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Date: Thu, 30 Dec 2010 13:48:23 -0600
From: Earl Hood <>
To: Jeff Breidenbach <>
Cc: oss-security <>, 
	"Steven M. Christey" <>, non customers <>,
Subject: Re: CVE Request -- MHonArc: Improper escaping of certain HTML
 sequences (XSS)

On Thu, Dec 30, 2010 at 1:01 PM, Jeff Breidenbach <> wrote:
> Earl,
> One of my hats is the Debian package maintainer for mhonarc. I'm tempted to
> disable HTML mail support by default rather than try to improve it. What do
> you think about the idea? What do you think about implementation?

Personally, I would like HTML disabled by default, but if I do, I,
and/or the user's list, will get burdened by messages of why
HTML email does not render correctly.

Many mhonarc users are not tech savvy, and I do not have the time and
resources to deal with the potential flood of emails.

A nice thing to have would be a whitelist-based filter, but such
a filter would depend on a robust HTML parser, and I'm not sure
one really exists for Perl.  Because of how different browsers allow
for different craziness to happen in HTML data, it is a non-trivial
task to generate a robust parser.  Because of this, it is still
likely someone could still bypass such a filter by exploiting
a weakness in the HTML parser.

I think the double pass of the current filter may be the best short-term
solution now, but the DoS aspect is a concern.  There are some degenerate
cases in the Perl regex engine (at least in the past) that I had to
work around with the current filter, and it appears there may still
be other degenerate cases.  The cases also varied depending on
the version of Perl being used.

Thinking about it a bit, the example provided in the original post
is definitely invalid HTML, and normal email clients would never create
such a thing.  Therefore, would it be sufficient to strip-out, or reject,
data that clear has invalid tags like:


For example, the sequence of "<scr<" is invalid.  It is simple
to provide a pre-check for such occurrences, and if it exists,
"reject" the data.  For example, the following regex, if true,
indicates bad HTML:


If a '<' occurs before a '>' after an initial '<', something is
not right.  The filter would return nothing, signally mhonarc
to use the next alternative part (if provided), or display no
content for the message.


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