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PowerShell 5.0 and Applocker. When security doesn’t mean security (part 2)

In the previous post, I tried to explain some inconsistences in the current implementation of Constrained PowerShell feature that is introduced in PowerShell 5.0: PowerShell 5.0 and Applocker. When security doesn’t mean security. After having a long email and twitter conversations I realized that many of readers blame me for being against Constrained PowerShell feature. It is not true. In this post, I would like to summarize what is going wrong now and how it should work in my opinion.


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PowerShell 5.0 and Applocker. When security doesn’t mean security

Problem description

A friend of mine asked why his PowerShell scripts (PowerShell profile) doesn’t execute properly in after upgrading to PowerShell 5.0. A brief investigation showed that interactive PowerShell console runs in Constrained Language mode, as the result many language features are stripped out and PowerShell profile isn’t loaded with the following error:

Windows PowerShell
Copyright (C) 2015 Microsoft Corporation. All rights reserved.
 
C:\Users\vpodans\Documents\WindowsPowerShell\Microsoft.PowerShell_profile.ps1 : Cannot dot-source this command because
it was defined in a different language mode. To invoke this command without importing its contents, omit the '.'
operator.
At line:1 char:1
+ . 'C:\Users\vpodans\Documents\WindowsPowerShell\Microsoft.PowerShell_ ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [Microsoft.PowerShell_profile.ps1], NotSupportedException
    + FullyQualifiedErrorId : DotSourceNotSupported,Microsoft.PowerShell_profile.ps1


PS C:\Users\vpodans> [math]::Sqrt(1)
Cannot invoke method. Method invocation is supported only on core types in this language mode.
At line:1 char:1
+ [math]::Sqrt(1)
+ ~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : MethodInvocationNotSupportedInConstrainedLanguage

PS C:\Users\vpodans> $ExecutionContext.SessionState.LanguageMode
ConstrainedLanguage
PS C:\Users\vpodans>

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Certificate Rules may not work in Software Restriction Policies

SYMPTOMS

Consider the following scenario. You configured Software Restriction Policies (SRP) to allow run all applications that are signed by the specific signer by creating a Certificate Rule against the signer certificate.

When the policy is refreshed on the client, user cannot run the application, because it is blocked by Software Restriction Policies.

 

CAUSE

This behavior occurs when the certificate is issued by a Certification Authority (CA) which do not include or provide incorrect issuer information in the Authority Information Access (AIA) extension in the signing certificate. In addition, intermediate CA certificate is not installed in the local store. SRP reads only signing certificate in the digital signature and ignores the rest certificates.


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Hacking and securing Software Restriction Policies

Hello folks, CryptoGuy is here again. Today I’ll talk about the most common ways to violate default SRP configurations and how to protect SRP against them.

System folders with write access

It is generally correct that regular users haven’t write permissions on system folders and we can safely allow to run any program from these folders (C:\Windows). However this statement is not fully correct. This is because there are folders where regular users have write and execute permissions. There are at least two default folders:

  1. System temp folder: C:\Windows\Temp
  2. Print spooler folder.

Here is a screenshot of Temp folder ACL:


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Software Restriction Policies — Rule ordering

Hello everyone! Last time I was busy on other stuff and haven’t enough time to continue the topic. Today I want to talk about SRP rule ordering and how rule conflicts are resolved.

When you define SRP rules, you may have 2 or more conflicting rules. For example, you have a rule that allows to run any software signed by a certain certificate. For some reasons you decided to block one or more specified applications that are signed by the allowed certificate. Or you have two path rules that points to the same file, but have opposite security levels. It is important to understand how SRP processes rules and decides resulting action (allowed or blocked).

Multiple policies

The first thing we should learn is how multiple policies are applied. As you already know (at least, I assume that you know, because you have to know this), in a domain environments you can define multiple policies at various levels. Normally, such policies are applied by following the following sequence: LSDOU (local, site, domain and GPO linked to an OU). The latest policy object applied becomes effective. However this is not true for SRP. When you look at RSOP (Resultant Set Of Policies) for other settings (for example, account lockout settings), you can see which policy wins:


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