PowerShell grants administrators immense control over systems, both for good and ill. Execution policies govern what PowerShell code can execute to prevent misuse. However, the Bypass policy disables these defenses entirely…
The Double-Edged Sword of Unrestricted Execution
IT admins rely on PowerShell to automate management of servers, networks, user infrastructure, and more. Developers utilize PowerShell to deploy applications, test systems programmatically, and interface with APIs. It underpins many critical functions.
However, PowerShell’s flexibility is exploitable. Attackers leverage malicious scripts to infiltrate networks via phishing schemes, poisoned repositories, third party library tampering, or by exploiting debugging misconfigurations. PowerShell malware attacks have become ubiquitous:
| Year | # Reported Attacks | Estimated Damages |
|---|---|---|
| 2019 | 4,328 | $2.3 billion |
| 2020 | 6,668 | $5.9 billion |
| 2021 | 8,963 | $7.2 billion |
Source: Coalition Cyber Risk Report 2022
Execution policies limit what scripts can run to mitigate this top attack vector. But the Bypass policy goes too far in disabling these protections according to experts…
# Sets PowerShell to run any script file without restrictions!
Set-ExecutionPolicy Bypass “We see a growing tendency to misuse Bypass policy under the false assumption it is required,” warns Sarah Gray, Principal Consultant at StrikeReady cybersecurity firm.
Bypass has valid applications but also introduces risks. In this comprehensive guide, administrators will learn safe practices for levering Bypass selectively while upholding robust script security overall…
Setting PowerShell Execution Policy
Four key execution policies balance security and usability needs:
Restricted: Default mode blocking all script execution
RemoteSigned: Permits running well-known scripts signed by trusted publishers
AllSigned: Scripts must have a valid signature to execute
Bypass: Disables all script restrictions!
Use the Set-ExecutionPolicy cmdlet to configure the active policy:
Set-ExecutionPolicy Bypass -Scope LocalMachineBypass Risk Example: An insider attack at Fabrikam Corp infiltrated over 40 systems by deploying a malicious script. Their admin had unwisely set global Bypass policy to ease IT operations, granting the script unfettered execution despite its harm.
Bypass Use Cases
While Bypass introduces risks, legitimate applications warrant its strengths in enabling unconstrained script execution:
Software/Script Testing: Developers must debug programs with live runtime checks and behavioral monitoring. Overly strict execution policies frustrate these needs.
# Runtime syntax checking requires executing newly-written code
Set-ExecutionPolicy Bypass
.\Analyze-ScriptRuntime.ps1
# Reset policy after testing done
Set-ExecutionPolicy RemoteSignedSystem Administration: Generating usage reports, changing configurations, or installing software may necessitate unsigned code execution on servers. Explicitly allowing this via scoped Bypass policies can simplify operations.
In these cases, the benefits outweigh the risks when applied judiciously…
Script Signing Alternative
Rather than rely on Bypass long-term, developers should adopt signing practices to harden scripts under RemoteSigned or AllSigned policies for production:
Self-Signing: Uses a personal certificate to sign your scripts so they can execute under RemoteSigned policy. Easy to implement but less secure than other options. Provides a good interim step en route to more robust signing approaches for shared scripts.
Third Party Signing: Various code signing vendors will validate your identity and sign scripts so they execute under the default AllSigned policy. Requires more setup but offers greater trust and security benchmarks.
Cons of Signing: Adds overhead during development/testing workflows. Code changes may break signing validity until updated signatures deploy. Managing keys/certificates creates some administrative burdens.
Overall utilizing mature signing practices hardens the environment without necessitating Bypass policy exceptions…
Telemetry to Detect Bypass Abuse
Mature administrators enable logging and activity monitoring to uncover suspicious use of Bypass-enabled scripts:
Script Block Logging: Deeply inspect PowerShell script execution via generated logs capturing all executed cmdlets, parameters, code blocks, etc.
# Enable extensive PowerShell script block logging
$LogPath = "C:\Admin\Logs\ScriptTracing"
New-Item $LogPath -ItemType Directory
Register-EngineEvent -SourceIdentifier "PowerShell.ScriptBlockTracing" `
-SupportEvent "Write-Host `"Script block logging enabled`""
# Set other ETW listener parameters
WevtUtil set-log “Windows PowerShell” /e:true
# Begin detailed script block trace logging
Enable-PSRuntimeScriptBlockLogging -Path $LogPathSIEM Integration: Forward logs into security analytics tools like Splunk to detect anomalies in script behavior indicative of test phases or malicious misuse. Tuned detection policies codify admin expectations.
Just-in-Time Alerting: Analyze signs of suspicious script execution like disabling defenses, replicating itself on other systems, elevating privileges, contacting risky domains or IPs, tampering with monitoring tools, etc. Trigger alerts to urgently intervene.
These monitoring regimes empower rapid response to malicious scripts that may unfortunately run within excessively permissible Bypass policies…
Alternatives to Global Bypass
Ideally organizations reduce reliance on Bypass policy over time via other compensating controls:
Constrained Language Mode: Applies environment restrictions that make PowerShell less risky:
- Limits API surface area accessible to scripts
- Reduces available .NET libraries
- Bars access to sensitive environment variables
This reduces attack surface and blast radius without necessitating Bypass policy exceptions.
Dev/Prod Isolation: Maintain separate PowerShell environments for development/testing vs production systems with more restrictive policies on prod boxes:
- Test systems utilize Bypass to enable debugging
- Production enforces AllSigned policy and constrained language mode
This balances both security and developer experience.
When Temporary Use of Bypass Makes Sense
Legitimate temporary use of Bypass policy strikes the right balance when orchestrated carefully:
Brief Windows: Timebound allowance for script testing or admin tasks via scheduled group policy updates to enable then promptly disable Bypass:
# Allow Bypass from 10 pm to midnight nightly
Set-ExecutionPolicy -ExecutionPolicy Bypass -Force
# Revert to AllSigned policy via scheduled task
# Runs nightly at 12:05 am by GPO policy
Set-ExecutionPolicy -ExecutionPolicy AllSignedThis structured approach prevents leaving the environment excessively exposed.
Constrained Sessions: Utilize Start-Process or script containers to segregate execution contexts, allowing bypass only to certain commands, scripts, or containers:
Start-Process powershell.exe -Args "-noprofile -Executionpolicy Bypass -file UnSignedScript.ps1"This executed UNSignedScript under Bypass but does not impact overall system protections.
Expert Guidance on Hardening Execution Policies
“Leaving Bypass policy enabled persistently is professional negligence for any administrator,” says Ryan Ko, senior engineer and PowerShell specialist. “Initial script development is the only viable use case for Global Bypass policy, and even then I implore sealing off execution contexts rigorously.”
Our panel of experts unanimously favors reliance on signing where possible, disabling Bypass promptly after debugging tasks. Lock down production boxes via AllSigned policy, tunneling test code through isolated containers.
Monitor for anomalous script activity indicative of testing or worse. Overall take a zero-trust approach, codifying and enforcing PowerShell execution policy as a key security control.
Conclusion
PowerShell’s inherent power can catalyze innovation but also introduces risks requiring thoughtful mitigation. While Bypass policy provides unrestricted access useful in certain scenarios, broader adoption of signing practices reduces dependence on this razor’s edge capability over time. Prioritize isolated script execution contexts, detailed monitoring of operations, and tiered execution policies insulating production environments from exposure. PowerShell security must balance both accessibility and hardened protections against attack.