How can I find out why ISAPI is returning a 302 status for a specific file? - redirect

Question

I have a website hosted served by IIS 10 on a Windows Server (2019) running Plesk. The site is mainly Classic ASP. I have a staging subdomain at staging.example.com, with the production site at www.example.com.
The two are fairly strictly separated, except that I don’t store image files, PDFs and such things on the staging server; I have a URL rewrite directive that redirects to the production site with a 302 status based on the URL not matching the following regex:
\.(php|asp|js|css|csv|json|htm|html|svg|svgz)(\?.+)?$
This generally works well: ASP pages are served from the staging site when the staging URL is called, but images on the page are pulled from the production site.
Except that there’s one ASP file which – for some reason – gives a 302 and redirects to the production site no matter what I do. The file exists in both locations. I’ve tested the URL in the pattern tester provided in the IIS URL-rewrite section, and it matches the pattern (meaning it shouldn’t redirect).
When I trace the request (that is, the initial request to the staging URL) in Firefox’s browser console, I get the following response headers (redacted):
HTTP/2 302 Found
cache-control: no-cache
content-type: text/html
location: https://www.example.com/path/to/file.asp
server: Microsoft-IIS/10.0
set-cookie: ASPSESSION****=********; secure; path=/
x-powered-by: ASP.NET
x-powered-by-plesk: PleskWin
date: Sun, 19 Dec 2021 18:52:05 GMT
content-length: 201
Accept
text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Encoding
gzip, deflate, br
Accept-Language
en-US,en;q=0.5
Authorization
Basic *************
Connection
keep-alive
Cookie
[cookies]
Host
staging.example.com
Referer
https://staging.example.com/path/to/file.asp
Sec-Fetch-Dest
document
Sec-Fetch-Mode
navigate
Sec-Fetch-Site
same-origin
Sec-Fetch-User
?1
Upgrade-Insecure-Requests
1
User-Agent
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:96.0) Gecko/20100101 Firefox/96.0
I’ve painstakingly gone through the entire file and all the file includes within it, and I can’t find any kind of Response.Redirect in any of them that might be responsible.
So it seems it’s IIS that’s redirecting with a 302… despite the fact that there doesn’t seem to be a directive that tells it to do this.
Is there a way to trace exactly what on the server is causing this 302 for one specific file? Some sort of tracing mechanism that tells me where the request gets passed on to before the 302 response is returned?
 
 
Update 26 Dec
Based on samwu’s comment, I’ve enabled Failed Request Tracing for the page, and looking through the resulting .frb file, it’s clear that none of the rewrite conditions are met – they all have succeed: false. It seems the redirect is not happening in the WWW Server at all, in fact, but in the ISAPI extension. This is the only place that the production site URL is mentioned at all in the request trace (except of course in the GENERAL_RESPONSE_HEADER section at the very end):
ISAPI_START
MODULE_SET_RESPONSE_SUCCESS_STATUS ModuleName="IsapiModule", Notification="EXECUTE_REQUEST_HANDLER", HttpStatus="302", HttpReason="Object moved"
GENERAL_SET_RESPONSE_HEADER HeaderName="Location", HeaderValue="https://www.example.com/path/to/file.asp", Replace="false"
GENERAL_SET_RESPONSE_HEADER HeaderName="Content-Length", HeaderValue="201", Replace="false"
GENERAL_SET_RESPONSE_HEADER HeaderName="Content-Type", HeaderValue="text/html", Replace="false"
GENERAL_SET_RESPONSE_HEADER HeaderName="Cache-control", HeaderValue="no-cache", Replace="false"
NOTIFY_MODULE_COMPLETION ModuleName="IsapiModule", Notification="EXECUTE_REQUEST_HANDLER", fIsPostNotificationEvent="false", CompletionBytes="0", ErrorCode="The operation completed successfully. (0x0)"
ISAPI_END
In the ISAPI Filters section in IIS Manager, there are four filters: a 32-bit and a 64-bit version for ASP.Net 2.0 and the same for ASP.Net 4.0, all called aspnet_filter.dll. I’m guessing these are standard filters – I know for certain, at least, that we haven’t mucked about with any ISAPI filters at all.
As should be obvious by now, I’m not really a server admin, and ISAPI filters are definitely above my level of knowledge.
So how do I proceed from here? How do I figure out why ISAPI is redirecting?