Full Report
Introduction From time to time I like to delve into malware analysis as a pastime and post interesting examples, and recently we received a malware sample that had a low-detection rate. Anti-Virus coverage was 15/43 (35.7%) based on a virustotal.com report and Norman sandbox did not detect any suspicious activity as shown in the report below: Norman sandbox report did not show any registry or network activity. This might be due to the use of virtual CPU or sandbox bypass techniques by the malware. Sunbelt sandbox was down at the time of the analysis.
Analysis Summary
# Tool/Technique: UDP Worm (Unnamed Variant)
## Overview
This is an analysis of a previously unseen malware sample, referred to here as a UDP Worm, characterized by low initial antivirus detection rates and sophisticated techniques to evade automated sandbox analysis, including potential virtual CPU evasion and code injection. It utilizes UDP for command and control communication.
## Technical Details
- Type: Malware (Worm/Botnet component)
- Platform: Windows (Inferred from registry paths and API usage like `Winlogon`, `VirtualAlloc`, `CreateRemoteThread`)
- Capabilities: Persistence via registry modification, C2 communication over UDP (custom protocol), sandbox evasion, code injection, URL redirection, USB infection, file download/execution, and cookie theft.
- First Seen: Prior to October 25, 2010 (Date of analysis publication).
## MITRE ATT&CK Mapping
- TA0001 - Initial Access
- T1078.001 - Valid Accounts: Local Accounts (Implied, as it copies to current user's AppData)
- TA0003 - Persistence
- T1547.001 - Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder
- TA0005 - Defense Evasion
- T1497.003 - Virtualization/Sandbox Evasion: System Checks (Implied via sandbox bypass)
- T1027 - Obfuscated Files or Information (Implied through packed/scrambled traffic and decryption)
- TA0011 - Command and Control
- T1071.004 - Application Layer Protocol: Generic Protocols (Uses custom UDP protocol)
- TA0008 - Lateral Movement
- T1021.007 - Remote Services: Server Message Block (SMB) (Implied if spreading via network shares, but primary lateral mechanism detailed is USB infection)
- TA0010 - Data Exfiltration
- T1041 - Exfiltration Over C2 Channel (Implied via cookie dumping)
## Functionality
### Core Capabilities
- **Persistence:** Copies itself to `%AppData%` and establishes persistence by adding a registry entry to `HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Taskman`, pointing to `%AppData%\ygmdrm.exe`.
- **C2 Communication:** Communicates via scrambled/encrypted UDP traffic on port 14000. The initial packets involve sequence number exchange (connect requests, command code 0x18).
- **Code Execution:** Injects secondary stage code into the `explorer.exe` process using `VirtualAlloc` and `CreateRemoteThread`.
### Advanced Features
- **Sandbox Evasion (GDI Calls):** Uses "indirect execution delays through crafted calls to GDI functions" to evade heuristic detection systems (virtual CPUs) that look for suspicious patterns in direct execution flow.
- **C2 Decryption/Decompression:** Implements a multi-stage decryption mechanism for received commands:
1. XOR decryption based on the formula: $D(\text{buff}[n]) = \text{buff}[n] \oplus (\text{buff}[n-1] \cdot 2^{(n-1 \;\text{AND}\; 3)})$.
2. Subsequent decompression of the decrypted buffer.
- **Remote Command Execution:** Capabilities gathered from string analysis include:
1. **Browser Homepage Redirection:** Commands specifically instruct the bot to change the victim's browser startup page to `"http://www.juniormind.com/"` (likely for SEO spam).
2. **USB Worm:** Capable of infecting USB drives via creating an `autorun.inf`.
3. **Download and Execute:** Ability to fetch and run secondary payloads.
4. **Cookie Theft:** Remote extraction of Firefox cookies.
## Indicators of Compromise
- File Hashes:
- ViralTotal Sample Hash: `3498a24cee736e4d0bb5969afa37e9e095a511229ae000f145937f77d957ec82` (SHA1 equivalent, linked from VT report)
- File Names:
- `%AppData%\ygmdrm.exe`
- Registry Keys:
- Key: `HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Taskman`
- Value: `%AppData%\ygmdrm.exe`
- Network Indicators:
- C2 Ports: 14000 (UDP)
- C2 Servers: `jebena[.]ananikolic[.]su`, `peer[.]pickeklosarsk[.]ru`, `teske[.]pornicarke[.]com`, `juice[.]losmibracala[.]org`, `92[.]241[.]190[.]237`
- Command Payload URL: `http://www[.]juniormind[.]com/`
- Behavioral Indicators:
- Crafted, delayed execution calls to GDI functions during initial analysis.
- Memory allocation via `VirtualAlloc` followed by execution via `call eax`.
- Code injection into `explorer.exe` via `CreateRemoteThread`.
- Initial UDP packets starting with `0x18` command code.
## Associated Threat Actors
- Not explicitly named in the article, but the sophisticated evasion techniques suggest a professional adversary, potentially engaged in large-scale operations (given the SEO redirection command).
## Detection Methods
- Signature-based detection: Low (35.7% coverage at the time of analysis).
- Behavioral detection: Failed against Norman Sandbox, suggesting standard behavioral heuristics were successfully evaded by GDI-based delays.
- YARA rules: Not provided.
## Mitigation Strategies
- **Prevention:** Implementing strict application whitelisting to prevent execution of unauthorized executables from `%AppData%`.
- **Hardening Recommendations:**
- Disable or monitor changes to the `Taskman` value under `Winlogon` keys, as this is a common persistence mechanism.
- Implement network segmentation and egress filtering to restrict connections to dynamic or unusual C2 ports (like UDP 14000).
- Deploy advanced EDR solutions capable of detecting complex code injection techniques (`CreateRemoteThread` into critical processes like explorer.exe).
- Scan USB storage devices before allowing files to execute, mitigating the `autorun.inf` vector.
## Related Tools/Techniques
- Other malware utilizing GDI function calls for sandbox evasion.
- Malware employing process injection into `explorer.exe` for stealth.
- Other custom UDP-based C2 protocols.