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Recently, there have been reports on security problems induced by
the interpretation of shell meta-characters embedded in MIME
parameters.  These reports were referring to Pine, but the problem
also applied when using mutt.

More precisely, a mailcap entry like this one would lead to
problems:

> text/test-mailcap-bug; cat %s; copiousoutput; \
>	test=test "`echo %{charset} | tr '[A-Z]' '[a-z]'`" != iso-8859-1

When expanded with a charset parameter of ``touch${IFS}ME``, a file
named "ME" would be created in the current directory.

While we don't completely agree that this is an actual MUA problem
(see below), we have implemented a couple of fixes for this:

- Backticks are handled specially when preparing % expandos for
  mailcap entries.  This fix will keep the current problem from
  occuring, but we are sure there are other possible mailcap entries
  where this doesn't help.

- We have added a configuration variable named $mailcap_sanitize,
  which is set by default.  If set, mutt will restrict possible
  characters in mailcap % expandos to a well-defined set of safe
  characters.  This is the safe setting, but we are not sure it
  doesn't break some more advanced MIME stuff.

>>>  DON'T UNSET THIS OPTION UNLESS YOU KNOW WHAT YOU ARE DOING.


Anyway, this problem is not necessarily a problem which should be
solved inside the MUA, as it's difficult (maybe impossible) to solve
there.  Additionally, there is more than one program which parses
mailcap.  So writing secure mailcap statements is generally a good
idea.  We encourage you to do this.

The most basic rule is this one:

>>>	     KEEP THE %-EXPANDOS AWAY FROM SHELL QUOTING.

Don't quote them with single or double quotes.  Mutt does this for
you, the right way, as should any other program which interprets
mailcap.  Don't put them into backtick expansions - as you have seen
above, this is a recipe for disaster.  Be highly careful with eval
statements, and avoid them if possible at all.

If you have to use the %-expandos' values in context where you need
quoting or backtick expansions, put that value into a shell variable
and reference the shell variable where necessary (possibly with the
proper quoting put around it, like in "$charset").

For example, a safe version of the mailcap statement above could
look like this:

> text/test-mailcap-bug; cat %s; copiousoutput; test=charset=%{charset} \
>         && test "`echo \"$charset\" | tr '[A-Z]' '[a-z]'`" != iso-8859-1