TMA-IIS-4.5

Ensure Double-Encoded requests will be rejected

MediumLevel 1Automatediis

Description

This Request Filter feature prevents attacks that rely on double-encoded requests and applies if an attacker submits a double-encoded request to IIS. When the double- encoded requests filter is enabled, IIS will go through a two iteration process of normalizing the request. If the first normalization differs from the second, the request is rejected and the error code is logged as a 404.11. requestFiltering allowDoubleEscaping must be false at server level and for 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; $paths = @('MACHINE/WEBROOT/APPHOST') + @(Get-Website | ForEach-Object { "IIS:\Sites\$($_.Name)" }); $bad = @($paths | Where-Object { $c = Get-WebConfiguration -Filter 'system.webServer/security/requestFiltering' -PSPath $_; $c.allowDoubleEscaping -ne $false }); if ($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' -Filter system.webServer/security/requestFiltering -Name allowDoubleEscaping -Value False

Properties

Operating systems: Any
Tags: iisrequest-filtering
Reference: CIS Microsoft IIS 10 Benchmark v1.2.1 - 4.5 Ensure Double-Encoded requests will be rejected

Check this host

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

shell

tmaegis scan --target iis