

Of course, by using the original e-mail content (which was legitimate when the client sent it), the spammers are trying to evade Bayesian filters, and at least in Mike's example they even managed to get SpamAssassin decrease the final score of the e-mail.

In any case, it's an arms race between spammers and content filter developers. Thanks Mike again for sending this interesting information (and sorry it took so long to analyze it).


I will be teaching next: Web App Penetration Testing and Ethical Hacking - SANS London April 2021


396 Posts
ISC Handler
Aug 6th 2008

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