![]() ![]() ![]() What does Constrained Language constrain?Ĭonstrained Language consists of a number of restrictions that limit unconstrained code execution on a locked-down system. We have described these dangers in much more detail in our post, “Writing Secure PowerShell Scripts” (coming soon). In either case it could allow a user to run arbitrary code in Full Language mode, thus bypassing the system policy protections. A vulnerability could allow code injection, or leak private functions not intended for public use. But if you create a custom PowerShell module that is not allowed by the policy then it will be considered untrusted and run with Constrained Language restrictions.Ĭonsequently, any PowerShell module marked as trusted in the policy needs to be carefully reviewed for security vulnerabilities. The UMCI policy allowing signed Windows files lets PowerShell run these modules in Full Language mode. This will allow your approved scripts to run in Full Language mode.įor example, all PowerShell module files shipped with Windows (e.g., Install-WindowsFeature) are trusted and signed. The solution to this is simple: add these scripts (or more effectively: your code signing authority that signed them) to your Device Guard policy. Since Constrained Language is so limited, you will find that many of the approved scripts that you use for advanced systems management no longer work. With a policy enforced even administrators are limited to what they can do on the system. These lockdown policies are important for high-value systems that need to be protected against malicious administrators or compromised administrator credentials. However, we will continue to enhance this for non-Windows platforms where feasible. So this is currently very much a Windows security feature. It does not work on non-Windows platforms. PowerShell’s detection of system policy enforcement through DeviceGuard is supported only for Windows platform running Windows PowerShell version 5.1 or PowerShell 7. So PowerShell Constrained Language mode becomes more interesting when working in conjunction with system-wide lockdown policies. PowerShell automatically detects when a UMCI policy is being enforced on a system and will run only in Constrained Language mode. For DeviceGuard UMCI the approved applications are determined via a UMCI policy. Application control solutions are an incredibly effective way to drastically reduce the risk of viruses, ransomware, and unapproved software. ![]() PowerShell Constrained Language mode was designed to work with system-wide application control solutions such as Device Guard User Mode Code Integrity (UMCI). These test hooks cannot override a Device Guard UMCI policy and can only be used when no policy enforcement is applied. ![]() In addition, there are also file naming conventions that enable FullLanguage mode on a script, effectively bypassing Constrained Language. This is unwise because an attacker can easily change the environment variable to remove this enforcement. While we have never documented this, some have discovered it and described this as an enforcement mechanism. As part of the implementation of Constrained Language, PowerShell included an environment variable for debugging and unit testing called _PSLockdownPolicy. A user can simply start another PowerShell session which will run in Full Language mode and have full access to PowerShell features. + FullyQualifiedErrorId : MethodInvocationNotSupportedInConstrainedLanguage + CategoryInfo : InvalidOperation: (:), RuntimeException Method invocation is supported only on core types in this language mode. PS C:\> ::WriteLine("Hello")Ĭannot invoke method. ![]()
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