Monday 26 January 2015

IIS Hosted Web API Not Caching Internal HTTP Responses

Executive Summary: Set “Load Profile” to true for the IIS application pool.

I had a fun afternoon last week trying to help a colleague work out why the responses to the internal HTTP requests from one of my team’s web APIs weren’t being cached by the .Net HTTP component. It appeared that this was all working fine in the pre-production environment, but not production itself.

Caching HTTP Responses

The code (which I’m not overly familiar with) uses one of the standard .Net HTTP request classes (WebRequest I believe) to make authentication calls to another internal, RESTful service. It was anticipated by the owner of this service that responses could be cached by the .Net framework thereby alleviating the need for our own service to manually implement a caching layer. Naturally the point of (briefly) caching the HTTP responses was to reduce load on the underlying authentication servers and they had made sure that their response headers were correctly configured to make this “all just work”.

In theory this was true, but in practice it turned out to be a little more complicated once the code as running under the real IIS worker process (w3wp.exe).

Administrator Rights

My colleague had already done a fair bit of analysis and experimentation with a spare production server and the wonderful Process Monitor tool from Sysinternals. By watching the w3wp.exe process whilst he sent requests with CURL he could see that the process was getting Access Denied errors when accessing parts of the Registry and file-system.

He had also tried configuring the IIS application pool to run under another non-service account, i.e. a support user’s account, and that appeared to work. Comparing the behaviours with Process Monitor for the two accounts he could verify that the problem was one of permissions, or so it seemed.

A natural response when seeing Access Denied problems is to grant the underprivileged account admin rights and that’s what he did and lo-and-behold things started working. However further experimentation suggested that this didn’t always fix the problem, which was mightily confusing.

At this point I joined the party...

Verifying Admin Rights

Past experience has taught me that just adding an account to the Administrators group is not always enough - it’s best to actually check the security attributes of the process in question to double-check that it really has become elevated. Conversely when removing rights I make the same check to ensure they’ve been revoked correctly.

For this check I used another of Sysinternals tools - Process Explorer. We hit an initial “Access Denied” for some process information [1] so I used the technique of starting it with “psexec -s -i” which was probably a bit of overkill (See “Process Explorer Failed to Display DLLs or Handles” for when this might be necessary). After right-clicking on w3wp.exe to show the process’s Properties and then switching to the Security tab I could see that the process was indeed a member of BUILTIN\Administrators.

It later transpired that the w3wp.exe process not correctly acquiring or relinquishing its power might have been the source of some of the earlier confusion. I too witnessed the removal of the account from the local Administrators group and after recycling the web site application pool found the w3wp.exe process still appeared to be a member. The process start time strongly suggested it had been restarted too when we recycled the app pool.

The Default Profile

We then switched back to Process Monitor and looked at the Registry and file-system access. The HTTP client was accessing various “Internet Settings” registry keys under the HKEY_USERS tree - HKU\.Default in particular. It was then reading and writing to an internet cache that was stored under “C:\Windows\Temp\Temporary Internet Files\...”.

I had always suspected that user profiles might have a role to play in this problem because I remembered back in the dim-and-distant past that the native WinInet library couldn’t (or perhaps shouldn’t) be used for making HTTP calls when running as a service. Microsoft later added another native component called WinHttp that could be used by services. IIRC the main problem back then was around configuring proxy settings, and whilst we weren’t suffering from that specific ailment this time, the registry keys it appeared to be accessing was pointing in the same direction.

We then revoked the admin rights from the service account, checked they had been revoked with Process Explorer, and then executed the test HTTP requests again so I could see whether the same registry and file-system access patterns occurred, and they did. A brief play with the REG command-line tool and DIR helped prove that, although the keys and folders existed, the unprivileged service account wouldn’t have access to them.


This suggested to me that the problem was likely down to the fact that the service account was being used by the w3wp.exe process without a user profile being loaded. As such, due to the lack of a profile, it was trying to do the same thing it would with just a default profile on hand, but it didn’t have the necessary rights to update the cache in the “C:\Windows\Temp\Temporary Internet Files” folders.

Changing the Application Pool setting for “Load Profile” to “true” seemed to fix the problem. We then applied this to another spare production server (after first proving that it was not behaving correctly) and that worked too.

Admin Rights Not Required

I think this tale highlights why the answer to many deployment issues can be solved by apparently making the service account a local administrator. The huge gain in power that right confers allows the process to read, and more importantly write, to so much of the system. Even if the correct answer is an entirely different setting (as in this case) taking the sledgehammer approach can get you running, but at a potentially disastrous cost [2].

Anyone doing service style deployments on Windows should know how to weld tools like NTRIGHTS and CACLS (or their more modern equivalents) to grant just the right amount of permissions for their processes.


[1] I’ve spent so many years supporting pre-Windows Vista systems that I still forget occasionally that I need to elevate admin tools. I usually start with a non-elevated command prompt for safety and then wonder later why a tool doesn’t work properly because I’ve forgotten that I’ve gone for safety first. Which of course is the whole point :-).

[2] If the service is compromised it might grant an intruder further access. Even if it’s an internal service putting it in a sandbox helps ensure it does what you think it should.

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