Full Report
In this blog, we dissect a sample of malware that makes clever use of batch scripting obfuscation—turns out it was a launcher for TrickBot!
Analysis Summary
# Tool/Technique: DOS Obfuscation (TrickBot Launcher)
## Overview
This technique involves using highly convoluted CMD/Batch scripting to mask malicious commands. By leveraging environment variable manipulation and string concatenation, threat actors can bypass static signature-based detection. In this specific case, the technique was used to deliver and execute a variant of the **TrickBot** malware.
## Technical Details
- **Type:** Malware Launcher / Obfuscation Technique
- **Platform:** Windows (cmd.exe)
- **Capabilities:** Anti-analysis, signature evasion, automated payload execution, environment variable manipulation.
- **First Seen:** November 2020 (Specific variant discussed).
## MITRE ATT&CK Mapping
- **TA0005 - Defense Evasion**
- **T1027 - Obfuscated Files or Information**
- **T1027.010 - Command Obfuscation**
- **TA0002 - Execution**
- **T1059.003 - Windows Command Shell**
## Functionality
### Core Capabilities
- **Variable Expansion:** The script defines short variable names (e.g., `set wjdk=set`) to recursively build further commands, hiding the actual intent of the script from text-based scanners.
- **String Fragmentation:** Commands like `C:\Users\` and `AppData` are broken into small, meaningless chunks (e.g., `Use`, `st`, `ar`) and reassembled only at runtime.
- **Payload Launching:** The final reassembled string executes the TrickBot binary, typically hidden within deep directory structures.
### Advanced Features
- **Environment Variable Abuse:** Uses `%variable%` substitution to create a "custom language" within the batch file, making manual de-obfuscation time-consuming.
- **Dynamic Command Execution:** The script does not contain hardcoded malicious strings; it constructs them in memory and executes them using the command interpreter.
## Indicators of Compromise
- **File Hashes:** *(Note: Specific hashes for this sample were not provided in the snippet, but it is associated with TrickBot "v100" samples from late 2020).*
- **File Names:** Frequently uses `.bat` or `.cmd` extensions with randomized or administrative-sounding names.
- **Registry Keys:** TrickBot commonly targets `HKCU\Software\Microsoft\Windows\CurrentVersion\Run` for persistence.
- **Network Indicators:**
- C2 communications typically involve hardcoded IP addresses (e.g., `185[.]142[.]99[.]x`).
- Requests often target `/mor60`, `/red6`, or `/tst` URI paths.
- **Behavioral Indicators:**
- `cmd.exe` spawning with an unusually long command line containing multiple `%` signs.
- `cmd.exe` creating executable files in `%AppData%` or `%Temp%` subdirectories.
## Associated Threat Actors
- **Wizard Spider** (The primary group behind TrickBot)
## Detection Methods
- **Signature-based detection:** While difficult, signatures can target the specific patterns of variable setting (e.g., `set %var%=set`).
- **Behavioral detection:** Monitor for `cmd.exe` processes that exhibit high entropy in their command-line arguments or frequent use of environment variable expansion to execute hidden commands.
- **YARA Rule Concept:**
yara
rule TrickBot_Batch_Obfuscation {
strings:
$set = "set "
$var = /%[a-z]{4,8}%/
condition:
filesize < 10KB and #set > 20 and #var > 20
}
## Mitigation Strategies
- **Prevention measures:** Implement Block rules for suspicious batch files in user-writable directories (e.g., `%AppData%`).
- **Hardening recommendations:** Use AppLocker or Windows Defender Application Control (WDAC) to restrict the execution of scripts to signed or approved locations.
- **Monitoring:** Enable "Command Line Process Auditing" (Event ID 4688) to capture the de-obfuscated commands at the moment of execution.
## Related Tools/Techniques
- **Invoke-DOSfuscation:** A framework by Daniel Bohannon that automates these types of CMD obfuscation techniques.
- **Gootloader:** Often uses similar multi-layer script obfuscation.
- **PowerShell Obfuscation:** A similar conceptual approach but applied to the `.ps1` language.