ExCobalt: GoRed, the hidden-tunnel technique

Introduction

While responding to an incident at one of our clients, the PT ESC CSIRT team discovered a previously unknown backdoor written in Go, which we attributed to a cybercrime gang dubbed ExCobalt.

ExCobalt focuses on cyberespionage and includes several members active since at least 2016 and presumably once part of the notorious Cobalt gang. Cobalt attacked financial institutions to steal funds. One of Cobalt’s hallmarks was the use of the CobInt tool, something ExCobalt began to use in 2022.

Over the past year, the PT ESC has recorded attacks linked to ExCobalt and investigated related incidents at Russian organizations in the following sectors:

• Metallurgy
• Telecommunications
• Mining
• Information technology
• Government
• Software development

This article discusses ExCobalt’s new tool, GoRed, how it evolved, and some of the tactics, techniques, and procedures that the group has used in its attacks.

The investigation begins

While investigating an incident recorded in March 2024 on one of our client’s Linux hosts, we discovered a file named scrond, compressed with UPX (Ultimate Packer for eXecutables).

The data in an unpacked sample, written in Go, included package paths containing the substring “red.team/go-red/”. This suggests that the sample is a proprietary tool named GoRed, allegedly by a certain Red Team.

Looking into the site, we were unable to find any meaningful connections or malicious activity. The site resembles a typical online business card, no longer actively used by its creators. All the information dates back to 2019, and the design is typical of many similar sites.

As for the GoRed backdoor, the following key features can be identified:

• Operators can connect to the backdoor and execute commands, similar to other C2 frameworks like Cobalt Strike or Sliver.
• GoRed uses the RPC protocol to communicate with its C2 server.
• Operators use DNS/ICMP tunneling, WSS, and QUIC to communicate with GoRed.
• GoRed can obtain credentials from compromised systems.
• It collects various types of information from compromised systems: details of active processes, host names, lists of network interfaces, file system structures, and so on.
• Operators use a variety of commands to conduct reconnaissance on the victim’s network.
• GoRed serializes, encrypts, archives, and sends data it collects to a special server dedicated to storing compromised data.

For a complete technical description of GoRed, see the further section “GoRed analysis”.

First version of GoRed and other malicious tools we found

As we analyzed GoRed, we found that we had already come across several versions of the backdoor while responding to incidents at several of our clients earlier. For example, in July 2023, as we were investigating an incident at a certain company, we discovered several different tools inside the attackers’ directories due to an error they had made, one of tools being the original version of GoRed.

Similarly, in an October 2023 incident, we found further tools inside public directories on the attackers’ network. A short name and description of each tool we found during the investigations are presented in the table below.

In addition to the tools obtained from the threat actors’ server directories, ExCobalt used the following tools:

• Mimikatz
• ProcDump
• SMBExec
• Metasploit
• rsocx

Modified versions of standard utilities

Besides the above tools found in public directories, we came across modified versions of standard Linux utilities in several incidents, something we first saw in November 2023.

The modification of utilities serves two purposes:

1. Modified ps and htop hide an active core process of the gsocket module and related processes in terminal output.

Let’s take ps as an example.

The simple_readproc function was modified.

The original code looks as follows:

The following code was added:

The is_target_proc function checks for every process whether its name is equal to any of the hardcoded names of malicious processes:

If the check returns true, the process will be hidden in the terminal output. Here’s the list of the names of malicious processes hidden this way:

2. Modified ss and netstat hide the active network connection of the core module of gsocket in terminal output.

Let’s take netstat as an example.

The first code modification was made in the main function.

The original code looks as follows:

First, code was added to copy all C2s from the data section to a global structure:

Second modification was in the tcp_do_one function, starting here:

Checks for the name of the malicious process and malicious connection were added to the code. If the check returns true, the process with one of the hardcoded names or network connections will be hidden:

Relation to ExCobalt

In November 2023, we discussed ExCobalt’s attacks on Russian companies.

In that report, we mentioned the domain lib.rpm-bin.link, where upon directory enumeration we found many of the tools described above—including col, the first version of GoRed.

Also, in a March 2024 incident, we observed infected hosts that contacted the threat actor’s servers: get.rpm-bin.link and leo.rpm-bin.link. Additionally, GoRed used a static_TransportConfig structure with the following C2s:

• leo.rpm-bin.link
• sula.rpm-bin.link
• lib.rest
• rosm.pro

In May 2023, Bi.Zone researchers released an analysis of attacks by Sneaking Leprechaun, whose tools showed some overlap with the above-mentioned files found inside the public directories.

Furthermore, our colleagues at Rostelecom-Solar in May 2024 released a report on the Shedding Zmiy activity cluster, which also correlated with ExCobalt. Case 7 in this report described the same attack and a GoRed stealer sample with a C2 at pkg.collect.net.in. This sample was designated as Bulldog Backdoor in the report.

GoRed analysis

Before we proceed to analyzing the current version of GoRed, we will provide a retrospective analysis of its evolution.

Versions we found

All the versions we found are shown in the table below.

This article examines the current version, 0.1.4.

Internal packages

First, we will describe the structure of the internal packages and their purpose to give you an understanding of GoRed functionality. In the backdoor’s data, we have found the following package paths containing the substring “red.team/go-red/”:

Let’s now proceed to analysis proper. Here’s a simplified diagram of the control flow to give you an understanding of how it works.

1. Start of execution

The control flow relies on the command line interface (CLI). However, before passing control to the CLI, the backdoor initializes a number of commands, which are described below.

• service command
• gecko subcommand

The service command, intended for gaining persistence in the system, is initialized first. The GoRed CLI command structure looks as follows:

    
struct cli_Command
{
    string Name;
    _slice_string Aliases;
    string Usage;
    string UsageText;
    string Description;
    string ArgsUsage;
    string Category;
    PTR_cli_BashCompleteFunc BashComplete;
    PTR_cli_BeforeFunc Before;
    PTR_cli_AfterFunc After;
    PTR_cli_ActionFunc Action;
    PTR_cli_OnUsageErrorFunc OnUsageError;
    _slice__ptr_cli_Command Subcommands;
    _slice_cli_Flag Flags;
    cli_FlagCategories flagCategories;
    bool SkipFlagParsing;
    bool HideHelp;
    bool HideHelpCommand;
    bool Hidden;
    bool UseShortOptionHandling;
    string HelpName;
    _slice_string commandNamePath;
    string CustomHelpTemplate;
    cli_CommandCategories categories;
    bool isRoot;
    cli_separatorSpec separator;
};

In terms of command identification, the most helpful fields in this structure are the following:

• Name: command name
• Usage: command description
• Action: function called when the command is executed
• Subcommands: subcommands for the current command

Next, the structure of the CLI itself is initialized into the variable app.

The app structure looks as follows:

    
struct cli_App
{
    string Name;
    string HelpName;
    string Usage;
    string UsageText;
    string ArgsUsage;
    string Version;
    string Description;
    string DefaultCommand;
    _slice__ptr_cli_Command Commands;
    _slice_cli_Flag Flags;
    bool EnableBashCompletion;
    bool HideHelp;
    bool HideHelpCommand;
    bool HideVersion;
    cli_CommandCategories categories;
    cli_FlagCategories flagCategories;
    PTR_cli_BashCompleteFunc BashComplete;
    PTR_cli_BeforeFunc Before;
    PTR_cli_AfterFunc After;
    PTR_cli_ActionFunc Action;
    PTR_cli_CommandNotFoundFunc CommandNotFound;
    PTR_cli_OnUsageErrorFunc OnUsageError;
    PTR_cli_InvalidFlagAccessFunc InvalidFlagAccessHandler;
    time_Time Compiled;
    _slice__ptr_cli_Author Authors;
    string Copyright;
    io_Reader Reader;
    io_Writer Writer;
    io_Writer ErrWriter;
    PTR_cli_ExitErrHandlerFunc ExitErrHandler;
    map_string_interface_ Metadata;
    PTR_func_map_string_string ExtraInfo;
    string CustomAppHelpTemplate;
    string SliceFlagSeparator;
    bool DisableSliceFlagSeparator;
    bool UseShortOptionHandling;
    bool Suggest;
    bool AllowExtFlags;
    bool SkipFlagParsing;
    bool didSetup;
    cli_separatorSpec separator;
    _ptr_cli_Command rootCommand;
};

Here, too, the most informative fields for identifying commands are as follows:

• Name: current command name
• Action: function called
• Commands: subcommands for the current command

The Commands field for the app structure is also initialized.

Next, the Logging field is retrieved from the embedded_Config structure described in the section below. After this, the control flow moves to the CLI.

2. Gaining persistence in the system

The first command to be executed is service. It achieves persistence in the system. It can be executed with the following options:

If there are no options from the table above, it gains persistence as a service with the name it received as an argument. To maintain presence in the compromised system, it creates environment variables whose names begin with “BB”, for example:

• BB_WS
• BB_QUIC
• BB_ICMP
• BB_DNS
• BB_START_DELAY

3. Initializing beacon mode

The control flow is then transferred to the gecko command. This command is the entry point for GoRed to run in beacon mode.

It can be executed with the following options:

Depending on the protocol received as an option, the command fetches the C2 from the transport configuration whose structure is described in the section below. It then begins initializing the beacon functionality.

To identify the victim, the malware first generates an ID, which is an MD5 hash of the computer’s MAC addresses and name, similarly to the case described in our Hellhounds: Operation Lahat article. The resulting hash sum is added to a field in the client structure, which stores all data required for communication with the C2.

4. Establishing an initial connection

After GoRed is initialized, it needs to connect to its C2. The number of connection attempts is defined in the backoff package.

The execution flow calls the function that registers the beacon functionality, after which the CLI commands are initialized.

Registration uses the RPC protocol. Data in the model_Beacon structure is sent to the server, and data in the model_Auth structure is used for authentication with the server.

    
struct model_RegisterBeaconRequest
{
    model_Beacon Beacon;
    model_Auth Auth;
};

struct model_Beacon
{
    string ID;
    string Hostname;
    string ClientIPs;
    _slice_string Tags;
    string OS;
    string Username;
};

struct model_Auth
{
    string Token;
    uuid_UUID ClientID;
    string Transport;
};

The fields in these structures have the following purposes:

After registering, GoRed runs the birdwatch command to monitor the file system.

The execution flow then sets a GoRed beacon mode heartbeat period (for signaling to C2).

The execution flow then runs a command to monitor the password file stored in /etc/shadow/.

Finally, the execution flow initializes all available commands and goes into heartbeat mode.

5. Entering listen and execute mode

At the final stage of initialization, GoRed starts listening for the operator’s commands that it previously initialized. It can execute both system and built-in commands. Commands can be set to run in the background.

6. Communications in beacon mode

GoRed uses the RPC protocol to communicate with its C2 in beacon mode.

It registers a custom codec to communicate via RPC.

The registered codec serializes data with CBOR and encrypts with AES-256-GCM (Secret field in embedded_Config) when sending, and does the reverse when receiving.

Configurations

GoRed contains two configuration blocks: built-in and transport.

Built-in configuration

This is the configuration of GoRed itself. It is encoded in Base64 and serialized with msgpack.

For versions 0.0.23-10-g4528ef3 through 0.1.4, the structure of the built-in configuration is as follows:

    
struct embedded_Config
{
    string Logging;
    string Token;
    uuid_UUID UserID;
    uuid_UUID ClientID;
    string ClientKey;
    string Version;
    _slice_string Tags;
    _slice_string Args;
    string Secret;
    _ptr_url_URL BackendAddress;
    _ptr_url_URL ProxyAddress;
};

The purposes of the built-in configuration fields are as follows:

Configuration structure for versions 0.0.9 through 0.0.13.

    
struct embedded_Config
{
    bool Debug;
    string Token;
    uuid_UUID ClientID;
    string ClientKey;
    string Version;
    _slice_string Tags;
    string Secret;
    string BasicAuthLogin;
    string BasicAuthPass;
    _ptr_url_URL BackendAddress;
    _ptr_url_URL ProxyAddress;
};

The fields in this version of the built-in configuration have the following purposes:

Built-in configuration for version 0.0.1.

    
struct embedded_Config
{
    bool Debug;
    uuid_UUID ClientID;
    string ClientKey;
    string Version;
    _ptr_url_URL BackendAddress;
};

Golang script for getting built-in configuration fields:

    
package main

import (
    "encoding/base64"
    "fmt"
    "net/url"

    "github.com/google/uuid"
    "github.com/vmihailenco/msgpack/v5"
)

type embedded_Config struct {
    Logging        string
    Token          string
    UserID         uuid.UUID
    ClientID       uuid.UUID
    ClientKey      string
    Version        string
    Tags           []string
    Args           []string
    Secret         string
    BackendAddress* url.URL
    ProxyAddress* url.URL
}

const config = `...`

func main() {
    var item map[string]any
    data, _ : = base64.StdEncoding.DecodeString(config)
    err : = msgpack.Unmarshal(data, &item)
    if err != nil{
        panic(err)
    }
    fmt.Print("Logging: ")
    fmt.Println(item["Logging"])
    fmt.Print("Token: ")
    fmt.Println(item["Token"])
    fmt.Print("UserID: ")
    fmt.Println(uuid.UUID(item["UserID"].([]byte)))
    fmt.Print("ClientID: ")
    fmt.Println(uuid.UUID(item["ClientID"].([]byte)))
    fmt.Print("ClientKey: ")
    fmt.Println(item["ClientKey"])
    fmt.Print("Version: ")
    fmt.Println(item["Version"])
    fmt.Print("Tags: ")
    fmt.Println(item["Tags"])
    fmt.Print("Args: ")
    fmt.Println(item["Args"])
    fmt.Print("Secret: ")
    fmt.Println(item["Secret"])
    fmt.Print("BackendAddress: ")
    fmt.Println(string(item["BackendAddress"].([]byte)))
    fmt.Print("ProxyAddress: ")
    fmt.Println(item["ProxyAddress"])
}

Python script for getting built-in configuration fields:

    
import msgpack

import base64

s = base64.b64decode('...')

config = msgpack.unpackb(s, raw = False)

print(config)

Transport configuration

The transport configuration looks as follows:

It has the following structure:

    
struct static_TransportConfig
{
    static_Transport Revsh;
    static_Transport RPC;
    static_Transport Proxy;
};

struct static_Transport
{
    _ptr_static_Address WS;
    _ptr_static_Address QUIC;
    _ptr_static_Address ICMP;
    _ptr_static_Address DNS;
    _ptr_static_Address TCP;
};

struct static_Address
{
    string Domain;
    string BackupIP;
    signed __int64 Port;
    string Proto;
};

The fields of the static_TransportConfig structure have the following purposes:

The fields of the static_Address structure have the following purposes:

Communication protocols

GoRed has several protocols for communicating with the operator.

DNS

DNS tunneling in GoRed can use Base64 or Base32. This option is selected during compilation.

An example of a domain used in an attack is 8E1A4QB4OGA66RPJCHL72DJGCKRMIOR8CDN3EDJBDOOAEQ3FEDQ5UQB4OGA66RP.JCHL6EDJGCKRMIOR8CDN3EDJBDLJG.rosm[.]pro.

Background commands

Background commands run continuously; some of them can be added to the background or removed, depending on the conditions in the table below.

Connecting in rev-proxy and rev-fwd mode

Before starting to act as a server, GoRed needs to initialize an embedded X.509 certificate, similarly to the case described in our Hellhounds: Operation Lahat. Part 2 article.

The backdoor also needs to collect information about the victim’s host by executing the CollectHostInfo function shown above. This produces the structure presented below, except for the Addr field.

    
struct proto_HostInfo
{
    string Addr;
    string OS;
    string Username;
    string Hostname;
    _slice_string IPs;
};

The fields in the structure have the following purposes:

A structure to identify GoRed as a server is initialized as follows:

The initialized structure looks as follows:

    
struct proto_BinInfo
{
    uuid_UUID ClientID;
    string Token;
    _slice_string Tags;
};

The fields in the structure have the following purposes:

Having received the proto_HostInfo and proto_BinInfo structures, GoRed uses their fields in a handshake message that it sends to the C2 at the address it gets from the transport configuration. The handshake message structure looks as follows:

    
struct proto_MsgGreeting
{
    proto_ConnectionMode Mode;
    string MachineID;
    proto_BinInfo BinInfo;
    _ptr_proto_HostInfo HostInfo;
};

The fields in the structure have the following purposes:

The handshake sequence used to register GoRed as a server looks as follows:

In response to the handshake message, the server sends a message with the following structure:

    
struct proto_MsgGreetingResponse
{
    string Greeting;
    _ptr_proto_BinInfo BinInfo;
    _ptr_proto_HostInfo HostInfo;
};

If the Greeting field contains the string “welcome”, the connection is considered successful, and GoRed starts running in server mode; if not, the connection cannot be established.

Issued commands

The operator uses the following commands to communicate with GoRed:

collect

Since this command collects system information and data for subsequent exfiltration, we decided to describe it in more detail:

Collected information

An example of the code used to collect information is shown below.

A complete list of collected information is provided in the table below.

Exfiltration

Before the data is sent, it is serialized with msgpack and encrypted with AES-256-GCM (Secret field in embedded_Config).

Next, after archiving the data, the backdoor sends it with a POST request to a URL generated by appending “/api/collection-result” to the BackendAddress field in embedded_Config.

It is also possible to update the model_CollectionConfig structure by sending a GET request to a URL generated by appending “/api/config” to the BackendAddress field in embedded_Config.

The model_CollectionConfig structure is a configuration for the collect command and has the following fields:

    
struct model_CollectionConfig
{
    _slice__slice_string Commands;
    _slice_string Files;
    _slice_string TreePaths;
};

The fields in the structure have the following purposes:

Conclusion

ExCobalt continues to demonstrate a high level of activity and determination in attacking Russian companies, constantly adding new tools to its arsenal and improving its techniques. Not only is it developing new attack methods, but it’s also actively improving its existing tools, such as the GoRed backdoor.

ExCobalt is apparently aiming for more sophisticated and productive methods of hacking and cyberespionage, seeing how GoRed has been acquiring new capabilities and features. These include expanded functionality for collecting victim data and increased secrecy both inside the infected system and in communications with C2 servers.

In addition, ExCobalt demonstrates flexibility and versatility by supplementing its toolset with modified standard utilities, which help the group to easily bypass security controls and adapt to changes in protection methods. The use of modified utilities is a sign that the members of the group have an in-depth understanding of the weaknesses of companies they attack, while leveraging vulnerabilities helps them to pursue sophisticated attacks on their targets.

Overall, the evolution of ExCobalt and its toolset, including GoRed and the modified utilities, highlights the need for organizations and cybersecurity professionals to continuously improve detection and protection techniques to combat this group as well as other similar cyberthreats.

Source: Original Post