Re: [PATCH] avoid insecure use of mail in man page example

[Jeff King <peff@peff.net>]

On Tue, Sep 28, 2021 at 04:46:52PM -0700, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > On Tue, Sep 28, 2021 at 08:16:48AM -0400, Joey Hess wrote:
> >
> >> As recently seen in fail2ban's security hole (CVE-2021-32749),
> >> piping user controlled input to mail is exploitable,
> >> since a line starting with "~! foo" in the input will run command foo.
> >> 
> >> This example on the man page pipes to mail. It may not be exploitable.
> >> git rev-list --pretty indents commit messages, which prevents the escape
> >> sequence working there. It's less clear if it might be possible to embed
> >> the escape sequence in a signed push certificate. The user reading the
> >> man page might alter the example to do something more exploitable.
> >> To encourage safe use of mail, add -E 'set escape'
> >
> > Seems like a good goal, but is "-E" portable?
> >
> > On my system, where "mail" comes from the bsd-mailx package, "-E" means
> > "do not send a message with an empty body" and your example command
> > barfs as it tries to deliver to the recipient "set escape".
> >
> > At least we'd want to make a note in the documentation saying what the
> > mysterious "set escape" is doing, and that not all versions of mail
> > would need / want it.
> 
> It is not the primary focus for this documentation page to teach how
> to send e-mails in the first place.  Instead of risking confused
> users rightly complain with "my 'mail' does not understand the -E
> option---what does this do?", I wonder if it is better to just change it to
> 
> 	git rev-list --pretty ...
> -   fi |
> -   mail -s ...    
> +   fi >>/var/log/update.log
> 
> so that it illustrates what's available *out* *of* *us* to the
> authors of the script, without having to teach them "mail" and other
> things we are responsible for.

Yeah, I'd agree that side-stepping the issue entirely is a good
direction. Doing it right is probably best left to tools like
git-multimail.

-Peff