TMA-IIS-3.3

Ensure custom error messages are not off

LowLevel 2Automatediis

Description

When an ASP.NET application fails and causes an HTTP/1.x 500 Internal Server Error, or a feature configuration (such as Request Filtering) prevents a page from being displayed, an error message will be generated. Administrators can choose whether or not the application should display a friendly message to the client, detailed error message to the client, or detailed error message to localhost only. customErrors mode must be RemoteOnly or On (not Off) in the root web.config and every site. PowerShell 5.1 with the WebAdministration module; run elevated.

How it is checked

Audit method: Shell command

shell

Import-Module WebAdministration -ErrorAction Stop; $paths = @('MACHINE/WEBROOT') + @(Get-Website | ForEach-Object { "IIS:\Sites\$($_.Name)" }); $bad = @($paths | Where-Object { $c = Get-WebConfiguration -Filter 'system.web/customErrors' -PSPath $_; [string]$c.mode -eq 'Off' }); if ($bad.Count -eq 0) { 'compliant' } else { 'noncompliant' }

Expected output: compliant

How it is fixed

Fix method: Guidance or infrastructure patch

Set-WebConfigurationProperty -PSPath 'MACHINE/WEBROOT/APPHOST/<site>' -Filter system.web/customErrors -Name mode -Value RemoteOnly

Properties

Operating systems: Any
Tags: iisinformation-disclosure
Reference: CIS Microsoft IIS 10 Benchmark v1.2.1 - 3.3 Ensure custom error messages are not off

Check this host

Run this on a machine you are allowed to audit. The scan is read-only.

shell

tmaegis scan --target iis --level 2