Payload Type Info

Payload Type information must be set and pulled from a definition either in Python or in GoLang. Below is a python example of the apfell agent:

from mythic_container.PayloadBuilder import *
from mythic_container.MythicCommandBase import *
from mythic_container.MythicRPC import *
import json


class Apfell(PayloadType):
    name = "apfell"
    file_extension = "js"
    author = "@its_a_feature_"
    supported_os = [SupportedOS.MacOS]
    wrapper = False
    wrapped_payloads = []
    note = """This payload uses JavaScript for Automation (JXA) for execution on macOS boxes."""
    supports_dynamic_loading = True
    c2_profiles = ["http", "dynamichttp"]
    mythic_encrypts = True
    translation_container = None # "myPythonTranslation"
    build_parameters = []
    agent_path = pathlib.Path(".") / "apfell"
    agent_icon_path = agent_path / "agent_functions" / "apfell.svg"
    agent_code_path = agent_path / "agent_code"

    build_steps = [
        BuildStep(step_name="Gathering Files", step_description="Making sure all commands have backing files on disk"),
        BuildStep(step_name="Configuring", step_description="Stamping in configuration values")
    ]

    async def build(self) -> BuildResponse:
        # this function gets called to create an instance of your payload
        resp = BuildResponse(status=BuildStatus.Success)
        return resp

There are a couple key pieces of information here:

• Line 1-2 imports all of basic classes needed for creating an agent

• line 7 defines the new class (our agent). This can be called whatever you want, but the important piece is that it extends the PayloadType class as shown with the ().

• Lines 8-27 defines the parameters for the payload type that you'd see throughout the UI.

  • the name is the name of the payload type

  • supported_os is an array of supported OS versions

  • supports_dynamic_loading indicates if the agent allows you to select only a subset of commands when creating an agent or not

  • build_parameters is an array describing all of the build parameters when creating your agent

  • c2_profiles is an array of c2 profile names that the agent supports

• Line 18 defines the name of a "translation container" which we will talk about in another section, but this allows you to support your own, non-mythic message format, custom crypto, etc.

• The last piece is the function that's called to build the agent based on all of the information the user provides from the web UI.

The PayloadType base class is in the PayloadBuilder.py file. This is an abstract class, so your instance needs to provide values for all these fields.

Build Parameters

Build parameters define the components shown to the user when creating a payload.

The BuildParameter class has a couple of pieces of information that you can use to customize and validate the parameters supplied to your build:

class BuildParameter:
    def __init__(
            self,
            name: str,
            parameter_type: BuildParameterType = None,
            description: str = None,
            required: bool = None,
            randomize: bool = None,
            format_string: str = "",
            crypto_type: bool = False,
            verifier_regex: str = None,
            default_value: any = None,
            choices: list[str] = None,
            dictionary_choices: list[DictionaryChoice] = None,
            value: any = None,
            verifier_func: callable = None,
    ):
        self.name = name
        self.verifier_func = verifier_func
        self.parameter_type = (
            parameter_type if parameter_type is not None else BuildParameterType.String
        )
        self.description = description if description is not None else ""
        self.required = required if required is not None else True
        self.verifier_regex = verifier_regex if verifier_regex is not None else ""
        self.default_value = default_value
        if value is None:
            self.value = default_value
        else:
            self.value = value
        self.choices = choices
        self.dictionary_choices = dictionary_choices
        self.crypto_type = crypto_type
        self.randomize = randomize
        self.format_string = format_string

• name is the name of the parameter, if you don't provide a longer description, then this is what's presented to the user when building your payload

• parameter_type describes what is presented to the user - valid types are:

  • BuildParameterType.String

  • BuildParameterType.ChooseOne

  • BuildParameterType.Array

  • BuildParameterType.Date

  • BuildParameterType.Dictionary

  • BuildParameterType.Boolean

• required indicates if there must be a value supplied. If no value is supplied by the user and no default value supplied here, then an exception is thrown before execution gets to the build function.

• verifier_regex is a regex the web UI can use to provide some information to the user about if they're providing a valid value or not

• default_value is the default value used for building if the user doesn't supply anything

• choices is where you can supply an array of options for the user to pick from if the parameter_type is ChooseOne

• dictionary_choices are the choices and metadata about what to display to the user for key-value pairs that the user might need to supply

• value is the component you access when building your payload - this is the final value (either the default value or the value the user supplied)

• verifier_func is a function you can provide for additional checks on the value the user supplies to make sure it's what you want. This function should either return nothing or raise an exception if something isn't right

As a recap, where does this come into play? In the first section, we showed a section like:

bbuild_parameters = [
        BuildParameter(name="string", parameter_type=BuildParameterType.String, 
                       description="test string", default_value="test"),
        BuildParameter(name="choose one", parameter_type=BuildParameterType.ChooseOne, 
                       description="test choose one",
                       choices=["a", "b"], default_value="a"),
        BuildParameter(name="choose one crypto", 
                       parameter_type=BuildParameterType.ChooseOne, 
                       description="choose one crypto",
                       crypto_type=True, choices=["aes256_hmac", "none"]),
        BuildParameter(name="date", parameter_type=BuildParameterType.Date, 
                       default_value=30, description="test date offset from today"),
        BuildParameter(name="array", parameter_type=BuildParameterType.Array, 
                       default_value=["a", "b"],
                       description="test array"),
        BuildParameter(name="dict", parameter_type=BuildParameterType.Dictionary, 
                       dictionary_choices=[
            DictionaryChoice(name="host", default_value="abc.com", default_show=False),
            DictionaryChoice(name="user-agent", default_show=True, default_value="mozilla")
        ],
                       description="test dictionary"),
        BuildParameter(name="random", parameter_type=BuildParameterType.String, 
                       randomize=True, format_string="[a,b,c]{3}",
                       description="test randomized string"),
        BuildParameter(name="bool", parameter_type=BuildParameterType.Boolean, 
                       default_value=True, description="test boolean")
    ]
    

Building

You have to implement the build function and return an instance of the BuildResponse class. This response has these fields:

• status - an instance of BuildStatus (Success or Error)

  • Specifically, BuildStatus.Success or BuildStatus.Error

• payload - the raw bytes of the finished payload (if you failed to build, set this to None or empty bytes like b'' in Python.

• build_message - any stdout data you want the user to see

• build_stderr - any stderr data you want the user to see

• build_stdout - any stdout data you want the user to see

• updated_filename - if you want to update the filename to something more appropriate, set it here. For example: the user supplied a filename of apollo.exe but based on the build parameters, you're actually generating a dll, so you can update the filename to be apollo.dll. This is particularly useful if you're optionally returning a zip of information so that the user doesn't have to change the filename before downloading. If you plan on doing this to update the filename for a wide variety of options, then it might be best to leave the file extension field in your payload type definition blank "" so that you can more easily adjust the extension.

The most basic version of the build function would be:

async def build(self) -> BuildResponse:
        # this function gets called to create an instance of your payload
        return BuildResponse(status=BuildStatus.Success)

Once the build function is called, all of your BuildParameters will already be verified (all parameters marked as required will have a value of some form (user supplied or default_value) and all of the verifier functions will be called if they exist). This allows you to know that by the time your build function is called that all of your parameters are valid.

Your build function gets a few pieces of information to help you build the agent (other than the build parameters):

From within your build function, you'll have access to the following pieces of information:

• self.uuid - the UUID associated with your payload

  • This is how your payload identifies itself to Mythic before getting a new Staging and final Callback UUID

• self.commands - a wrapper class around the names of all the commands the user selected.

  • Access this list via self.commands.get_commands()

for cmd in self.commands.get_commands():
    command_code += open(self.agent_code_path / "{}.js".format(cmd), 'r').read() + "\n"

• self.agent_code_path - a pathlib.Path object pointing to the path of the agent_code directory that holds all the code for your payload. This is something you pre-define as part of your agent definition.

  • To access "test.js" in that "agent_code" folder, simply do: f = open(self.agent_code_path / "test.js", 'r').

  • With pathlib.Path objects, the / operator allows you to concatenate paths in an OS agnostic manner. This is the recommended way to access files so that your code can work anywhere.

• self.get_parameter("parameter name here")

  • The build parameters that are validated from the user. If you have a build_parameter with a name of "version", you can access the user supplied or default value with self.get_parameter("version")

• self.selected_os - This is the OS that was selected on the first step of creating a payload

• self.c2info - this holds a list of dictionaries of the c2 parameters and c2 class information supplied by the user. This is a list because the user can select multiple c2 profiles (maybe they want HTTP and SMB in the payload for example). For each element in self.c2info, you can access the information about the c2 profile with get_c2profile() and access to the parameters via get_parameters_dict(). Both of these return a dictionary of key-value pairs.

• • the dictionary returned by self.c2info[0].get_c2profile() contains the following:

    • name - name of the c2 profile

    • description - description of the profile

    • is_p2p - boolean of if the profile is marked as a p2p profile or not

  • the dictionary returned by self.c2info[0].get_parameters_dict()contains the following:

    • key - value

    • where each key is the key value defined for the c2 profile's parameters and value is what the user supplied. You might be wondering where to get these keys? Well, it's not too crazy and you can view them right in the UI - Name Fields.

    • If the C2 parameter has a value of crypto_type=True, then the "value" here will be a bit more than just a string that the user supplied. Instead, it'll be a dictionary with three pieces of information: value - the value that the user supplied, enc_key - a base64 string (or None) of the encryption key to be used, dec_key - a base64 string (or None) of the decryption key to be used. This gives you more flexibility in automatically generating encryption/decryption keys and supporting crypto types/schemas that Mythic isn't aware of. In the HTTP profile, the key AESPSK has this type set to True, so you'd expect that dictionary.

    • If the C2 parameter has a type of "Dictionary", then things are a little different.

      • Let's take the "headers" parameter in the http profile for example. This allows you to set header values for your http traffic such as User-Agent, Host, and more. When you get this value on the agent side, you get an array of values that look like the following: {"User-Agent": "the user agent the user supplied", "MyCustomHeader": "my custom value"}. You get the final "dictionary" that's created from the user supplied fields.

  • One way to leverage this could be:

for c2 in self.c2info:
    c2_code = ""
    try:
        profile = c2.get_c2profile()
        c2_code = open(
            self.agent_code_path
            / "c2_profiles"
            / "{}.js".format(profile["name"]),
            "r",
        ).read()
        for key, val in c2.get_parameters_dict().items():
            if key == "AESPSK":
                c2_code = c2_code.replace(key, val["enc_key"] if val["enc_key"] is not None else "")
            elif not isinstance(val, str):
                c2_code = c2_code.replace(key, json.dumps(val))
            else:
                c2_code = c2_code.replace(key, val)
    except Exception as p:
        build_msg += str(p)
        pass

Finally, when building a payload, it can often be helpful to have both stdout and stderr information captured, especially if you're compiling code. Because of this, you can set the build_essage ,build_stderr , and build_stdout fields of the BuildResponse to have this data. For example:

    async def build(self) -> BuildResponse:
        # this function gets called to create an instance of your payload
        resp = BuildResponse(status=BuildStatus.Success)
        # create the payload
        build_msg = ""

        #create_payload = await MythicRPC().execute("create_callback", payload_uuid=self.uuid, c2_profile="http")
        try:
            command_code = ""
            for cmd in self.commands.get_commands():
                try:
                    command_code += (
                            open(self.agent_code_path / "{}.js".format(cmd), "r").read() + "\n"
                    )
                except Exception as p:
                    pass
            base_code = open(
                self.agent_code_path / "base" / "apfell-jxa.js", "r"
            ).read()
            await SendMythicRPCPayloadUpdatebuildStep(MythicRPCPayloadUpdateBuildStepMessage(
                PayloadUUID=self.uuid,
                StepName="Gathering Files",
                StepStdout="Found all files for payload",
                StepSuccess=True
            ))
            base_code = base_code.replace("UUID_HERE", self.uuid)
            base_code = base_code.replace("COMMANDS_HERE", command_code)
            all_c2_code = ""
            if len(self.c2info) != 1:
                resp.build_stderr = "Apfell only supports one C2 Profile at a time"
                resp.set_status(BuildStatus.Error)
                return resp
            for c2 in self.c2info:
                c2_code = ""
                try:
                    profile = c2.get_c2profile()
                    c2_code = open(
                        self.agent_code_path
                        / "c2_profiles"
                        / "{}.js".format(profile["name"]),
                        "r",
                    ).read()
                    for key, val in c2.get_parameters_dict().items():
                        if key == "AESPSK":
                            c2_code = c2_code.replace(key, val["enc_key"] if val["enc_key"] is not None else "")
                        elif not isinstance(val, str):
                            c2_code = c2_code.replace(key, json.dumps(val))
                        else:
                            c2_code = c2_code.replace(key, val)
                except Exception as p:
                    build_msg += str(p)
                    pass
                all_c2_code += c2_code
            base_code = base_code.replace("C2PROFILE_HERE", all_c2_code)
            await SendMythicRPCPayloadUpdatebuildStep(MythicRPCPayloadUpdateBuildStepMessage(
                PayloadUUID=self.uuid,
                StepName="Configuring",
                StepStdout="Stamped in all of the fields",
                StepSuccess=True
            ))
            resp.payload = base_code.encode()
            if build_msg != "":
                resp.build_stderr = build_msg
                resp.set_status(BuildStatus.Error)
            else:
                resp.build_message = "Successfully built!\n"
        except Exception as e:
            resp.set_status(BuildStatus.Error)
            resp.build_stderr = "Error building payload: " + str(e)
        return resp

Depending on the status of your build (success or error), either the message or build_stderr values will be presented to the user via the UI notifications. However, at any time you can go back to the Created Payloads page and view the build message, build errors, and build stdout for any payload.

When building your payload, if you have to modify files on disk, then it's helpful to do this in a "copy" of the files. You can make a temporary copy of your code and operate there with the following sample:

agent_build_path = tempfile.TemporaryDirectory(suffix=self.uuid)
# shutil to copy payload files over
copy_tree(self.agent_code_path, agent_build_path.name)
# now agent_build_path.name maps to the root folder for your agent code

Build Steps

The last thing to mention are build steps. These are defined as part of the agent and are simply descriptions of what is happening during your build process. The above example makes some RPC calls for SendMythicRPCPayloadUpdatebuildStep to update the build steps back to Mythic while the build process is happening. For something as fast as the apfell agent, it'll appear as though all of these happen at the same time. For something that's more computationally intensive though, it's helpful to provide information back to the user about what's going on - stamping in values? obfuscating? compiling? more obfuscation? opsec checks? etc. Whatever it is that's going on, you can provide this data back to the operator complete with stdout and stderr.

Execution flow

So, what's the actual, end-to-end execution flow that goes on? A diagram can be found here: #what-happens-for-building-payloads.

1. PayloadType container is started, it connects to Mythic and sends over its data (by parsing all these python files)

2. An operator wants to create a payload from it, so they go to "Create Components" -> "Create Payload"

3. The operator selects an OS type that the agent supports (ex. Linux, macOS, Windows)

4. The operator selects all c2 profiles they want included

   1. and for each c2 selected, provides any c2 required parameters

5. The operator selects this payload type

6. The operator fills out/selects all of the payload type's build parameters

7. The operator selects all commands they want included in the payload

8. Mythic takes all of this information and sends it to the payload type container

The container sends the BuildResponse message back to the Mythic server.