1.13.2.9 Ensure 'Signature Warning' is set to Enabled:Always warn about invalid signatures

Information

This policy setting controls how Outlook warns users about messages with invalid digital signatures.
If you enable this policy setting, you can choose from three options for controlling how Outlook users are warned about invalid signatures:
- Let user decide if they want to be warned. This option enforces the default configuration.
- Always warn about invalid signatures.
- Never warn about invalid signatures.
If you disable or do not configure this policy setting, if users open e-mail messages that include invalid digital signatures, Outlook displays a warning dialog. Users can decide whether they want to be warned about invalid signatures in the future. The recommended state for this setting is: Enabled:Always warn about invalid signatures.

Rationale:

By default, if users open e-mail messages that include invalid digital signatures, Outlook displays a warning dialog box. Users can decide whether they want to be warned about invalid signatures in the future.
If users are not notified about invalid signatures, they might be prevented from detecting a fraudulent signature sent by a malicious person.

Solution

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

User Configuration\Administrative Templates\Microsoft Outlook 2016\Security\Cryptography\Signature Warning

Then set the Signature Warning option to Always warn about invalid signatures.

Impact:

Enabling this setting could cause some disruptions for Outlook users who receive a lot of messages signed with invalid signatures. These users will see a warning dialog box every time they open such a message.

See Also

https://workbench.cisecurity.org/files/553

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-8(1)

Plugin: Windows

Control ID: 7ce184c14fd43be32fbb2335c8a880ac0b62964becd389093428bb4601e7bd21