Vim: Unfixed Vulnerabilities in Tar Plugin Version 20

["Jan Minář" <rdancer () rdancer org>]

Vim: Unfixed Vulnerabilities in Tar Plugin Version 20

1. SUMMARY

Product  : Vim -- Vi IMproved
Version  : Vim >= 7.0 (possibly older), present in 7.2c.002
           autoload/tar.vim >= 9 (possibly older), present in version 20
Impact   : Arbitrary code execution
Wherefrom: Local, remote
CVE      : CVE-2008-2712
Original : http://www.rdancer.org/vulnerablevim-tarplugin-update.html

The Vim Tar Plugin vulnerabilities published in our previous advisories
have been addressed, but the changes do not provide fix for all attack
vectors.  We analyses some of the vulnerabilities remaining in
``$VIMRUNTIME/autoload/tar.vim''.


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

 ``When one edits a *.tar file, this plugin will handle displaying a
   contents page.  Select a file to edit by moving the cursor atop
   the desired file, then hit the <return> key.  After editing, one may
   also write to the file.''

                -- Tar File Interface (pi_tar.txt)


3. ATTEMPTED FIX

These are all the ``execute'' and system() calls in the current code
(autoload/tar.vim version 20, 2008-07-30) code.  It can be seen that all
the vulnerable statements have been changed.  Unfortunately, not all the
changes provide a sufficient fix.  (We analyse the vulnerabilities in
section 4 below):

    133    let tarfile=substitute(system("cygpath -u
".s:Escape(tarfile,0)),'\n$','','e')
    138    exe "silent r! gzip -d -c -- ".s:Escape(tarfile,1)." |
".g:tar_cmd." -".g:tar_browseoptions." - "
    141    exe "silent r! cat -- ".s:Escape(tarfile,1)."|gzip -d -c
-|".g:tar_cmd." -".g:tar_browseoptions." - "
    144    exe "silent r! bzip2 -d -c -- ".s:Escape(tarfile,1)." |
".g:tar_cmd." -".g:tar_browseoptions." - "
    147    exe "silent r! ".g:tar_cmd." -".g:tar_browseoptions."
".s:Escape(tarfile,1)
    163    exe "r ".fnameescape(a:tarfile)
    198    let tarfile=substitute(system("cygpath -u
".s:Escape(tarfile,0)),'\n$','','e')
    223    let tarfile=substitute(system("cygpath -u
".s:Escape(tarfile,0)),'\n$','','e')
    244    exe "silent r! gzip -d -c -- ".s:Escape(tarfile,1)."|
".g:tar_cmd." -".g:tar_readoptions." - ".s:Escape(fname,1).decmp
    247    exe "silent r! cat -- ".s:Escape(tarfile,1)." | gzip -d -c
- | ".g:tar_cmd." -".g:tar_readoptions." - ".s:Escape(fname,1).decmp
    250    exe "silent r! bzip2 -d -c -- ".s:Escape(tarfile,1)."|
".g:tar_cmd." -".g:tar_readoptions." - ".s:Escape(fname,1).decmp
    253    exe "silent r! ".g:tar_cmd." -".g:tar_readoptions."
".s:Escape(tarfile,1)." -- ".s:Escape(fname,1).decmp
    262   exe "file tarfile::".fnameescape(fname)
    308    exe "cd ".fnameescape(tmpdir)
    332    call system("gzip -d -- ".s:Escape(tarfile,0))
    336    call system("gzip -d -- ".s:Escape(tarfile,0))
    341    call system("bzip2 -d -- ".s:Escape(tarfile,0))
    359      let dirpath = substitute(system("cygpath
".s:Escape(dirpath, 0)),'\n','','e')
    368    exe "w! ".fnameescape(fname)
    370     let tarfile = substitute(system("cygpath
".s:Escape(tarfile,0)),'\n','','e')
    375    call system("tar --delete -f ".s:Escape(tarfile,0)." --
".s:Escape(fname,0))
    384     call system("tar -".g:tar_writeoptions."
".s:Escape(tarfile,0)." -- ".s:Escape(fname,0))
    391      call system(compress)
    407     exe "e! ".fnameescape(tarfile)
    419   exe "cd ".fnameescape(curdir)
    431    call system("/bin/rm -rf -- ".s:Escape(a:fname,0))
    434     call system("/bin/rm -rf -- ".s:Escape(a:fname,0))
    436     call system("del /S ".s:Escape(a:fname,0))

This is the listing from section ``3.4.2.3.1.'' of the original
advisory[1], for reference:

     99   exe "$put ='".'\"'." Browsing tarfile ".a:tarfile."'"
    107    let tarfile=substitute(system("cygpath -u ".tarfile),'\n$','','e')
    112    exe "silent r! gzip -d -c ".g:tar_shq.tarfile.g:tar_shq."|
".g:tar_cmd." -".g:tar_browseoptions." - "
    115    exe "silent r! bzip2 -d -c ".g:tar_shq.tarfile.g:tar_shq."|
".g:tar_cmd." -".g:tar_browseoptions." - "
    118    exe "silent r! ".g:tar_cmd." -".g:tar_browseoptions."
".g:tar_shq.tarfile.g:tar_shq
    134    exe "r ".a:tarfile
    169    let tarfile=substitute(system("cygpath -u ".tarfile),'\n$','','e')
    192    let tarfile=substitute(system("cygpath -u ".tarfile),'\n$','','e')
    199    exe "silent r! gzip -d -c ".g:tar_shq.tarfile.g:tar_shq."|
".g:tar_cmd." -".g:tar_readoptions." - '".fname."'"
    202    exe "silent r! bzip2 -d -c ".g:tar_shq.tarfile.g:tar_shq."|
".g:tar_cmd." -".g:tar_readoptions." - '".fname."'"
    205    exe "silent r! ".g:tar_cmd." -".g:tar_readoptions."
".g:tar_shq.tarfile.g:tar_shq." ".g:tar_shq.fname.g:tar_shq
    208   exe "file tarfile:".fname
    278    call system("gzip -d ".tarfile)
    282    call system("gzip -d ".tarfile)
    287    call system("bzip2 -d ".tarfile)
    303      let dirpath = substitute(system("cygpath ".dirpath),'\n','','e')
    312    exe "w! ".fname
    314     let tarfile = substitute(system("cygpath ".tarfile),'\n','','e')
    319    call system("tar --delete -f '".tarfile."' '".fname."'")
    335      call system(compress)
    351     exe "e! ".tarfile


4. VULNERABILITIES

4.1. Untrusted File Names Interpreted as Optional Argument

4.1.1. POSIX Systems

The POSIX end-of-options double-dash (--) is missing from some of the
commands invoked by system() -- line 244 a.o.:

    244    exe "silent r! gzip -d -c -- ".s:Escape(tarfile,1)."|
".g:tar_cmd." -".g:tar_readoptions." - ".s:Escape(fname,1).decmp

The resulting command looks like this:

    gzip -d -c -- TARBALL | tar -OPxf - MEMBER

MEMBER can be interpreted by tar(1) as a command line option.  This can
be still used to execute arbitrary shell commands (cf. e.g. the
``--compress-program'' option of tar(1)).


4.1.2. Other Systems

With implementations of tar(1) (and other programs) that do not
understand the double-dash convention, another mechanism must be used to
prevent the file name from being interpreted as command line options.
At the same time, the current code may confuse such programs.

It is not possible for Vim to know the invocation syntax of external
programs.  As the double-dash security measure may not be present in any
given external command, the security of commands that pass untrusted
input to these external commands is not be guaranteed.


4.2 Unspecified Behaviour of system() and  ``!''

4.2.1. The system() Function

system(), does not invoke /bin/sh to run the commands, as does the C
Standard Library function of the same name.  Rather, it uses the program
specified in the Vim internal option 'shell'.  The full details of how
system() works can be found in the Vim Manual:

  ``system({expr} [, {input}])                          *system()* *E677*
    [...]
    The command executed is constructed using several options:
    'shell' 'shellcmdflag' 'shellxquote' {expr} 'shellredir' {tmp} 'shellxquote'
    ({tmp} is an automatically generated file name).  For Unix and OS/2
    braces are put around {expr} to allow for concatenated commands.''

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

As the particularities of how this program interprets the command can
not be known, it is inherently impossible to say anything meaningful as
to whether there are security issues.  In fact, it is not possible to
say anything about how the command will be interpreted, or if it will be
interpreted at all.  In the absence of a baseline specification, the
behaviour of system() as implemented by Vim can only be described as
"unspecified".

By setting the respective options to known values, it may be possible to
reach the C Standard Library system() functionality.  There will still
be problems on systems without /bin/sh, and on systems where /bin/sh is
not POSIX-conformant.


4.2.2. The ``!'' Command

The same applies to the ``!'' command, as used e.g. on line 138:

    138    exe "silent r! gzip -d -c -- ".s:Escape(tarfile,1)." |
".g:tar_cmd." -".g:tar_browseoptions." - "

The ``r!'' means the ``read'' command reads the output of the ``!''
command, which in turn executes shell commands, in a way similar to
system().


5. EXPLOIT

No exploit code is provided.


6. REFERENCES

[1] Collection of Vulnerabilities in Fully Patched Vim 7.1
    http://www.rdancer.org/vulnerablevim.html


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 were written by various
parties.  Those parties may hold copyright, and those portions may be
copied according to their respective licenses.


8. HISTORY

2008-08-08 Sent to: <bugs () vim org>, <vim-dev () vim org>,
           <full-disclosure () lists grok org uk>,
           <bugtraq () securityfocus com>,
           Charles E Campbell, Jr (Vim Tar Plugin Maintainer)
           <drchip () campbellfamily biz>