Vim: Arbitrary Code Execution in Commands: K, Control-], g]

[=?UTF-8?Q?Jan_Min=C3=A1=C5=99?=]

Vim: Arbitrary Code Execution in Commands: K, Control-], g]

1. SUMMARY

Product : Vim -- Vi IMproved
Versions : 3.0--current, possibly older
Impact : Arbitrary code execution
Wherefrom: Local
Original : http://www.rdancer.org/vulnerablevim-K.html

Insufficient sanitization can lead to Vim executing arbitrary commands
when performing keyword or tag lookup. Ben Schmidt discovered this
vulnerability[1].


2. BACKGROUND

``Vim is an almost compatible version of the UNIX editor Vi. Many new
features have been added: multi-level undo, syntax highlighting,
command line history, on-line help, spell checking, filename
completion, block operations, etc.''

-- Vim README.txt

``[Normal mode command] K [...] Run a program to lookup the keyword
under the cursor. The name of the program is given with the
'keywordprg' (kp) option (default is "man").''

-- Vim Reference Manual (``various.txt'')

``[Normal mode command] CTRL-] [...] Jump to the definition of the
keyword under the cursor.''

-- Vim Reference Manual (``tagsrch.txt'')


3. VULNERABILITIES

``src/normal.c'':

5514 if (cmdchar == '*')
5515 aux_ptr = (char_u *)(p_magic ? "/.*~[^$\\" : "/^$\\");
5516 else if (cmdchar == '#')
5517 aux_ptr = (char_u *)(p_magic ? "/?.*~[^$\\" : "/?^$\\");
5518 else if (cmdchar == 'K' && !kp_help)
--> 5519 aux_ptr = (char_u *)" \t\\\"|!";
5520 else
5521 /* Don't escape spaces and Tabs in a tag with a backslash */
--> 5522 aux_ptr = (char_u *)"\\|\"";
5523
5524 p = buf + STRLEN(buf);
5525 while (n-- > 0)
5526 {
5527 /* put a backslash before \ and some others */
5528 if (vim_strchr(aux_ptr, *ptr) != NULL)
5529 *p++ = '\\';
5530 #ifdef FEAT_MBYTE
5531 /* When current byte is a part of multibyte character, copy all bytes
5532 * of that character. */
5533 if (has_mbyte)
5534 {
5535 int i;
5536 int len = (*mb_ptr2len)(ptr) - 1;
5537
5538 for (i = 0; i < len && n >= 1; ++i, --n)
5539 *p++ = *ptr++;
5540 }
5541 #endif
5542 *p++ = *ptr++;
5543 }
5544 *p = NUL;
5545
5546 /*
5547 * Execute the command.
5548 */
5549 if (cmdchar == '*' || cmdchar == '#')
5550 {
5551 if (!g_cmd && (
5552 #ifdef FEAT_MBYTE
5553 has_mbyte ? vim_iswordp(mb_prevptr(ml_get_curline(), ptr)) :
5554 #endif
5555 vim_iswordc(ptr[-1])))
5556 STRCAT(buf, "\\>");
5557 #ifdef FEAT_CMDHIST
5558 /* put pattern in search history */
5559 add_to_history(HIST_SEARCH, buf, TRUE, NUL);
5560 #endif
5561 normal_search(cap, cmdchar == '*' ? '/' : '?', buf, 0);
5562 }
5563 else
--> 5564 do_cmdline_cmd(buf);

The variable ``aux_ptr'' contains characters to be escaped. Line 5519
for the ``K'' command, line 5522 for the ``Control-]'' and ``g]''
commands. Both values leave out characters that must be escaped. The
command is assembled, and on line 5564, it is executed as a regular Ex
command. No special shell escaping is done for the ``K'' command,
although the string is passed to shell for execution.


3.1. Keyword Lookup -- The ``K'' Command

3.1.1. Shell Commands and Ex Commands

Because the string passed to the shell for execution is not sanitized,
it is possible to specify arbitrary shell commands where Vim expects an
argument for the keyword program. Same applies to arbitrary Ex commands.


3.1.2. Keyword Program Command Line Switches

It is possible to specify command line switches for the keyword program
in place of the argument. The gravity of this vulnerability depends on
the keyword program selected. GNU man, the default keyword program in
many installations, supports for example the ``--pager'' option (cf.
the GNU man(1) manual page). This allows arbitrary command execution.


3.2. Tag Lookup -- the ``Control-]'' and ``g]'' Commands

Insufficient sanitization of an Ex command argument allows specifying
additional arbitrary Ex commands in place of the argument.


3.3. Unknown Shell/Keyword Program

Because the syntax of the shell that is being used to execute the
commands is not known beforehand, there may be other unknown
vulnerabilities, that are present depending on the shell being used.
Ditto for the man(1) program, and other keyword programs.


4. EXPLOIT

Copy-and-paste these examples into separate files:

;xclock
vim: set iskeyword=;,@

Place your cursor on ``xclock'', and press K. xclock appears.

;date>>pwned
vim: set iskeyword=1-255

Place your cursor on ``date'' and press K. File ``pwned'' is created in
the current working directory.

Please note: If modeline processing is disabled, set the 'iskeyword'
option manually.

See the thread on the Vim Developers' mailing list for some other
examples[2].


5. PATCH

A patch that fixes some of the vulnerabilities has been developed[3].


6. REFERENCES

[1] Ben Schmidt discovered this vulnerability in:
Message-Id: <48AB91B3.9000709@yahoo.com.au>
http://groups.google.com/group/vim_dev/msg/6ad2d5b50a96668e

[2] http://groups.google.com/group/vim_dev/browse_thread/thread/1434d0812b5c817e/6ad2d5b50a96668e

[3] http://groups.google.com/group/vim_dev/msg/dd32ad3a84f36bb2


7. COPYRIGHT

This advisory is Copyright 2008 Jan Minar <rdancer@rdancer.org>

Copying welcome, under the Creative Commons ``Attribution-Share Alike''
License http://creativecommons.org/licenses/by-sa/2.0/uk/

Code included herein, and accompanying this advisory, may be copied
according to the GNU General Public License version 2, or the Vim
license. See the subdirectory ``licenses''.

Various portions of the accompanying code may have been written by
various parties. Those parties may hold copyright, and those portions
may be copied according to their respective licenses.


8. HISTORY

2008-08-22 Sent to: <bugs@vim.org>, <vim-dev@vim.org>,
<full-disclosure@lists.grok.org.uk>,
<bugtraq@securityfocus.com>
2008-08-20 Ben Schmidt reported this vulnerability to <vim-dev@vim.org>

[Michael Wojcik]

> From: rdancer@gmail.com [mailto:rdancer@gmail.com] On Behalf
> Of Jan Minár
> Sent: Friday, 22 August, 2008 10:26
> To: bugs@vim.org; vim-dev@vim.org;
> full-disclosure@lists.grok.org.uk; bugtraq@securityfocus.com
> Subject: Vim: Arbitrary Code Execution in Commands: K, Control-], g]
>
> Vim: Arbitrary Code Execution in Commands: K, Control-], g]

This report greatly overstates the danger of this bug. It's worth reading the discussion from the Vim Dev list (Minár's [2] below).


> 3.1. Keyword Lookup -- The ``K'' Command
>
> 3.1.1. Shell Commands and Ex Commands
>
> Because the string passed to the shell for execution is not
> sanitized, it is possible to specify arbitrary shell commands
> where Vim expects an argument for the keyword program. Same
> applies to arbitrary Ex commands.

The K command is designed to execute an arbitrary program. The user can set the program by setting the keywordprg option. Minár's exploits require setting vim options to implausible values, either using a modeline (which no sensible user ever allows on untrustworthy files, and no truly security-conscious user enables at all) or monumental user stupidity. Given that, why not simply set keywordprg? Or do anything else that a modeline allows?

> 3.1.2. Keyword Program Command Line Switches
>
> It is possible to specify command line switches for the
> keyword program in place of the argument. The gravity of
> this vulnerability depends on the keyword program selected.
> GNU man, the default keyword program in many installations,
> supports for example the ``--pager'' option (cf.
> the GNU man(1) manual page). This allows arbitrary command execution.

As does setting PAGER in the environment before vim starts, which is an equally plausible attack.

Schmidt did accidentally discover an issue with unescaped characters and the K command - specifically with Visual-K and an unconventional setting of keywordprg, used in a manner for which it was not intended (selecting a URL and using K to pass it to a browser). See Minár's [1]. So it's not impossible for someone to encounter this bug while operating in a manner they think is sensible.

But very few users will create the necessary conditions, so the attack surface is vanishingly small; and users who do that sort of thing with untrustworthy data are going to shoot themselves in the foot sooner or later. No vim required.

It'd be much better to focus on vim security issues that have some chance of exploitation, like the netrw problems that Minár recently documented. This sort of thing is just noise.

> [1] Ben Schmidt discovered this vulnerability in:
> Message-Id: <48AB91B3.9000709@yahoo.com.au>
> http://groups.google.com/group/vim_dev/msg/6ad2d5b50a96668e
>
> [2]
> http://groups.google.com/group/vim_dev/browse_thread/thread/14
> 34d0812b5c817e/6ad2d5b50a96668e
>
> [3] http://groups.google.com/group/vim_dev/msg/dd32ad3a84f36bb2

--
Michael Wojcik
Principal Software Systems Developer, Micro Focus