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

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'

Signed-off-by: Joey Hess <redacted>
---
 Documentation/git-receive-pack.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index 014a78409b..cdaae75365 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt

@@ -183,7 +183,7 @@ do
 		echo "New commits:"
 		git rev-list --pretty "$nval" "^$oval"
 	fi |
-	mail -s "Changes to ref $ref" commit-list@mydomain
+	mail -E 'set escape' -s "Changes to ref $ref" commit-list@mydomain
 done
 # log signed push certificate, if any
 if test -n "${GIT_PUSH_CERT-}" && test ${GIT_PUSH_CERT_STATUS} = G

@@ -191,7 +191,7 @@ then
 	(
 		echo expected nonce is ${GIT_PUSH_NONCE}
 		git cat-file blob ${GIT_PUSH_CERT}
-	) | mail -s "push certificate from $GIT_PUSH_CERT_SIGNER" push-log@mydomain
+	) | mail -E 'set escape' -s "push certificate from $GIT_PUSH_CERT_SIGNER" push-log@mydomain
 fi
 exit 0
 ----

-- 
2.33.0

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