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* bug#37656: 27.0.50; Arbitrary code execution with special `mode:'
@ 2019-10-15 21:05 adam plaice
  2019-10-15 22:23 ` Clément Pit-Claudel
                   ` (3 more replies)
  0 siblings, 4 replies; 20+ messages in thread
From: adam plaice @ 2019-10-15 21:05 UTC (permalink / raw)
  To: emacs-devel; +Cc: 37656

Since the bug allows an attacker to execute arbitrary code if the
victim opens a payload file, and hence opening any file from an
untrusted source becomes dangerous, it seems to be rather
serious.

The bug relies on the fact that flymake-mode can execute arbitrary
code, that minor modes (in particular, flymake-mode) can be set with
local variables (with `mode:') and that when a minor-mode is set in
this way, the major-mode is not unset. (See the linked bug or below
for details.)

I'm not sure whether I should be bringing greater attention to it,
but given that it's already in the open, and malicious actors can
find it (or just come up with it themselves, as it's not a particularly
complex idea), increasing the likelihood of getting it fixed hopefully
outweighs the disadvantages.

I'd offer to provide a patch, but I'm neither very proficient with
Emacs lisp, nor a security expert.  I also haven't signed any copyright
papers.


Some thoughts on potential solutions (from a well-intentioned, but
possibly misguided layman):

AFAICT the easiest way to prevent this specific bug would be to
prevent more than one mode being set by the file and directory
local-variables machinery.

Perhaps also only allowing major modes to be set with `mode' in local
variables (and only allowing minor-modes to be set with `eval', as is
already encouraged in the manual), might decrease the "attack surface"
for similar such attacks.

I'm not sure whether any major modes are "unsafe" (in the way flymake
is), but possibly it might make sense to mark major modes as safe,
similarly to the way variables are, though that would be a far more
extensive change.

Thank you,
Adam

PS Should Emacs have some policies on reporting security issues? I
was encouraged (via an earlier e-mail exchange) to post the bug to
debbugs, as normal, but it might perhaps be useful if the process
(specifically for security vulnerabilities, not bugs in general) were
mentioned in the manual.

> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37656
>
> * To reproduce:
>
> 1. Create a file, say `~/foobar', (it could have an arbitrary
> extension) with the following contents:
>
> -*- mode: emacs-lisp; mode: flymake -*-
>
> (eval-when-compile
>   (with-temp-file "~/emacs_flymake_security_bug"
>       (insert "Could have also executed any code.")))
>
> 2. Open the file with emacs:
>
> emacs -Q ~/foobar
>
> 3. Inspect ~/emacs_flymake_security_bug:
>
> cat ~/emacs_flymake_security_bug
>
> * Expected result
>
> ~/emacs_flymake_security_bug does not exist.
>
> * Actual result
>
> ~/emacs_flymake_security_bug does exist.
>
> * Further information
>
> This relies on the "deprecated" feature of allowing `mode: ' to be
> repeated more than once, to also specify minor modes.  Just having:
>
> -*- mode: flymake -*-
>
> in, say, `~/foobar.el' would not trigger the security bug.  There may,
> however, be alternative ways of triggering it, that I haven't come up
> with.
>
>
> This was "inspired" by a very similar bug (concerning an external
> package, editorconfig), described here:
>
> https://illikainen.dev/blog/2019-10-06-editorconfig
>
> Thank you and best regards,
> Adam
>
>



^ permalink raw reply	[flat|nested] 20+ messages in thread
* bug#37656: 27.0.50; Opening file with specially crafted local variables can cause arbitrary code execution Inbox x
@ 2019-10-08  8:48 adam plaice
  2019-10-15 21:05 ` bug#37656: 27.0.50; Arbitrary code execution with special `mode:' adam plaice
  0 siblings, 1 reply; 20+ messages in thread
From: adam plaice @ 2019-10-08  8:48 UTC (permalink / raw)
  To: 37656

* To reproduce:

1. Create a file, say `~/foobar', (it could have an arbitrary
extension) with the following contents:

-*- mode: emacs-lisp; mode: flymake -*-

(eval-when-compile
  (with-temp-file "~/emacs_flymake_security_bug"
      (insert "Could have also executed any code.")))

2. Open the file with emacs:

emacs -Q ~/foobar

3. Inspect ~/emacs_flymake_security_bug:

cat ~/emacs_flymake_security_bug

* Expected result

~/emacs_flymake_security_bug does not exist.

* Actual result

~/emacs_flymake_security_bug does exist.

* Further information

This relies on the "deprecated" feature of allowing `mode: ' to be
repeated more than once, to also specify minor modes.  Just having:

-*- mode: flymake -*-

in, say, `~/foobar.el' would not trigger the security bug.  There may,
however, be alternative ways of triggering it, that I haven't come up
with.


This was "inspired" by a very similar bug (concerning an external
package, editorconfig), described here:

https://illikainen.dev/blog/2019-10-06-editorconfig

Thank you and best regards,
Adam


In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2019-10-07 built on adam
Repository revision: 9839466b231b6384055b9b137405730876413cbe
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description: Ubuntu 16.04.6 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules --without-pop'

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 44045 5448)
 (symbols 48 5971 1)
 (strings 32 15685 1582)
 (string-bytes 1 506409)
 (vectors 16 9198)
 (vector-slots 8 123144 8510)
 (floats 8 19 25)
 (intervals 56 186 0)
 (buffers 1000 11)
 (heap 1024 12431 1138))





^ permalink raw reply	[flat|nested] 20+ messages in thread

end of thread, other threads:[~2019-10-16 21:02 UTC | newest]

Thread overview: 20+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2019-10-15 21:05 bug#37656: 27.0.50; Arbitrary code execution with special `mode:' adam plaice
2019-10-15 22:23 ` Clément Pit-Claudel
2019-10-15 22:27 ` Stefan Kangas
2019-10-15 22:27 ` Stefan Kangas
2019-10-15 22:55   ` Stefan Kangas
2019-10-15 23:17     ` Stefan Kangas
2019-10-15 23:17     ` Stefan Kangas
2019-10-16  7:58       ` Eli Zaretskii
2019-10-16 11:51         ` Adam Plaice
2019-10-16 17:09           ` Eli Zaretskii
2019-10-16 19:09             ` Phil Sainty
2019-10-16 19:34               ` Eli Zaretskii
2019-10-16 21:02               ` Adam Plaice
2019-10-16  0:35     ` Adam Plaice
2019-10-16  7:57       ` Eli Zaretskii
2019-10-16  0:55     ` Phil Sainty
2019-10-16  0:55     ` Phil Sainty
2019-10-16  7:59     ` Eli Zaretskii
2019-10-16  6:43 ` Eli Zaretskii
  -- strict thread matches above, loose matches on Subject: below --
2019-10-08  8:48 bug#37656: 27.0.50; Opening file with specially crafted local variables can cause arbitrary code execution Inbox x adam plaice
2019-10-15 21:05 ` bug#37656: 27.0.50; Arbitrary code execution with special `mode:' adam plaice

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