[Full-Disclosure] CSS in E-Mails possible E-Mail-Validity Check for Spammers?
To: email@example.com Date: Wed, 03 Nov 2004 02:28:30 +0100
This might be a minor problem in times of e-mail-collecting viruses and
massive hijacking of SOHO-PCs. Still I wonder what you think
Mozilla Mail 1.7.1 (W98) and 1.7.3 (W98) (didn't check different
versions) automatically load CSS-files which are linked from within an
html-page sent in an e-mail, even though plug-ins and loading of images
in e-mails are turned off. Of course, this only happenes, when you click
the mail and when HTML-Mails are enabled. Mozilla tries to display the
page and loads the CSS.
I think you all know, how this enables spammers to use HTTP-requests for
CSS-files to check the validity of e-mails-addresses: Instead of
embedding an image with an identification code assigned to the
receipients e-mail-address in the address or as a parameter to the
request, they can now embed an external style sheet definition in
HTML-code with the same "functionality". Analyzing the requests on the
server will show the codes corresponding to valid e-mail-addresses.
I used the "send page"-function of the Mozilla browser to to send a page
to my own e-mail-account. When I click the e-mail, ethereal shows the
HTTP-GET www.myserver.com/css/standard.css .
How dangerous is this? What about possible CSS-exploits?
Workaround suggestions ;-)
- Cut your internet connection before reading any suspicious e-mails,
you can probably live without the CSS.
- turn off HTML in E-Mails (not possible in Mozilla?)
Full-Disclosure - We believe in it.