1.13.3.1.5 Ensure 'Remove file extensions blocked as Level 1' is set to Disabled

Information

This policy setting controls which types of attachments (determined by file extension) Outlook prevents from being delivered.

Outlook 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.

If you enable this policy setting, you can specify the removal of file type extensions as that Outlook classifies as Level 1--that is, to be blocked from delivery--by entering them in the text field provided separated by semicolons.

If you disable or do not configure this policy setting, Outlook classifies a number of potentially harmful file types (such as those with .exe, .reg, and .vbs extensions) as Level 1 and blocks files with those extensions from being delivered.

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.

Solution

To implement the recommended configuration state, set the following Group Policy setting to Disabled.

User Configuration\Administrative Templates\Microsoft Outlook 2013\Security\Security Form Settings\Attachment Security\Remove file extensions blocked as Level 1\: Removed Extensions:

Impact:

Disabling this setting will cause any extensions that are already on the list will be ignored, which means that Outlook will block access to them again. This configuration could cause disruptions for users who are accustomed to sending and receiving such files.

See Also

https://workbench.cisecurity.org/files/552

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-18(3)

Plugin: Windows

Control ID: 1242e5281cf1f878dfcd7da60e2dde3144434f9726fd87d4b6134ebf2c5bf5b1