Getting 403 forbidden error when trying to connect to customer sharepoint site using C# CSOM despite having access via web/Teams

Question

I am writing a C# console application that lists and then downloads files from a customer's SharePoint site. When I try to connect to their SharePoint site, I get a 403 Forbidden error. The same code works find when I connect to one of my company's SharePoint site.

Both sites use the same AD/Office 365 credentials (myusername@mycompany.com). I can navigate to the customer's SharePoint site in a browser and am able to view and download the files there using those credentials. But when I try to do the same thing via code, I get the 403 error.

Here is the code I am using:

using (ClientContext context = new ClientContext("https://customer.sharepoint.com/sites/SiteToReadFilesFrom/"))  
{  
    context.AuthenticationMode = ClientAuthenticationMode.Default;  

    string account = "myusername@mycompany.com";  
    string password = "mypassword";  

    var secret = new SecureString();  
    foreach (char c in password)  
    {  
        secret.AppendChar(c);  
    }  
    context.Credentials = new SharePointOnlineCredentials(account, secret);  
    context.Load(context.Web);  
          
    context.ExecuteQuery();      <!-- throws the error  
}  

If I switch the URL in the ClientContext to one of my SharePoint sites, it works fine. And I have access to the site and can view/download via a web browser (or Teams - this is a SharePoint site attached to a Team that I was added to).

I feel like I'm missing something obvious here. Is there a way to specify which AD Directory I am authenticating against? I have no control over the customer's site (or AD for that matter) so I have to solve this on my side unless is is a very simple request to change something on their side (like permissions).

Any and all help would be greatly appreciated.

Answer 1

Hi @Matt Thesing ,

According to my research and testing, I can use the code you provided well and successfully connect to SharePoint. Please try to do a troubleshooting:

1. Make sure you have permission to this site.
2. Make sure you are using the latest Microsoft.SharePoint.Client dlls

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