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There is a variation of this approach that is considered legitimate sometimes called picket-fence mitigation. A picket fence has holes in it, so we put another one behind it. And if there are still holes then we keep adding fences until there are no more holes. This is equivalent to dumping truckloads of lawn gnomes on our property until the house is surrounded by a 30-foothigh wall of ceramic gnomes. It ll work, but it s not very attractive.
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The moral of this story is that, when securing a system, don t add unnecessary or inappropriate checks. We have to look at the system as a whole. This is particularly true when writing code. The more lawn gnomes we add, the more code we add to the system. Each new line of code introduces new possibilities for errors, and these errors can, in turn, become additional vulnerabilities. Blacklisting/whitelisting Short and sweet blacklisting is saying who s bad and whitelisting is saying who s good. In general, whitelisting is preferred. Assume that the world is bad and we only trust people we know. This is inherently the most secure approach to use with PowerShell. The number of people we trust to give us scripts to run is much smaller than the number of people we don t trust to run scripts. PowerShell supports the use of script signing to verify the identity of a script publisher and also validate the integrity of a published script. This is discussed at length in section 13.4. Authentication authorization and roles Authentication is verifying the identity of the user. Authorization is determining whether the user is authorized to perform an action. Finally, roles are groupings of activities for which authorization can be granted. By grouping multiple activities into a role, it
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CHAPTER 1 3
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SECURITY, SECURITY, SECURITY
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becomes easier to manage authorization. When users are assigned a particular role, they are automatically authorized to perform all of the activities associated with that role. PowerShell depends primarily on the operating system for authentication and authorization, and currently has no special support for roles, unfortunately. A PowerShell script operates with the capabilities associated with the security token of the user who is running the script. We will see an example later in the chapter where it is possible to run a program from PowerShell as a different user, however. Input validation The rule is that we must validate any input received from outside our script. In scripting, this is the second most important rule for writing secure scripts. (The most important rule is don t run unknown or untrusted scripts .) Most scripting environments have the ability to dynamically compile and execute code (this is one of the things that makes them dynamic languages). It s tempting to use this capability to simplify our code. Say the user needs to do some arithmetic calculations in her script. In PowerShell, we could just pass this code directly to the Invoke-Expression cmdlet and let it evaluate the expression.
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PS (1) > $userInput = "2+2"
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Now we ll use Invoke-Expression to execute the command:
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PS (2) > invoke-expression $userInput 4
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Wasn t that easy! But what if the user types the following
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PS (3) > $userInput = "2+2; 'Hi there'" PS (4) > invoke-expression $userInput 4 Hi there
It still executed the calculation, but it also executed the code after the semicolon. In this example, it was a harmless statement. But it might have been something like
$userInput = "2+2; del rec force c:\"
If this statement were executed, it would try to delete everything on your C: drive. Which would be bad. There are other places where you need to do input validation. If the user is supplying a path you should make sure that it s a path that the user actually should have access to. For example:
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