From 9e7c81e6dc85b6abe46436359a06b5056c16c743 Mon Sep 17 00:00:00 2001 From: Thomas Roessler Date: Thu, 11 Feb 1999 12:38:59 +0000 Subject: [PATCH] Add a note on how to write mailcaps. --- README.SECURITY | 65 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) create mode 100644 README.SECURITY diff --git a/README.SECURITY b/README.SECURITY new file mode 100644 index 00000000..c2446804 --- /dev/null +++ b/README.SECURITY @@ -0,0 +1,65 @@ +$Id$ + +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 $mailcap_sanitize, + which is set by default. If set, mutt will restrict possible + characters in mailcap % expandos to a well-defined set of safe + characters. This is the safe setting, but we are not sure it + doesn't break some more advanced MIME stuff. + +>>> DON'T UNSET THIS OPTION UNLESS YOU KNOW WHAT YOU ARE DOING. + + +Anyway, this problem is not necessarily a problem which should be +solved inside the MUA, as it's difficult (maybe impossible) to solve +there. Additionally, there is more than one program which parses +mailcap. So writing secure mailcap statements is generally a good +idea. We encourage you to do this. + +The most basic rule is this one: + +>>> KEEP THE %-EXPANDOS AWAY FROM SHELL QUOTING. + +Don't quote them with single or double quotes. Mutt does this for +you, the right way, as should any other program which interprets +mailcap. Don't put them into backtick expansions - as you have seen +above, this is a recipe for desaster. Be highly careful with eval +statements, and avoid them if possible at all. + +If you have to use the %-expandos' values in context where you need +quoting or backtick expansions, put that value into a shell variable +and reference the shell variable where necessary (possibly with the +proper quoting put around it, like in "$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 + +The "charset=%{charset}" assignment is risk-free since mutt performs +the necessary quoting steps here. Using it inside the backtick +expansion is safe, too, since the variable's value is not itself +subject to any further expansion (but note that it _is_ subject to +word splitting). -- 2.40.0