Title

S/Mime interoperability with external clients for message handling must be configured. (Cat II impact)

Discussion

In some situations, administrators might wish to use an external program, such as an add-in, to handle S/MIME message decryption. If your organization works with encrypted messages that the decryption functionality in Outlook cannot handle appropriately, this setting can be used to configure Outlook to hand S/MIME messages off to an external program for decryption. If no external program has been authorized however, misconfiguring this setting could allow unauthorized and potentially dangerous programs to handle encrypted messages, which could compromise security.

Check Content

Verify the policy value for User Configuration -> Administrative Templates -> Microsoft Outlook 2013 -> Security -> Cryptography "S/MIME interoperability with external clients" is set to "Enabled (Handle internally)". Procedure: Use the Windows Registry Editor to navigate to the following key: HKCU\Software\Policies\Microsoft\Office\15.0\outlook\security Criteria: If the value ExternalSMime is REG_DWORD = 0, this is not a finding.

Fix Text

Set the policy value for User Configuration -> Administrative Templates -> Microsoft Outlook 2013 -> Security -> Cryptography "S/MIME interoperability with external clients" to "Enabled (Handle internally)".

Additional Identifiers

Rule ID: SV-54003r1_rule

Vulnerability ID: V-17790

Group Title: DTOO257 - No S/Mime interop w/ external clients

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.