Wrap the hex escaped encoder output in bash -c "...".

* This is because on Ubuntu/Debian systems `sh` is actually `dash`,
  which does not support `$'\xXX...'` style strings. So we must ensure
  the hex escaped command is executed under `bash`.

lib/ronin/payloads/encoders/builtin/shell/hex_escape.rb

@@ -47,7 +47,7 @@ class HexEscape < ShellCommandEncoder
             Encodes the arguments of a given command String as hex escaped shell
             strings then executes it as a command in a sub-shell.
 
-              ls -la -> $'\\x6c\\x73' $'\\x2d\\x6c\\x61'
+              ls -la -> bash -c "$'\\x6c\\x73' $'\\x2d\\x6c\\x61'"
 
             Note: supports bash, zsh, mksh, but *not* the dash shell which is
             the default system shell on Ubuntu and Debian.
@@ -61,9 +61,11 @@ class HexEscape < ShellCommandEncoder
           # @return [String]
           #
           def encode(command)
-            Shellwords.shellsplit(command).map { |arg|
+            escaped_command = Shellwords.shellsplit(command).map { |arg|
               "$'#{Support::Encoding::Shell.encode(arg)}'"
             }.join(' ')
+
+            %{bash -c "#{escaped_command}"}
           end
 
         end

spec/encoders/builtin/shell/hex_escape_spec.rb

@@ -16,14 +16,14 @@
 
   describe "#encode" do
     let(:command) { "echo PWNED" }
-    let(:encoded) { "$'\\x65\\x63\\x68\\x6f' $'\\x50\\x57\\x4e\\x45\\x44'" }
+    let(:encoded) { %{bash -c "$'\\x65\\x63\\x68\\x6f' $'\\x50\\x57\\x4e\\x45\\x44'"} }
 
     it "must encode each argument in the given command string into a hex strings" do
       expect(subject.encode(command)).to eq(encoded)
     end
 
     it "must return a valid shell command", :integration do
-      expect(`bash -c "#{subject.encode(command)}"`).to eq("PWNED#{$/}")
+      expect(`#{subject.encode(command)}`).to eq("PWNED#{$/}")
     end
   end
 end