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2.5.14.3.1.6 Ensure 'Remove file extensions blocked as Level 2' is set to 'Disabled'

Information

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.

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 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.

Impact:

Disabling this setting enforces the default configuration, and is therefore unlikely to cause usability issues for most users.

Important: For this setting to apply, the Outlook Security Mode setting must be enabled in User Configuration\Administrative Templates\Microsoft Outlook 2016\Security\Security with Use Outlook Security Group Policy selected, as set in this benchmark in Section 2.5.14.3.

Solution

To establish the recommended configuration via GP, set the following UI path to Disabled:

User Configuration\Administrative Templates\Microsoft Outlook 2016\Security\Security Form Settings\Attachment Security\Remove file extensions blocked as Level 2

Default Value:

Disabled. (Outlook does not classify any file type extensions as Level 2.)

See Also

https://workbench.cisecurity.org/benchmarks/12129

Item Details

Category: SYSTEM AND INFORMATION INTEGRITY

References: 800-53|SI-3, 800-53|SI-8

Plugin: Windows

Control ID: 40ab528346a5d68f2ba14f04578705ba294b410348d58e47d20e5f065e6dc7a5