HTTP Error 502.5 - ANCM Out-Of-Process Startup Failure

What’s actually happening

The page returns a 502.5 instead of your app. This is the out-of-process hosting model talking: the ASP.NET Core Module (aspnetcorev2_outofprocess.dll) spawns dotnet.exe as a reverse-proxy backend, and that child process died on startup or never bound to its assigned port. The in-process equivalent is 500.30/500.31, so seeing 502.5 specifically tells you hostingModel is OutOfProcess. The error page itself carries almost no detail; the real reason is in the stdout log or the Windows Event Log.

Common causes

• The targeted .NET runtime isn't installed on the box — web.config asks for Microsoft.AspNetCore.App 8.0.x but only 6.0.x is present, so the host throws 'The specified framework was not found.'
• processPath or arguments in web.config are wrong: dotnet.exe not on PATH for the app pool identity, or arguments pointing at a .dll that isn't in the publish output.
• The app throws during Program.cs / Startup before Kestrel binds — a bad connection string, missing appsettings.json, or a DI registration that blows up in the constructor.
• Bitness mismatch: a 32-bit published app under an app pool with 'Enable 32-Bit Applications = False' (or the reverse).
• Port conflict or the app exited non-zero immediately; ANCM waits for the port and reports the process started-then-stopped.

How to fix it

1. Read the actual exception from the Event LogOpen Event Viewer > Windows Logs > Application and filter on source 'IIS AspNetCore Module V2', EventID 1010. The message names the failing application path and an ErrorCode like 0x800700c1 (bad image / bitness) or a framework-not-found string. This is faster than guessing.
2. Turn on stdout loggingIn web.config set stdoutLogEnabled="true" and stdoutLogFile=".\logs\stdout", create the logs folder, and give the app pool identity (e.g. IIS AppPool\YourPool) write access. Reproduce the request, then read the newest .\logs\stdout_*.log — the unhandled exception and stack trace land there.
3. Run the app by hand from the deploy folderOpen a cmd in the publish directory and run: dotnet .\YourApp.dll (framework-dependent) or .\YourApp.exe (self-contained). You'll see the crash immediately with full output, with IIS removed from the equation.
4. Install or retarget the runtimeRun dotnet --list-runtimes on the server. If the version your app needs is missing, install the ASP.NET Core Hosting Bundle for that major version and run iisreset. Alternatively publish self-contained (dotnet publish -c Release --self-contained -r win-x64) so the runtime ships with the app.
5. Verify processPath/arguments and bitnessConfirm <aspNetCore processPath="dotnet" arguments=".\YourApp.dll" /> matches the actual DLL name, and that the app pool's 32-bit setting matches how the app was built. After any web.config change, recycle the app pool.