BufferRegister should be validated.

If BufferRegister is in lower-case, then gen_decoder_prefix will
return nil.  When the return value is nil, other functions like
gen_decoder() will backtrace due to a "undefined method "+" for nil"
error.  Therefore, this input should NOT be case-sensitive.

Also, if for some reason the user supplies an invalid BufferRegister,
the function should be aware of that and warn the user about the
bad input.

Author: sinn3r

lib/rex/encoder/alpha2/unicode_mixed.rb

@@ -39,7 +39,12 @@ def self.gen_decoder_prefix(reg, offset)
 			'EDI'   => 'WWYA' + mod,         # push edi, pop edi
 		}
 
-		return regprefix[reg]	
+		prefix = regprefix[reg.upcase]
+		if prefix.nil?
+			raise "Critical: Invalid register"
+		end
+
+		return prefix
 	end
 
 	def self.gen_decoder(reg, offset)