242 lines
6.7 KiB
Ruby
242 lines
6.7 KiB
Ruby
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require 'msf/core'
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module Msf
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module Encoders
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module X86
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#
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# This encoder is pretty lame. It has a huge size overhead. Alas, it
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# does produce tolower safe and UTF8 safe payloads. The decoder itself is
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# split into three distinct chunks. The first chunk is the header, the second
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# chunk is the inline-decoding, and the third chunk is where the decoded data
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# is persisted. Unlike most encoders, this encoder does not use any branch
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# instructions and instead runs into the decoded data after it completes due
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# to the fact that it is decoding inline.
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#
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# The basic approach taken to implement the encoder is this. First, the
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# decoder header assumes that a register (ecx) points to the first byte
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# in the decoder stub. It then proceeds to calculate the offset to the
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# third chunk of the decoder (the persisted data) and updates the context
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# register (ecx) to point to the first byte of the third chunk of the decoder
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# stub. Following that, the second chunk of the decoder begins executing
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# which uses a series of add or subtract operations on the third chunk of the
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# decoder to produce the actual opcodes of the encoded payload. For each four
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# bytes of encoded data, a sub or add instruction is used in combination with
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# complementary information stored in the third chunk of the decoder.
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#
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# For example, in order to produce 0x01fdfeff one could do the following:
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#
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# 0x5e096f7c
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# - 0x5c0b707d
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# ------------
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# 0x01fdfeff
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#
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# After all of the inline decoding operations complete, the payload should
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# simply fall through into the now-decoded payload that was stored in the
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# third chunk of the decoder.
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#
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# The following is an example encoding of:
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#
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# "\xcc\x41\xcc\x41\xcc\x41\xcc\x41\xff\xfe\xfd\x01\xff\x02\x82\x4c"
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#
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# 00000000 6A04 push byte +0x4
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# 00000002 6B3C240B imul edi,[esp],byte +0xb
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# 00000006 60 pusha
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# 00000007 030C24 add ecx,[esp]
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# 0000000A 6A11 push byte +0x11
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# 0000000C 030C24 add ecx,[esp]
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# 0000000F 6A04 push byte +0x4
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# 00000011 68640F5F31 push dword 0x315f0f64
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# 00000016 5F pop edi
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# 00000017 0139 add [ecx],edi
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# 00000019 030C24 add ecx,[esp]
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# 0000001C 6870326B32 push dword 0x326b3270
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# 00000021 5F pop edi
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# 00000022 0139 add [ecx],edi
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# 00000024 030C24 add ecx,[esp]
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# 00000027 687D700B5C push dword 0x5c0b707d
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# 0000002C 5F pop edi
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# 0000002D 2939 sub [ecx],edi
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# 0000002F 030C24 add ecx,[esp]
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# 00000032 6804317F32 push dword 0x327f3104
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# 00000037 5F pop edi
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# 00000038 2939 sub [ecx],edi
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# 0000003A 030C24 add ecx,[esp]
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# 0000003D 68326D105C push dword 0x5c106d32
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# 00000042 0F610F punpcklwd mm1,[edi]
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# 00000045 7C6F jl 0xb6
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# 00000047 095E03 or [esi+0x3],ebx
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# 0000004A 3401 xor al,0x1
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# 0000004C 7F db 0x7F
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#
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class AvoidUtf8 < Msf::Encoder
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# Assign this encoder an average ranking because it should not
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# be used preferably, but it should be used more preferably than
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# low ranking encoders, like alpha.
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Rank = AverageRanking
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def initialize
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super(
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'Name' => 'Avoid UTF8/tolower',
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'Version' => '$Revision: 3425 $',
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'Description' => 'UTF8 Safe, tolower Safe Encoder',
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'Author' => 'skape',
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'Arch' => ARCH_X86,
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'License' => MSF_LICENSE,
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'EncoderType' => Msf::Encoder::Type::NonUpperUtf8Safe,
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'Decoder' =>
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{
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'KeySize' => 4,
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'BlockSize' => 4,
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})
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end
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#
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# Returns the decoder stub that is adjusted for the size of
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# the buffer being encoded
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#
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def decoder_stub(state)
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len = ((state.buf.length + 3) & (~0x3)) / 4
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# Grab the number of additional bytes that we need to adjust by in order
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# to get the context register to point immediately after the stub header
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off = (datastore['BufferOffset'] || 0).to_i
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decoder =
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"\x6a" + [len].pack('C') + # push len
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"\x6b\x3c\x24\x0b" + # imul 0xb
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"\x60" + # push edx
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"\x03\x0c\x24" + # add ecx, [esp]
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"\x6a" + [0x11+off].pack('C') + # push byte 0x11 + off
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"\x03\x0c\x24" + # add ecx, [esp]
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"\x6a\x04" # push byte 0x4
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# encoded sled
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state.context = ''
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return decoder
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end
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def encode_block(state, block)
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buf = try_add(state, block)
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if (buf.nil?)
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buf = try_sub(state, block)
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end
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if (buf.nil?)
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raise BadcharError.new(state.encoded, 0, 0, 0)
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end
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buf
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end
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#
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# Appends the encoded context portion.
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#
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def encode_end(state)
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state.encoded += state.context
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end
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#
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# Generate the instructions that will be used to produce a valid
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# block after decoding using the sub instruction in conjunction with
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# two UTF8/tolower safe values.
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#
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def try_sub(state, block)
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buf = "\x68";
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vbuf = ''
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ctx = ''
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carry = 0
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block.each_byte { |b|
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# It's impossible to reach 0x7f, 0x80, 0x81 with two subs
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# of a value that is < 0x80 without NULLs.
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return nil if (b == 0x80 or b == 0x81 or b == 0x7f)
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x = 0
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y = 0
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attempts = 0
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prev_carry = carry
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begin
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carry = prev_carry
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if (b > 0x80)
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diff = 0x100 - b
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y = rand(0x80 - diff - 1).to_i + 1
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x = (0x100 - (b - y + carry))
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carry = 1
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else
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diff = 0x7f - b
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x = rand(diff - 1) + 1
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y = (b + x + carry) & 0xff
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carry = 0
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end
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attempts += 1
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# Lame.
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return nil if (attempts > 512)
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end while (is_badchar(state, x) or is_badchar(state, y))
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vbuf += [x].pack('C')
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ctx += [y].pack('C')
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}
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buf += vbuf + "\x5f\x29\x39\x03\x0c\x24"
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state.context += ctx
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return buf
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end
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#
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# Generate instructions that will be used to produce a valid block after
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# decoding using the add instruction in conjunction with two UTF8/tolower
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# safe values.
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#
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def try_add(state, block)
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buf = "\x68"
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vbuf = ''
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ctx = ''
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block.each_byte { |b|
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# It's impossible to produce 0xff and 0x01 using two non-NULL,
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# tolower safe, and UTF8 safe values.
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return nil if (b == 0xff or b == 0x01)
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attempts = 0
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begin
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xv = rand(b - 1) + 1
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attempts += 1
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# Lame.
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return nil if (attempts > 512)
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end while (is_badchar(state, xv) or is_badchar(state, b - xv))
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vbuf += [xv].pack('C')
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ctx += [b - xv].pack('C')
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}
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buf += vbuf + "\x5f\x01\x39\x03\x0c\x24"
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state.context += ctx
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return buf
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end
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def is_badchar(state, val)
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((val >= 0x41 and val <= 0x5a) or val >= 0x80) or Rex::Text.badchar_index([val].pack('C'), state.badchars)
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end
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end
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end end end
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