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Vulnerability in Emacs python integration

From: Chong Yidong
Subject: Vulnerability in Emacs python integration
Date: Fri, 05 Sep 2008 13:16:38 -0400
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.3 (gnu/linux)

Romain Francoise has found a security risk in a feature of GNU Emacs
related to how Emacs interacts with Python.  Emacs version 22.3, which
was just released, contains a fix.  A patch for earlier version of Emacs
is described below.

The vulnerability may allow an attacker to run malicious code if the
user runs the Emacs command `run-python' while the current directory is
world-writable, or if the user toggles `eldoc-mode' and visits a Python
source file in a world-writable directory.

Description of the vulnerability:

The Emacs command `run-python' launches an interactive Python
interpreter.  After the Python process starts up, Emacs automatically
sends it the line

import emacs

which normally imports a script named emacs.py which is distributed with
Emacs.  This script, which is typically located in a write-protected
installation directory with other Emacs program files, defines various
functions to help the Python process communicate with Emacs.

The vulnerability arises because Python, by default, prepends '' to the
module search path, so modules are looked for in the current directory.
If the current directory is world-writable, an attacker may insert
malicious code by adding a fake Python module named emacs.py into that

Furthermore, emacs.py imports other non-built-in Python modules, such as
`inspect'.  The same vulnerability exists for these import statements.

By default, merely visiting and editing a *.py source file does not
launch a Python subprocess; you either have to call `M-x run-python', or
enable Emacs code that calls `run-python' automatically, such as

The Python developers, in a private communication, have stated that they
do not regard this module-importing behavior as a security problem for
Python per se, because running a python script in a world-writable
directory is itself a security hazard.  In the Emacs context, however,
it's much less obvious that it's unsafe to call `run-python' while the
current directory is world-writable; therefore, the problem discussed
here can be regarded as a security risk.

The following patch, against the Emacs 22.2 source tree, removes '' from
sys.path in the command-line arguments for invoking the Python process.
(Because `sys' is a "built-in module", an attacker cannot insert
malicious code by adding sys.py to the current directory.)

*** emacs/lisp/progmodes/python.el    8 May 2008 03:42:10 -0000      1.89
--- emacs/lisp/progmodes/python.el    24 Aug 2008 19:47:09 -0000     1.90
*** 1547,1553 ****
    ;; invoked.  Would support multiple processes better.
    (when (or new (not (comint-check-proc python-buffer)))
!     (let* ((cmdlist (append (python-args-to-list cmd) '("-i")))
             (path (getenv "PYTHONPATH"))
                    (process-environment        ; to import emacs.py
                                                (cons (concat
--- 1547,1555 ----
    ;; invoked.  Would support multiple processes better.
    (when (or new (not (comint-check-proc python-buffer)))
!     (let* ((cmdlist
!           (append (python-args-to-list cmd)
!                                        '("-i" "-c" "import sys; 
               (path (getenv "PYTHONPATH"))
                      (process-environment      ; to import emacs.py
                                                (cons (concat

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