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Introduction:
A novel file format dubbed “CaRT” (Compressed and RC4 Transport) is being weaponized by threat actors to cloak malware, bypassing security solutions that fail to recognize its magic number. Developed by Canada’s CSE and utilized by the Thorium platform, this format encrypts files to prevent execution or quarantine, requiring a specific Python tool for decryption back into a dangerous executable.
Learning Objectives:
- Identify the CaRT file format by its “CART” magic header and understand its purpose in malware transport.
- Utilize the `cart` Python library to safely decrypt and analyze a malicious payload contained within a CaRT file.
- Implement defensive strategies to detect and block CaRT-encapsulated malware within your enterprise environment.
You Should Know:
1. What is the CaRT File Format?
The CaRT (Compressed and RC4 Transport) format is a specialized encapsulation mechanism designed to securely store and transfer files. Its primary legitimate use is within the Thorium platform, where it converts uploaded files into this encrypted format to prevent accidental execution or analysis by intermediary systems. However, this very property has been co-opted by malicious actors. The format combines compression with RC4 stream cipher encryption, creating a blob that appears inert to most security tools. The most immediate identifier is the file’s “magic number”—the first four bytes of the file, which are `43 41 52 54` in hex, corresponding to the ASCII string “CART”. This replaces the expected header of the original file (e.g., `MZ` for a Windows PE executable).
Step-by-step guide explaining what this does and how to use it.
Step 1: Initial File Identification. When you encounter a suspicious file, the first step is to check its header. On a Linux system, use the `file` command or a hex viewer.
Command: `file suspicious_document.pdf.exe`
Expected Output if it’s a CaRT file: `suspicious_document.pdf.exe: data` (The `file` command won’t recognize it).
Command: `xxd -l 4 suspicious_document.pdf.exe`
Expected Output: `00000000: 4341 5254 CART`
Step 2: Understand the Obfuscation. A security product scanning the file will see the “CART” header, not the underlying “MZ” header of a Windows executable or the `%PDF` of a document. Since it’s not a recognized executable format and is encrypted, the security product may ignore it, allowing it to pass through undetected.
Step 3: The Attacker’s Final Step. The recipient (the victim) is instructed or socially engineered to “decrypt” the file using the `cart` Python package, which restores the original, fully functional malware.
- How to Unpack a CaRT File for Analysis
To analyze the malicious payload hidden inside a CaRT file, security analysts and incident responders must first unpack it. This requires the official `cart` Python library, which contains the necessary decryption routines.
Step-by-step guide explaining what this does and how to use it.
Step 1: Isolate the File. Perform all analysis in a secure, isolated malware analysis sandbox or virtual machine with no network access.
Step 2: Install the `cart` Python Package. The tool is publicly available via the Python Package Index (PyPI).
Command: `pip install cart`
Step 3: Execute the Unpacking Tool. The `cart` package installs a command-line utility. Use it to unpack the suspicious file.
Command: `cart unpack suspicious_document.pdf.exe`
This command will extract the original file, typically with a `.decrypted` extension or its original internal name.
Step 4: Analyze the Decrypted Payload. You can now analyze the resulting file with standard security tools.
Command: `file suspicious_document.pdf.exe.decrypted`
Expected Output: `suspicious_document.pdf.exe.decrypted: PE32 executable (GUI) Intel 80386, for MS Windows`
This file can now be scanned by antivirus, analyzed in a disassembler, or detonated in a controlled sandbox.
3. Building Defenses: Signature and Heuristic Detection
While the CaRT format itself is uncommon, its use in attacks means defenders must proactively create detection mechanisms. Relying on a multi-layered defense is crucial.
Step-by-step guide explaining what this does and how to use it.
Step 1: Create File Signature Rules. Use YARA, a powerful pattern-matching tool, to scan for the CaRT magic header.
Create a YARA rule file named `cart_malware.yar`:
rule CaRT_Encapsulated_File
{
meta:
description = "Detects files with the CaRT (Compressed and RC4 Transport) magic header"
author = "Your DFIR Team"
date = "2023-10-27"
strings:
$magic = { 43 41 52 54 } // "CART" in hex
condition:
$magic at 0
}
Command to scan a directory: `yara -r cart_malware.yar /path/to/scanned/directory`
Step 2: Implement Network Traffic Analysis. The “CART” string is plaintext at the beginning of the file. If such files are being downloaded over HTTP/S, Intrusion Detection Systems (IDS) like Suricata can be configured to alert on the transfer.
Example Suricata rule:
`alert http $HOME_NET any -> $EXTERNAL_NET any (msg:”SUSPICIOUS – CaRT File Download”; flow:established,from_client; http.response.body; content:”CART”; startswith; depth:4; classtype:trojan-activity; sid:1000001; rev:1;)`
Step 3: Endpoint Detection and Response (EDR) Tuning. Configure your EDR to monitor for the execution of the `cart` command-line tool, as its use on a typical user endpoint is highly suspicious.
4. Advanced Hunting: Detecting the `cart` Tool Execution
The presence or execution of the `cart` Python package on an endpoint is a critical indicator of compromise (IoC). Security teams should hunt for this activity.
Step-by-step guide explaining what this does and how to use it.
Step 1: Windows Event Log Hunting. On Windows systems, you can query for process creation events that involve `cart.exe` or Python scripts running the `cart` module.
Example KQL Query for Microsoft Defender for Endpoint:
DeviceProcessEvents | where FileName =~ "cart.exe" or ProcessCommandLine has "python" and ProcessCommandLine has "cart" | project Timestamp, DeviceName, FileName, FolderPath, ProcessCommandLine
Step 2: Linux Auditd Monitoring. On Linux, you can use Auditd rules to log executions of the `cart` command.
Command to add a rule: `auditctl -a always,exit -F path=/usr/local/bin/cart -F perm=x`
Search the audit logs: `ausearch -k cart_execution`
5. The Bigger Picture: Protecting Against Obfuscation Layers
The CaRT case is not an isolated incident but part of a broader trend of using custom, “exotic” obfuscation to bypass defenses. Defenders must assume that attackers will continuously change their methods.
Step-by-step guide explaining what this does and how to use it.
Step 1: Focus on Behavior, Not Just Signatures. Implement security tools that analyze file behavior during execution (sandboxing) and monitor for malicious activity on the endpoint (EDR), rather than relying solely on static file signatures.
Step 2: Harden User Environments. Restrict the ability to install Python packages from PyPI on standard user workstations through application whitelisting policies (e.g., AppLocker, Windows Defender Application Control).
Example AppLocker PowerShell rule to block `cart.exe`:
`New-AppLockerPolicy -RuleType Path -User Everyone -Action Deny -Path “C:\Users\\AppData\Local\Packages\PythonSoftwareFoundation.Python.\LocalCache\local-packages\Python\Scripts\cart.exe” -XmlVersion 1.0`
Step 3: Security Awareness. Educate users about the dangers of running unknown tools or scripts to “open” or “decrypt” files received from untrusted sources.
What Undercode Say:
- The CaRT format represents a significant evolution in malware delivery, leveraging a legitimate, albeit niche, encryption tool for obfuscation. Its detection bypass is not a flaw in antivirus engines but a fundamental feature of its design.
- The barrier to entry for using this technique is low. Any attacker can `pip install cart` and begin wrapping their payloads, making this an accessible and potent tool for a wide range of threat actors.
Analysis: The emergence of CaRT in the wild underscores a critical shift in the attacker playbook. Instead of developing custom packers from scratch, which can be time-consuming and buggy, actors are increasingly appropriating legitimate, specialized cryptographic tools. This provides them with a robust, tested, and initially undetected method of transport. The initial success of this technique hinges on the lack of widespread awareness and signatures for the “CART” header. While the defense community will quickly adapt by adding signatures for the header and monitoring for the tool, this event serves as a powerful reminder that our defensive perimeters must be built to handle “unknown-unknowns.” Security postures that rely heavily on static indicators of compromise (IoCs) are inherently fragile. A resilient defense requires a multi-layered strategy combining robust IoC scanning, deep behavioral analysis, stringent application control, and continuous user education to counter the ever-expanding toolkit of the modern cyber adversary.
Prediction:
The use of CaRT is a precursor to a wider trend of “legitimate tool abuse” in cyber-espionage and targeted attacks. We predict that state-sponsored and financially motivated actors will increasingly seek out and weaponize other little-known but powerful cryptographic and data transformation libraries from government, academic, and corporate sources. The next wave of such attacks may involve obfuscation formats that are dynamically generated, use less recognizable magic bytes, or require multiple decryption steps with keys fetched from remote command-and-control servers, making static analysis and signature-based detection even more ineffective. The arms race will shift further towards the endpoint, where behavioral detection and application control will become the most critical layers of defense.
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IT/Security Reporter URL:
Reported By: Stephan Berger – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
