bug#47616: 27.1; hardening mail-envelope-from

bug#47616: 27.1; hardening mail-envelope-from

[Francesco Potortì]

in mail-utils.el the function mail-fetch-field thus notes in the doc
string:

  The buffer should be narrowed to just the header, else false
  matches may be returned from the message body.

In fact, both sendmail-send-it and smtp-send-it use mail-envelope-from,
which calls mail-fetch-field without narrowing, which in fact causes a
false match if:

- you forward a message with "From: " at begining of line
- message-forward-as-mime is nil
- mail-specify-envelope-from is t
- mail-envelope-from is 'header

In this case, both sendmail-send-it and smptmail-send-it try to see if
they should set the From: field and the sender, and both get a false
match from mail-envelope-from.

Apparently, the problem with sendmail-send-it is corrected later in the
code (I don't know where) so the mail is sent correctly, which is why I
had never realised this until I started using smtpmail-send-it, which
sets a wrong From: header copied from the forwarded message.

Hardening mail-envelope-from from sendmail.el by narrowing to the
headers, as the doc says, corrects the problem that I observed.

(defun mail-envelope-from ()
  "Return the envelope mail address to use when sending mail.
This function uses `mail-envelope-from'."
  (or (if (eq mail-envelope-from 'header)
	  (nth 1 (mail-extract-address-components
		  (save-restriction
		    (save-excursion
		      (goto-char (point-max))
		      (re-search-backward
		       (concat "^" (regexp-quote mail-header-separator) "\n")
		       nil t)
		      (narrow-to-region (point-min) (point))
		      (mail-fetch-field "From")))))
	mail-envelope-from)
      user-mail-address))

This introduces a small semantic change for the meaning of the
mail-envelope-from variable.  Currently, the docs says:

If non-nil, designate the envelope-from address when sending mail.
This only has an effect if `mail-specify-envelope-from’ is non-nil.
The value should be either a string, or the symbol `header’ (in
which case the contents of the "From" header of the message
being sent is used), or nil (in which case the value of
‘user-mail-address’ is used).

The last two lines should be instead:

...
being sent is used, if one exists).  If the value is nil, or if it is
`header' and no "From" header is found in the message, the value of
‘user-mail-address’ is used.

bug#47616: 27.1; hardening mail-envelope-from

[Francesco Potortì]

>(defun mail-envelope-from ()
>  "Return the envelope mail address to use when sending mail.
>This function uses `mail-envelope-from'."
>  (or (if (eq mail-envelope-from 'header)
>	  (nth 1 (mail-extract-address-components
>		  (save-restriction
>		    (save-excursion
>		      (goto-char (point-max))
>		      (re-search-backward
>		       (concat "^" (regexp-quote mail-header-separator) "\n")
>		       nil t)
>		      (narrow-to-region (point-min) (point))
>		      (mail-fetch-field "From")))))
>	mail-envelope-from)
>      user-mail-address))

This one is better (I had forgotten about mail-header-end)

(require 'sendmail)
(defun mail-envelope-from ()
  "Return the envelope mail address to use when sending mail.
This function uses `mail-envelope-from'."
  (or (if (eq mail-envelope-from 'header)
	  (let ((from-field (save-restriction
			      (narrow-to-region (point-min) (mail-header-end))
			      (mail-fetch-field "From"))))
	    (when from-field
	      (nth 1 (mail-extract-address-components from-field))))
	mail-envelope-from)
      user-mail-address))

bug#47616: 27.1; hardening mail-envelope-from

[Lars Ingebrigtsen]

Francesco Potortì <pot@gnu.org> writes:

> Hardening mail-envelope-from from sendmail.el by narrowing to the
> headers, as the doc says, corrects the problem that I observed.

Thanks -- I don't think we should change mail-envelope-from itself here,
because it may conceivably be called from other contexts.  Instead the
callers in sendmail/smtpmail should be altered to narrow to the headers
before calling it, and I've now done this in Emacs 28.  (This uncovered
a similar bug in smtpmail.el, too.)

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#47616: 27.1; hardening mail-envelope-from

[Francesco Potortì]

>Francesco Potortì <pot@gnu.org> writes:
>> Hardening mail-envelope-from from sendmail.el by narrowing to the
>> headers, as the doc says, corrects the problem that I observed.
>
>Thanks -- I don't think we should change mail-envelope-from itself here,
>because it may conceivably be called from other contexts.  Instead the
>callers in sendmail/smtpmail should be altered to narrow to the headers
>before calling it, and I've now done this in Emacs 28.  (This uncovered
>a similar bug in smtpmail.el, too.)

That makes sense, in principle.  I would argue for adding a comment to
mail-envelope-from stating that since it calls mail-fetch-field it
should be called only after narrowing to the headers.  Or maybe even
adding a note in the doc string, as done in mail-fetch-field.

bug#47616: 27.1; hardening mail-envelope-from

[Lars Ingebrigtsen]

Francesco Potortì <pot@gnu.org> writes:

> That makes sense, in principle.  I would argue for adding a comment to
> mail-envelope-from stating that since it calls mail-fetch-field it
> should be called only after narrowing to the headers.  Or maybe even
> adding a note in the doc string, as done in mail-fetch-field.

Good idea.  I've now mentioned this in the doc string in Emacs 28.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#47616: 27.1; hardening mail-envelope-from

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 07 May 2021 13:17:27 +0200
> Cc: 47616@debbugs.gnu.org
> 
> Francesco Potortì <pot@gnu.org> writes:
> 
> > That makes sense, in principle.  I would argue for adding a comment to
> > mail-envelope-from stating that since it calls mail-fetch-field it
> > should be called only after narrowing to the headers.  Or maybe even
> > adding a note in the doc string, as done in mail-fetch-field.
> 
> Good idea.  I've now mentioned this in the doc string in Emacs 28.

Did you forget to push?

bug#47616: 27.1; hardening mail-envelope-from

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> Did you forget to push?

Yup.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no