CS-EXTC2-NTP

An NTP tunnel for Cobalt Strike beacons using External-C2

TOC:

• CS-EXTC2-NTP
• Video Demo
• How it works
  • Payload Retrieval (initial)
    • Step-by-step flow diagram
  • Beacon Loop
    • Beacon loop flow diagram
• Extension Fields
  • Base Packet Format
  • Rationale
  • Payload Retrieval & Execution Extension Fields
  • The Beacon Loop Extension Fields

Demo video:

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(Sorry for the quality, github limits to 10 MB, a full quality version is in the repo at misc/CS-EXTC2-NTP_Proof.mp4)

https://github.com/user-attachments/assets/17e7db18-4e81-42e9-93b6-88df9935ab41

How it works

The tunnel itself is fairly simple. Every packet is a normal NTP packet, and all the data hides in extension fields.

Additionally, There are two main jobs the Client and Controller have:

1. Payload Retrieveal: The initial Payload Retirival from the TeamServer
2. The Beacon Loop: The continuous comms between Client, Controller, and TeamServer

Payload Retireval:

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1. Client sends a getIdPacket packet to get a client ID

2. Controller responds with a idPacket packet containing the client ID. Client saves this for all further outbound packets

3. Client sends a packet with a giveMePayload extension.

   1. The data in this packet will either be 0x86, or 0x64, depending on the architecture of the host. This must be manually set in the client, this is not dynamically determined.

4. Controller reaches out to TeamServer to get the payload.

5. In the NTP response (to step 3), Controller returns a packet with a sizePacket extension, denoting the size of the payload.

6. The client initiates chunking by iterating over the inbound payload size, retrieving chunks until the entire payload has been received.

   1. The data in this packet is 0x00, which denotes "keep sending me chunks of the payload"

7. The packet back from the Controller is a dataFromTeamserver packet, which contains a chunk of payload.

8. (not shown below) Once the entire payload has been retrieved, it is in injected into a new thread, and run.

   1. Note: This uses a basic CreateThread injection method. It’s going to be detected — please modify or replace with your preferred technique. (Code is located in injector.cpp)

Beacon Loop:

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1. Client reads beacon data from the named pipe.
2. Client sends a packet with a sizePacket extension, which contains the size of the data retreived from the beacon.
3. Controller responds with sizePacketAcknowledge
4. Client then initiates chunking by iterating over the beacon data size, sending chunks with dataForTeamserver extensions until the entire beacon data has been sent to the Controller
5. Controller sends sizePacketAcknowledge each chunked packet to signify it made it.
6. Once the Controller has all the data, it forwards the data onto the TeamServer.
7. The controller then gets the response of the teamserver.
8. Client then sends a packet with the getDataFromTeamserverSize extension.
9. The Controller responds with the size of the data from the Teamserver, via a packet with a sizePacket extension.
10. Client then initiates chunking by sending a packet with the getDataFromTeamserver extension.
11. The Controller responds with a packet with the dataFromTeamserver extension. This packet contiains a chunk of data of the TeamServers response.
12. Once the Client has all the data, it forwards the data onto the beacon, via the named pipe. It then loops to step 1.

Extension Fields

All the extension fields used in the project.

Base Packet Format:

Before exploring the individual extension fields, it's important to understand their underlying structure.

All Extension Field packets follow this format:

Bytes 0-1: Extension Field ID
Bytes 2-3: Size of extension field data
Bytes 4-7:  Session ID for NTP packet
Bytes 8-X: (optional) Additional Data (buffered with 0x00 to 4 bit boundary)

If we include the 48 byte NTP packet, this turns into:

Bytes 0-47: NTP Packet
Bytes 48-49: Extension Field ID
Bytes 50-51: Size of extension field data
Bytes 52-55:  Session ID for NTP packet
Bytes 56-X: (optional) Additional Data (buffered to 4 bit boundary)

Rationale:

Why extension fields, and why this exact structure? Two reasons:

1. RFC 5905 defines the structure of NTP extension fields

   • RFC 5905 Section 7.5

2. The packets show up as NTP in wireshark (other structs that I tried can throw a malformed packet error)

So, if I want these packets to look legit, they need to be structured as such.

sizePacket

This extension field is used to communicate the size of an inbound message. This is crucial for chunk based communication.

Bytes 0-1: 0x51, 0x2E
Bytes 2-3: Size of Data
Bytes 4-7: Unique ID
Bytes 8-11 Size of total data to be sent 

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sizePacketAcknowledge

An acknowledge packet that is used to say the Controller recieved your message. Should probaly have been named "packetAcknowledge" instead of sizePacketAcknowledge", but it was first used for acknowledging size packets, and I haven't updated it yet. Either way, if/when it gets changed, the header of 0x51, 0x2E will stay the same.

Bytes 0-1: 0x51, 0x2E
Bytes 2-3: Size of Data
Bytes 4-7: Blank (0xFF,0xFF,0xFF,0xFF)

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giveMePayload

Bytes 0-1:  0x00, 0x01
Bytes 2-3: Size of Data
Bytes 4-7: ClientID
Bytes 8: Architechure (0x86, 0x64, or 0x00) 0x00 = continue sending payload, 0x86/0x64 are their own respective arch.

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getIdPacket

Used by the client to get an ID from the Controller.

Bytes 0-1:  0x12, 0x34
Bytes 2-3: Size of Data
Bytes 4-7: ClientID - Blank (0xFF,0xFF,0xFF,0xFF)

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idPacket

Used in response to a getIdPacket from the Controller to give the client an ID. This ID is stored in the Data section of the extension field, NOT in the ClientID field (which is blank).

Bytes 0-1: 0x1D, 0x1D
Bytes 2-3: Size of Data
Bytes 4-7: ClientID - Blank (0xFF,0xFF,0xFF,0xFF)
Bytes 8-11: ClientID for the client. 

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dataFromTeamserver

Used in response from the Controller to tunnel data back in. Contains data that came from the teamserver.

Bytes 0-1: 0x02, 0x04
Bytes 2-3: Size of Data
Bytes 4-7: ClientID - Blank (0xFF,0xFF,0xFF,0xFF)
Bytes 4-end of packet: Chunked data from teamserver

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getDataFromTeamserver

This extension field is used to requset TeamServer data that is stored on the controller. Used in chunking

Bytes 0-1: 0x00, 0x02
Bytes 2-3: Size of Data
Bytes 4-7: Unique ID

dataFromTeamserver

The response from the Controller, with data from the teamserver, meant for the client. Used in Chunking

Bytes 0-1: 0x02, 0x04
Bytes 2-3: Size of Data
Bytes 4-7: Unique ID
Bytes 8-end of packet: Chunked data from the teamserver for the client

getDataFromTeamserverSize

A size packet that is specifically for getting the size of the teamserver data, for the client.

Bytes 0-1: 0x03, 0x04
Bytes 2-3: Size of Data
Bytes 4-7: Unique ID
Bytes 8-11: Size of Teamserver Data for client

dataForTeamserver

Data meant for the teamserver, from the client. Used in chunking

Bytes 0-1: 0x02, 0x05
Bytes 2-3: Size of Data
Bytes 4-7: Unique ID
Bytes 8-end of packet: Chunked data from the client for the teamserver