The main difference between submitting a response with a post_response and submitting responses with get_tasking is that in a get_tasking message with a responses key, you’ll also get back additional tasking that’s available. With a post_response message and a responses key, you won’t get back additional tasking that’s ready for your agent. You can still get socks, rpfwd, interact, and delegates messages as part of your message back from Mythic, but you won’t have a tasks key.

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Message Request

The contents of the JSON message from the agent to Mythic when posting tasking responses is as follows: 
Base64( CallbackUUID + JSON(
{
	"action": "post_response",
	"responses": [
		{
			"task_id": "uuid of task",
			... response message (see below)
		},
		{
			"task_id": "uuid of task",
			... response message (see below)
		}
	], //if we were passing messages on behalf of other agents
	"delegates": [
		{"message": agentMessage, "c2_profile": "ProfileName", "uuid": "uuid here"},
		{"message": agentMessage, "c2_profile": "ProfileName", "uuid": "uuid here"}
		]
}
)
)
There are two things to note here:
  • responses - This parameter is a list of all the responses for each tasking.
    • For each element in the responses array, we have a dictionary of information about the response. We also have a task_id field to indicate which task this response is for. After that though, comes the actual response output from the task.
      • If you don’t want to hook a certain feature (like sending keystrokes, downloading files, creating artifacts, etc), but just want to return output to the user, the response section can be as simple as: {"task_id": "uuid of task", "user_output": "output of task here"}
      • You can find many fields to send in the hooking features section, but outside of that you can set:
        • completed - boolean field to indicate that the task is done or not
        • status - string field to indicate the current status of the task. If the task completes successfully, you can set this to success, otherwise you can use it to indicate a generic error mesage to the user. If you start the status with error: then in the Mythic UI that status message will turn red to help indicate an error. Any other status you set will appear as blue text.
If you want to return a more verbose error message, then you can set completed: true, status: "error: auth failed, and then user_output: "some more complex output that displays in the body of the UI under the task where you can have much more room.
  • Each response style is described in Hooking Features. The format described in each of the Hooking features sections replaces the ... response message piece above
    • To continue adding to that JSON response, you can indicate that a command is finished by adding "completed": true or indicate that there was an error with "status": "error".
  • delegates - This parameter is not required, but allows for an agent to forward on messages from other callbacks. This is the peer-to-peer scenario where inner messages are passed externally by the egress point. Each of these messages is a self-contained “Agent Message”.
Anything you put in user_output will go directly to the user to see. There’s no additional processing that happens. If you want to perform additional processing on the response, then instead of user_output use the process_response key. This will allow you to perform additional processing on whatever is passed through the process_response key - from here, if you want to register something for the user to see, you’ll need to use MythicRPCCreateResponse (you can use any MythicRPC at this point to register files, create credentials, etc).

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Message Response

Mythic responds with the following message format for post_response requests: 
Base64( CallbackUUID + JSON(
{
	"action": "post_response",
	"responses": [
		{
			"task_id": UUID,
			"status": "success" or "error",
			"error": 'error message if it exists'
		}
	],
	//if we were passing messages on behalf of other agents
	"delegates": [
		{"message": agentMessage, "c2_profile": "ProfileName", "uuid": "uuid here"},
		{"message": agentMessage, "c2_profile": "ProfileName", "uuid": "uuid here"}
		]
}
)
)
If your initial responses array to Mythic has something improperly formatted and Mythic can’t deserialize it into GoLang structs, then Mythic will simply set the responses array going back as empty. So, you can’t always check for a matching response array entry for each response you send to Mythic. In this case, Mythic can’t respond back with task_id in this response array because it failed to deserialize it completely.
There are two things to note here:
  • responses - This parameter is always a list and contains a success or error + error message for each task that was responded to.
  • delegates - This parameter contains any responses for the messages that came through in the first message
This message format also can take in socks, rpfwd, interact, alerts, edges, and delegates keys with their data as well. Just like with the get_tasking message, you can send all of that data along with each message.