An SSRS IFrame/CORS infinite redirect loop error and a quick and easy solution

The redirect loop looks like this and, in Edge, will display the error message: "[domain] redirected you too many times"

If you are trying to render the SSRS ReportViewer control within an <iframe>, you may run into a CORS issue that manifests in a series of 302 (Found) responses and an infinite redirect loop between ReportViewer control (ReportViewer.aspx) and Logon.aspx.

As of SSRS 2022, without an explicit instruction to allow CORS, ReportViewer cannot be rendered within an <iframe> on an origin different than the origin of the report server.

If you are using custom authentication, the solution is easy enough. Just add cookieSameSite="None" and enableCrossAppRedirects="true" to the authentication <forms> tag in the report server's web.config.

 <authentication mode="Forms">  
   <forms loginUrl="logon.aspx" name="sqlAuthCookie" cookieSameSite="None" timeout="60" path="/" enableCrossAppRedirects="true" requireSSL="true">  
   </forms>   
 </authentication>  

You may also need to enable CORS in your client app. In ASP.NET Core 8, this can be achieved through the following in your application startup code:

 app.UseCors(builder => builder  
 .WithOrigins("https://localhost", "https://[domain]")  
 .AllowAnyMethod()  
 .AllowAnyHeader());