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Re: The Dangers of Allowing Users to Post Images
From: "Jeffrey W. Baker" <jwbaker () acm org>
Date: Fri, 22 Jun 2001 14:13:40 -0700 (PDT)



On Tue, 19 Jun 2001, John Percival wrote:

I'm going to try and throw another issue into this discussion now too:
denial of service. We have discussed it for attacking remote servers, but
not for the client viewing the image. It's something else that I spotted
while I was playing around with this issue just now.

If you have images that include a mailto:me () my host somewhere com source,
then the default handler for mailto: links is opened up. Be that Outlook,
Netscape Composer, Eudora, or whatever else you care to use.

So if someone embedded 100 (arbitrary figure) mailto: images in a page, then
this would do a lot of harm to the user's computer. At best, it would get
very busy for a few minutes creating new emails, and would be a pain to
clear up. At worst, it could bring the whole system crashing down.

This is a user agent problem.  Since the mailto scheme can't be used to
fetch an image, script, style sheet, object, or anything else, mailto URIs
should be ignored in the img, object, link, script, and other elements.

I just checked Mozilla and it ignores them.

HTML is loaded with these kinds of hazards.  <img src="file:///dev/zero">,
microsoft's con\con problems, etc.  Careful user agent design is required.

-jwb


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