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Execution policy and code signing
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When signing a script, the signature is encrypted using the private key. Only the script author has access to that key, and only the public key can decrypt the signature. The signature contains a copy of the script. When PowerShell runs the script, it uses the author s public key (which is included along with the signature) to decrypt the signature. If the decryption fails, the signature was tampered with, and the script won t run. If the copy of the script within the signature doesn t match the clear-text copy, the signature is considered broken, and the script won t run.
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Figure 14.4 illustrates the entire process that PowerShell goes through when trying to run a script. You can see how the AllSigned execution policy is thus somewhat more secure: under that setting, only scripts containing a signature will execute, meaning that you ll always be able to identify a script s author. Of course, you ll also have to sign every script you want to run, and re-sign them any time you change them, which can be inconvenient.
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Is certificate from a trusted CA
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Figure 14.4 The process PowerShell follows when attempting to execute a script
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Is the script unmodified
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Security alert!
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14.4 Other security measures
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PowerShell has two other key security measures that are in effect at all times, and that should not be modified. First, the .PS1 filename extension (which is what the shell uses to identify PowerShell scripts) isn t considered an executable file type by Windows. Double-clicking a .PS1 file will normally open it in Notepad for editing, rather than attempting to execute it. This configuration is intended to help prevent users from unknowingly executing a script, even if the execution policy would allow it. Second, you can t run a script within the shell by simply typing its name. The shell never searches the current directory for scripts, so if you have a script named test.ps1, simply changing to its folder and typing test or test.ps1 won t run the script. Here s an example:
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PS C:\> test The term 'test' is not recognized as the name of a cmdlet, function, scrip t file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again. At line:1 char:5 + test <<<< + CategoryInfo : ObjectNotFound: (test:String) [], CommandNo tFoundException + FullyQualifiedErrorId : CommandNotFoundException Suggestion [3,General]: The command test was not found, but does exist in t he current location. Windows PowerShell doesn't load commands from the curr ent location by default. If you trust this command, instead type ".\test". See "get-help about_Command_Precedence" for more details. PS C:\>
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As you can see, PowerShell does detect the script but warns you that you have to type either an absolute or relative path in order to run the script. Because the script is located in C:\, you could run either C:\test, which is an absolute path, or run .\test, which is a relative path that points to the current folder. The purpose of this security feature is to guard against a type of attack called command hijacking. The attack involves putting a script into a folder, and giving it the same name as a built-in command, such as Dir. With PowerShell, you never put a path in front of a command name. So if you run Dir, you know you re running the command. If you run .\Dir, you know you re running a script named Dir.ps1.
14.5 Other security holes
As I ve already written, PowerShell s security is primarily focused on preventing users from unknowingly running an untrusted script. There s nothing to stop a user from manually typing commands into the shell, or even from copying the entire contents of a script and pasting them into the shell (although the commands might not have the exact same effect when run in that fashion). It s a little more difficult to convince a
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