Second Life of a Hungarian SharePoint Geek

March 23, 2014

HTTP Error: ‘401–Unauthorized’ When Accessing Exchange Web Services via PowerShell 2.0

Filed under: Exchange, PowerShell — Tags: , — Peter Holpar @ 20:25

Last week I had to create a tool to automate the synchronization of an Exchange 2010 folder with a SharePoint 2010 list. Formerly I had some experience with Exchange Web Services and its Managed API, and downloaded the code samples for Exchange 2013 to re-use a few classes of the examples. As my developer account had no access to Exchange, I used explicit credentials of my standard account via the NetworkCredential class in the test (user name hardcoded, password read from the command line). The C# code in Visual Studio 2012 of the proof-of-concept application was running without error, but I thought it’s a good idea to migrate the code to PowerShell, as if the requirements happened to change (that is rather likely in this case), it would be easier to modify the functionality without a recompilation. This idea cost me a few hours debugging later.

I found a few samples on the web, like this one, that illustrate the basic steps in PowerShell. I used the EWS Managed API version 2.1, and PowerShell 2.0, that support .NET CLR version 2 (handy when we want to use the SharePoint 2010 server side object library in the same process). Rewriting the code in PowerShell seemed a trivial task, however when I started the tool I got a 401 Unauthorized error on the first command that tried to access an Exchange resource. I double checked the user name in the code, but it was OK. I found no significant difference when comparing the network traffic generated by the .NET version vs. the PowerShell version up to the point of the error message. I altered the code to use the three-string-parameter constructor of the NetworkCredential class (user name, password, domain name) instead of the two-string-parameter constructor version (domainname\username, password), as I had previously issues from using the two-string-parameter version. But the error remained, so I altered the code back. When I logged in with my other user, that has access to Exchange, and used the default credentials (service.UseDefaultCredentials = $true) the PowerShell code gave no error more.

I read the password from the console using the Read-Host Cmdlet as described here (to tell the truth I simply copied the code and did not check how it works and what that might cause):

$password = Read-Host -assecurestring "Please enter your password"

I assumed that there might be a problem with the password I typed in, so decided to echo back it via Write-Host $password. To my great surprise instead of my password (or a mistyped version of that) the result was: “System.Net.NetworkCredential”. Then I realized the switch assecurestring used with Read-Host, that means not only that the text typed in will be replaced with asterisks, but also that it will be represented internally as SecureString, and not as String. After reading the password without assecurestring, the password was stored as string, but the error remained until I changed the NetworkCredential constructor to the three-string-parameter version again. So as usually, the mystic error was a result of a dual mistake, using SecureString instead of String, and using the wrong version of the NetworkCredential constructor.

The sample application from Microsoft (mentioned above) are working flawlessly with a NetworkCredential constructor that accepts a String for domainname\username, and a SecureString parameter for password, but these samples using the .NET version 4.0. After changing the .NET version of the project to 3.5 in Visual Studio, I had the same issues, but at least the IDE gave me a compilation error because of using a wrong constructor parameter. “Thanks” to the flexibility of PowerShell, it did not warned me because of the SecureString type (I assume it used the “System.Net.NetworkCredential” string as password) and did its best to perform the authentication and failed first there.


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