Vim: Flawed Fix of Arbitrary Code Execution Vulnerability in filetype.vim

2008-07-24T00:00:00
ID SECURITYVULNS:DOC:20220
Type securityvulns
Reporter Securityvulns
Modified 2008-07-24T00:00:00

Description

  1. SUMMARY

Product : Vim -- Vi IMproved Version : Tested with Vim 7.2b.10, filetype.vim 2008-07-17 Impact : Arbitrary code execution Wherefrom: Local and remote CVE : CVE-2008-2712 Original : http://www.rdancer.org/vulnerablevim-filetype.vim.updated.html http://www.rdancer.org/vulnerablevim-filetype.vim.updated.patch http://www.rdancer.org/vulnerablevim-latest.tar.bz2

This is an update of a previous advisory[1]. Vim patch 7.1.300 which purported to fix the ``filetype.vim'' vulnerability did not fix the vulnerability.

  1. 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

``Problem: Value of asmsyntax argument isn't checked for valid characters. Solution: Only accepts letters and digits.''

            -- Vim Patch 7.1.300[2]
  1. VULNERABILITY

This is the ``filetype.vim'' vulnerability, described in the sections 3.4.2.1. and 3.4.2.2. of the original advisory[1]. It can lead to arbitrary code execution upon Vim opening a crafted file. The file can be either local or remote, and the filename must match one of the following glob patterns:

*.asm
*.s
*.S
*.a
*.A
*.mac
*.lst (with the exception of /boot/grub/menu.lst)
*.i
  1. PURPORTED FIX

Quoting the original advisory[1]:

``[A]bsent sanitization on line 190, followed by the execute statements at filetype.vim lines 181 or 1267:

``The code looks in the first five lines [of the file being opened] for a statement of the form ``asmsyntax=FOO'', where FOO can contain any characters except Tab and Space. FOO is then executed, without any sanitization.''

   187        let head = " ".getline(1)." ".getline(2)." ".getline(3)."

".getline(4). 188 \" ".getline(5)." " 189 if head =~ '\sasmsyntax=\S\+\s' 190 let b:asmsyntax = substitute(head, '.\sasmsyntax=\(\S\+\)\s.','\1', "") [... logical flow of the code then jumps to line 181 ...] 181 exe "setf " . b:asmsyntax [... or line 1267 ...] *1267 exe "setf " . b:asmsyntax

Patch 7.1.300 changed the regular expression in the substitute() call on line 190:

let b:asmsyntax = substitute(head,

'.\sasmsyntax=\([a-zA-Z0-9]\+\)\s.','\1', "")

This would work if substitute() were a matching function -- returning a matching string, or an empty string if the pattern failed to match. But substitute() always returns its first argument -- substituting the matching string (if any). If the pattern fails to match, substitute() returns its first argument as-is:

                  | pattern matches | no match
------------------+-----------------+--------------------
substitute()      | alter match     | return as-is
------------------+-----------------+--------------------
matching function | return match    | return empty string

The previous line of code (line 189) remains unchanged, leaving two different regular expressions. It is easy to create a payload matching the first regular expression, but not the second one. As a matter of fact, the payload in the test suite[3] that accompanied the original advisory did just that.

It may be also worth noting that the failure to sanitize the input may not have been fatal if the ``execute'' statements on lines 181 and 1276 were updated to use the fnameescape() function to sanitize the arguments.

  1. EXPLOIT

The exploit needed a small update in order to work with the current Vim. It produces error messages, and the exploit text is not hidden. Making the exploit fully compatible would be just a matter of spending some more time. The updated exploit is called ``filetype.vim.updated'':

-------------------------------------------
-------- Test results below ---------------
-------------------------------------------
Vim version 7.2b, included patches: 1-10
filetype.vim revision date: 2008 Jul 17
zip.vim version: v21
netrw.vim version: v127
-------------------------------------------
filetype.vim
  strong  : EXPLOIT FAILED
  weak    : EXPLOIT FAILED
filetype.vim.updated

--> strong : VULNERABLE --> weak : VULNERABLE tarplugin : EXPLOIT FAILED tarplugin.updated: EXPLOIT FAILED tarplugin.v2: EXPLOIT FAILED zipplugin : EXPLOIT FAILED zipplugin.v2: EXPLOIT FAILED xpm.vim xpm : EXPLOIT FAILED xpm2 : EXPLOIT FAILED remote : EXPLOIT FAILED gzip_vim : EXPLOIT FAILED netrw : EXPLOIT FAILED netrw.v2 : EXPLOIT FAILED netrw.v3 : EXPLOIT FAILED netrw.v4 : EXPLOIT FAILED netrw.v5 : VULNERABLE shellescape: EXPLOIT FAILED

  1. PATCH

A copy of a patch that fixes this vulnerability can be found at the URL below[4].

  1. REFERENCES

[1] Collection of Vulnerabilities in Fully Patched Vim 7.1 http://www.rdancer.org/vulnerablevim.html [2] Patch 7.1.300 http://groups.google.com/group/vim_dev/msg/5a882ab234f02377 http://ftp.vim.org/pub/vim/patches/7.1/7.1.300 [3] The Vulnerable Vim Test Suite http://www.rdancer.org/vulnerablevim-latest.tar.bz2 [4] Proposed patch http://www.rdancer.org/vulnerablevim-filetype.vim.updated.patch

  1. 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.

  1. HISTORY

2008-07-23 Sent to: <bugs@vim.org>, <vim-dev@vim.org>, <full-disclosure@lists.grok.org.uk>, <bugtraq@securityfocus.com>