Details
This policy setting controls which types of attachments (determined by file extension)
must be saved to disk before users can open them. Files with specific extensions can be
categorized as Level 1 (users cannot view the file) or Level 2 (users can open the file after
saving it to disk). Users can freely open files of types that are not categorized as Level 1 or
Level 2.
If you enable this policy setting, you can specify a list of attachment file types to classify as
Level 2, which forces users to actively decide to download the attachment to view it.
If you disable or do not configure this policy setting, Outlook does not classify any file type
extensions as Level 2.
Important- This policy setting only applies if the ‘Outlook Security Mode’ policy setting
under ‘Microsoft Outlook 2010SecuritySecurity Form Settings’ is configured to ‘Use
Outlook Security Group Policy.’ The recommended state for this setting is- Disabled.
*Rationale*
Malicious code is often spread through e-mail. Some viruses have the ability to send copies
of themselves to other people in the victim’s Address Book or Contacts list, and such
potentially harmful files can affect the computers of unwary recipients.
Outlook 2010 uses two levels of security to restrict users’ access to files attached to e-mail
messages or other items. Files with specific extensions can be categorized as Level 1 (users
cannot view the file) or Level 2 (users can open the file after saving it to disk). Users can
freely open files of types that are not categorized as Level 1 or Level 2.
By default, Outlook classifies a number of potentially harmful file types as Level 1. (See
Attachment file types restricted by Outlook 2010 for the complete list.) Outlook does not
classify any file types as Level 2 by default, so this setting is not particularly useful in
isolation. Typically, if there are extensions on the Level 2 list they would have been added
by using the ‘Add file extensions to block as Level 2’ setting, through which they can be
removed.
The combined lists of blocked and restricted file extensions that Outlook uses are actually
built by combining various policies together. If a machine policy classifies an extension as
Level 2, this setting could be used to remove the extension from the list in some situations.
As with Level 1 extensions, though, removing restrictions on potentially dangerous
extensions can make it easier for users to open dangerous files, which can significantly
reduce security.
Solution
To implement the recommended configuration state, set the following Group Policy setting
to Disabled.
User ConfigurationAdministrative TemplatesMicrosoft Outlook 2010SecuritySecurity
Form SettingsAttachment SecurityRemove file extensions blocked as Level 2- Removed
Extensions-
Impact-Disabling this setting enforces the default configuration, and is therefore unlikely to cause
usability issues for most users.
Supportive Information
The following resource is also helpful.
This security hardening control applies to the following category of controls within NIST 800-53: System and Communications Protection.This control applies to the following type of system Windows.