Security: Mutt and mailcap rules

Cameron Simpson cs at
Sat Jun 22 22:55:38 UTC 2019

On 22Jun2019 12:24, vincent lefevre <vincent at> wrote:
>FYI, due to incorrect mailcap rules, which use '%s' or similar
>instead of just %s, the filename quoting system in Mutt eventually
>makes the filename *unquoted*, i.e. reversing its purpose, e.g.
>  "less ''/var/tmp/_.txt''"
>I've reported a general bug in Debian:

Just a tangent because we've been talking about sanitising filenames 

Interestingly, a reading of mailcap(5) on Ubuntu and of RFC 1524 shows 
that it says _nothing_ about shell quoting.

All the text says that %s shall be replaced with a filename containing 
the body (or attachment). Now, clearly that needs to be Bourne shell 
safe, but the text of the RFC and the manual entry say nothing about how 
that is achieved.

My inference from reading them is that the supplied filename is itself 
shell safe without quotes - it would need to contain no shell 
punctuation or whitespace. And in that form, it wouldn't matter if the 
mailcap recipe contained quotes:

  less '%s'

works as:

  less 'foo.txt'

and a recipe without quotes:

  less %s

works as:

  less foo.txt

with such a filename.

Of course, _all_ the mailcap examples in both these documents are 
without quotes, suggesting that a tool can put in a shell quoted string 
safely as mutt endeavours to do, allow it to preserve a presupplied 
filename closely.

Returning to the quotes-in-mailcap-recipes issue, I'd be all for mutt 
noticing _and warning_ about mailcap entries with '%s' in them, and 
maybe doing an aggressive filename sanitisation at that point to provide 
an _unquoted_ but safe filename regardless of the source filename. One 
which would be safe in quotes or not.

Our obligation here is to pass the filename correctly to the recipe 
command. The preferred way is a shell-quoted string replacing %s because 
that lets us preserve any suggested/provided filename most accurately.  
But we _can_ notice a '%s' easily and provide a 
safe-if-quoted-or-unquoted filename, and I think therefore that we ought 

[... mutt code specifics snipped ...]
>But it appears that the filename is usually or always sanitized.
>The code is not enough documented about that.
>If the filename is expected to be always sanitized, this should
>probably be double-checked here to be sure and potentially avoid
>future security bugs.

I think we should try to be robust against '%s' in a mailcap recipe if 
we notice it, because we can provide an unquoted-and-more-sanitised 
filename in that circumstance which won't be misused but a recipe with 
erroneous extra quotes.

I'm happy to try to make some time to understand the mutt code and 
suggest a patch if there's agreement about this.

Cameron Simpson <cs at>

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