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

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.

-Peff