Internet Explorer’s Explicit Security Zone Mappings

[Updated 15 May 2012 to correct a bug involving precedence of Computer policies over User policies.]

I recently worked with some customers who wanted to enumerate which web sites had been assigned to which Internet Explorer security zones.  I.e., they wanted to know which web sites had been assigned to the Intranet zone, which to Trusted Sites, etc.  In the course of this work I uncovered some surprising complexities about site-to-zone assignment rules that had not yet been documented.  This blog post describes those discoveries.  Later today I will post an updated version of IEZoneAnalyzer that lists the sites that have been configured and whether those settings are in effect or ignored. [Update: it's been posted.]

[I'm not happy with the way the blog software has reformatted this document; rather than spend the day fighting it I'm attaching the original Word doc to this post.]

Overview

Internet Explorer applies a set of rules to associate web sites (URLs) with security zones, based on criteria such as whether the server has a dot in its name.  In addition, group policies, computer settings and user preferences can be used to map specific URLs to specific zones.  For example, you could explicitly add “https://www.contoso.com” to the Trusted Sites zone.  Such site-to-zone mappings are defined under one or more ZoneMap key hierarchies in the registry.  There are five different locations where ZoneMap key hierarchies can be defined, but only one or two of them will be in effect at any particular point in time.  Exactly which settings under which ZoneMap keys are effective depends on a number of circumstances:

· Whether Site-To-Zone-Assignment lists are configured in Computer Configuration and/or User Configuration group policies;

· Whether the “Security Zones: Use only machine settings” group policy is configured (a.k.a., Security_HKLM_only);

· Whether Internet Explorer’s Enhanced Security Configuration (ESC) is enabled (Server only);

and, quite surprisingly:

· Whether or not the program is a 32-bit process on 64-bit Windows; a.k.a., “Windows On Windows 64” or WOW64.

Yes, that’s right – in some circumstances, a 32-bit process and a 64-bit process on the same computer can see the same site mapped to different security zones.

Also, my testing indicates that there is a bug that results in all URLs being treated as “Internet” zone when both ESC and a Computer or User Site-To-Zone-Assignment list are enabled.

Explicit Site To Zone Rules

The rules for selecting ZoneMap keys are listed below.  Each table shows some combination of the four circumstances described in the overview; following each table is the key or keys that are in effect in those circumstances.  There are separate settings under each ZoneMap key for “ESC on” and “ESC off”.  If ESC is on, only those settings under the EscDomains and EscRanges subkeys are used; if ESC is off, only the settings under the Domains and Ranges subkeys are used.

Note that in the tables below, WOW64 set to “Yes” means a 32-bit process on a 64-bit version of Windows.  WOW64 set to “No” means either a 32-bit process on a 32-bit version of Windows or a 64-bit process on a 64-bit version of Windows.

Combines results from

HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

User preferences (in HKCU) take precedence over computer preferences

Combines results from

HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

User preferences (in HKCU) take precedence over computer preferences

HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

User site-to-zone assignments are ignored if present

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

User site-to-zone assignments are ignored if present

HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

Combines results from

HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

HKCU\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

Computer policies (in HKLM) take precedence over User policies

HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

User site-to-zone assignments are ignored if present

HKCU\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMap

What About “ZoneMapKey”?

IT administrators trying to apply site-to-zone settings by directly manipulating registry values often discover two “ZoneMapKey” registry keys that appear to be more interesting than they actually are: specifically, HKCU\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMapKey and HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\ZoneMapKey.  Values under these keys look like the site-to-zone assignments applied through group policy, and in fact they are.  However, these keys are not used directly by Internet Explorer, and if you directly set values there, they will have no effect.  The ZoneMapKey entries are just a temporary writing place for the Group Policy engine, which writes entries there as specified by Group Policy, and then parses them into corresponding ZoneMap subkey settings that are used by Internet Explorer.

Explicit security zone mapping rules 1.1.docx