Security Considerations

To understand the security of ExcelWriter it is necessary to have background knowledge in IIS and ASP security.

The security context is not important if you are streaming files to the browser, or returning the resultant spreadsheet to memory. In both of these cases, ExcelWriter does not write any intermediate files to the hard disk of the server. Since everything is performed in memory, security considerations are not relevant.

The security context is important if you want to,

• Save a generated spreadsheet on the server's hard disk
• Open an existing spreadsheet
• Embed images in a spreadsheet

ASP provides three levels of authentication:

• Anonymous
• Basic
• NT Challenge/Response

When a user hits a web page, the security of the page is verified. If there are no restrictions, such as NTFS permissions, anonymous access is used. When there are NTFS permissions in place, IIS will negotiate with the browser and determine if the user can be authenticated.

IIS uses a special NT account, typically called IUSR_MachineName, as the default anonymous account. An administrator can change this default account. It is possible to have different anonymous accounts for each virtual server or web application on the server.

With Basic and NT Challenge/Response, an authentication mechanism is in place that identifies the user who is requesting the page. With either Basic or NT Challenge/Response, IIS/ASP will actually perform an NT login for that specific user.

This means that a given page is executing as a specific NT user: either IUSR_MachineName or the actual authenticated NT domain user. This technique is known as impersonation and is a standard feature of NT Services.

This has strong implications for ExcelWriter if you are trying to save the generated spreadsheet on the hard disk of the web server. When ExcelWriter attempts to create, read or write files, it will be executing in the security context provided by IIS/ASP.

If a non-authenticated user is creating a spreadsheet, ExcelWriter will only be able to write to locations that are accessible by the IUSR_MachineName account.

If an authenticated user is creating a spreadsheet, ExcelWriter will only be able to write to locations determined by that specific user's NT domain permissions.

For more information, see Microsoft KB Article Q207671.

Copyright © 2003, SoftArtisans, Inc.