Monthly Archives: April 2016

How a spamfilter can help you to drop a shell

A while ago i discovered a cross-site scripting vulnerability (XSS) in the McAfee E-mail Gateway (MEG) 7.6.4. I reported this vulnerability to McAfee, they fixed it within a few months. The security advisory can be found over here. MEG is an application that can be used to filter out malicious attachments from e-mails, however due to the vulnerability an attacker is able to abuse this functionality to drop a malicious file.

The McAfee E-mail Gateway is replacing a malicious file with a warning HTML-file (1_warning.html). I saw that this HTML-file is displaying the filename of the replaced malicious file (e.g. malicious.xlsx). This made me curious, I decided to check whether it was possible to use this filename to perform a cross-site scripting attack because it was used within HTML-context.

In order to check whether the XSS was present, i created a malicious Excel document. The file needs to be malicious in order to be replaced by the McAfee E-mail Gateway. The file was named “file<IMG SRC=x onerror=”alert(‘XSS’)”>jem.xls“. I e-mailed this file to a e-mailbox that was protected by McAfee E-mail Gateway. When opening the warning HTML-file, the following behavior became clear:

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