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Introduction:
The Huntress Security Operations Center (SOC) has recently detected active exploitation of three novel techniques – BlueHammer, RedSun, and UnDefend – originating from the “Nightmare-Eclipse” framework. These methods leverage low-privilege user directories to stage malicious binaries, renaming them to evade detection while executing post‑enumeration commands like `whoami /priv` and cmdkey /list. This article dissects the observed attack chain, provides hands‑on detection and hardening steps across Windows and Linux environments, and offers actionable training insights for security teams.
Learning Objectives:
- Understand the execution flow and indicators of compromise (IoCs) for BlueHammer, RedSun, and UnDefend techniques.
- Learn how to detect similar exploitation attempts using native Windows commands, PowerShell, and Sysmon.
- Implement defensive controls, including advanced Windows Defender configurations, application whitelisting, and privilege auditing.
You Should Know:
- Anatomy of the Attack: From Pictures Folder to Quarantine
The observed intrusions begin with a threat actor placing binaries in low‑privilege user directories:
– `C:\Users\
\Pictures\FunnyApp.exe` (April 10 – blocked as <code>Exploit:Win32/DfndrPEBluHmrBZ</code>)
- `C:\Users\[bash]\Downloads\[two‑letter folder]\RedSun.exe` (April 16 – triggered EICAR alert)
- Later renamed to `z.exe` before execution.
The actor first runs enumeration commands to assess privilege and credential exposure. Use the following commands on a live Windows system to check for similar artifacts:
[bash]
Find any FunnyApp.exe, RedSun.exe, or z.exe in user-writable paths
dir /s C:\Users\Pictures\FunnyApp.exe
dir /s C:\Users\Downloads\RedSun.exe
dir /s C:\Users\z.exe
Check for recently created two-letter subfolders in Downloads
dir /ad C:\Users\Downloads\??
Review Windows Defender protection history for the specific detection IDs
Get-MpThreatDetection | Where-Object {$<em>.ThreatID -eq "Exploit:Win32/DfndrPEBluHmrBZ" -or $</em>.Resources -like "FunnyApp" -or $_.Resources -like "RedSun"}
Step‑by‑step guide to detect and respond:
- Run the above `dir` commands across all endpoints via remote administration or RMM.
- Export Defender logs with
Get-MpThreatDetection | Export-Csv defender_alerts.csv. - If any binary is found, isolate the host (e.g.,
Set-NetFirewallProfile -All -Enabled False) and capture memory with `dumpit.exe` orwinpmem. - Submit suspicious files to a sandbox (e.g., Joe Sandbox, ANY.RUN) using their APIs or web UI.
2. Enumerating Privilege Leaks: The Threat Actor’s Checklist
Before dropping the final payload, the actor used whoami /priv, cmdkey /list, and net group. These commands reveal:
– Enabled privileges (e.g., SeDebugPrivilege, SeTakeOwnershipPrivilege) that can be abused for token manipulation.
– Stored credentials via `cmdkey /list` (Windows Credential Manager).
– Domain group memberships with `net group` (or net localgroup).
Step‑by‑step guide to audit and harden:
- Monitor for suspicious enumeration: Create a PowerShell script to log execution of these commands.
Windows Event Log monitoring for whoami, cmdkey, net group
$filter = @{
LogName = 'Microsoft-Windows-Sysmon/Operational'
ID = 1 Process creation
}
Get-WinEvent -FilterHashtable $filter | Where-Object {
$<em>.Message -match 'whoami|cmdkey.\/list|net group'
} | Select-Object TimeCreated, @{n='CommandLine';e={$</em>.Properties[bash].Value}}
- Remove unnecessary privileges: For service accounts and standard users, use `secedit` or Local Security Policy to strip high‑integrity privileges.
- Clear cached credentials (where safe): `cmdkey /delete:TargetName` and disable Credential Manager via GPO: `Computer Config > Admin Templates > System > Credentials Delegation > Allow delegating saved credentials` – set to disabled.
- Restrict net group execution using AppLocker: create a rule to block `net.exe` and `net1.exe` for non‑admin users.
-
Linux Parallels: Adapting the Kill Chain to Cross‑Platform Defenses
Although the attack targets Windows, similar TTPs apply to Linux systems (e.g., staged payloads in `~/Pictures/` or ~/Downloads/, enumeration with id -a, sudo -l, find / -perm -4000 2>/dev/null). Use the following commands to hunt for suspicious activity on Linux:
Find recently created executables in user directories
find /home -type f -executable -mtime -7 -path "/Pictures/" -o -path "/Downloads/" 2>/dev/null
Check for renamed binaries (e.g., a two-letter name)
find /home -type f -executable -regextype posix-egrep -regex "./[a-z]{2}.exe" -ls
Monitor process execution for enumeration patterns
auditctl -a always,exit -F arch=b64 -S execve -k enum_commands
ausearch -k enum_commands -c whoami --format text
Step‑by‑step guide for Linux hardening:
1. Disable `sudo` for non‑admin users via `/etc/sudoers`.
2. Use `systemd‑analyze security` to review unit files.
- Deploy Osquery to monitor file writes to low‑privilege folders and process creation for
whoami,id,sudo -l. - Implement mandatory access control with AppArmor or SELinux (e.g., `aa-genprof` for user binaries).
-
API Security & Cloud Hardening: Preventing Payload Staging via Sync Tools
Threat actors often abuse cloud sync clients (OneDrive, Google Drive, Dropbox) to drop payloads into `Downloads` or `Pictures` folders. This technique bypasses web filters because the file syncs over legitimate API calls.
Step‑by‑step guide to secure cloud sync:
- Audit connected apps in Microsoft 365 (Graph API):
Connect-MgGraph -Scopes "Application.Read.All", "Policy.Read.All" Get-MgApplication | Where-Object {$<em>.DisplayName -like "Drive" -or $</em>.DisplayName -like "Sync"} - Restrict file types synced to endpoints via Cloud App Security policies (e.g., block `.exe` from OneDrive to local
Downloads). - Enable Defender for Cloud Apps anomaly detection for mass download or unusual folder writes.
- Use Windows Defender Attack Surface Reduction (ASR) rules:
– Rule ID `D4F940AB-401B-4EFC-AADC-AD5F3C50688A` (Block executable files from running unless they meet prevalence, age, or trusted list criteria).
– Rule ID `92E97FA1-2EDF-4476-BDD6-9DD0B4DDDC7B` (Block process creations originating from PSExec and WMI commands).
- Vulnerability Exploitation & Mitigation: Why No CVE (Yet)?
As noted in the LinkedIn comments, these techniques (BlueHammer, RedSun, UnDefend) currently lack a CVE identifier, yet they are being actively used in the wild. This highlights the importance of behavior‑based detection over signature‑only approaches.
Step‑by‑step guide to emulate and test your defenses:
- Download proof‑of‑concept tools from the original GitHub repositories (search
Nightmare-Eclipse/BlueHammer,RedSun, `UnDefend` – exercise caution in isolated lab). - Run the attack simulation inside a Windows 10/11 VM with Defender enabled.
3. Observe the detection:
- BlueHammer should trigger
Exploit:Win32/DfndrPEBluHmrBZ. - RedSun should generate an EICAR test file alert.
- Test your EDR/SIEM correlation rules using the following Sigma rule snippet:
title: BlueHammer Execution Attempt status: experimental logsource: product: windows service: sysmon detection: selection: EventID: 11 FileCreate TargetFilename|contains: - '\Pictures\FunnyApp.exe' - '\Downloads\RedSun.exe' - '\z.exe' condition: selection
- Mitigation: Deploy Windows Defender `Set-MpPreference -AttackSurfaceReductionRules_Ids` with the ASR rules listed above, and block execution from `%USERPROFILE%\Pictures` and `%USERPROFILE%\Downloads` via AppLocker or WDAC (Windows Defender Application Control).
6. Training Courses & IR Playbook Development
To build team readiness against such emerging threats, integrate the following training modules:
– SANS FOR500: Windows Forensic Analysis – covers binary staging and privilege enumeration artifacts.
– Huntress free IR training (available on their website) – real‑world SOC case studies.
– MITRE ATT&CK mapping: T1087 (Account Discovery), T1055 (Process Injection), T1105 (Ingress Tool Transfer).
Step‑by‑step internal exercise:
- Create a lab with a domain‑joined Windows 10 VM.
2. Simulate the attack using the PoC binaries.
- Have trainees collect logs with KAPE (
kape.exe --tsource C: --tdest D:\output --targets WindowsLogs) and parse with Chainsaw or DeepBlueCLI. - Write a one‑page incident response playbook titled “Suspicious Binary in User Folder.”
What Undercode Say:
- Proactive hunting beats reactive patching. The absence of a CVE didn’t stop Huntress from detecting BlueHammer and RedSun – behavior analysis (file writes to unusual folders, followed by enumeration commands) is the key.
- Low‑privilege folders are the new beachhead. Attackers know that `Pictures` and `Downloads` are often excluded from strict execution policies. Enforce “block executable from Downloads” via ASR rule
D4F940AB-401B-4EFC-AADC-AD5F3C50688A. - Enumeration commands are your canary. Use Sysmon Event ID 1 to alert on `whoami /priv` and `cmdkey /list` – these are rarely run by legitimate users. Pair with UEBA to reduce noise.
Prediction:
Over the next 12 months, we will see a surge in “fileless” variants of BlueHammer and RedSun that live entirely in registry run keys or WMI event subscriptions, bypassing file‑based detection. Additionally, threat actors will begin abusing cloud storage APIs to directly inject malicious binaries into user folders without touching the endpoint’s browser. Organizations that fail to implement ASR rules and user‑space audit policies will face repeated compromises, while those adopting MITRE ATT&CK‑based behavioral detection will stay ahead. The Huntress SOC’s disclosure sets a new standard for transparency – expect more vendors to publish real‑time attack telemetry instead of waiting for CVEs.
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Reported By: The Huntress – Hackers Feeds
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