CIS.M365.2.1.9: Ensure that DKIM is enabled for all Exchange Online Domains


ID	`CIS.M365.2.1.9`
Severity	Medium
Suite	CIS Microsoft 365 Foundations Benchmark
Product	Defender
Related products	Exchange Online
Feature	Email authentication
Security area	Email and collaboration protection
Resource type	Domain
User impact of fixing	Low
Effort to fix	Medium
Benchmark	CIS Microsoft 365 Foundations Benchmark 7.0.0, `2.1.9` (L1, E3 Level 1)
Framework controls	`cis-m365:2.1.9` `nist-800-53-r5:SC-8` Transmission Confidentiality and Integrity
MITRE ATT&CK	T1566, T1684.002
Reads from	Exchange Online

What this checks and how to fix it

2.1.9 (L1) Ensure that DKIM is enabled for all Exchange Online Domains

DKIM is one of the trio of Authentication methods (SPF, DKIM and DMARC) that help prevent attackers from sending messages that look like they come from your domain. DKIM lets an organization add a digital signature to outbound email messages in the message header. When DKIM is configured, the organization authorizes it’s domain to associate, or sign, its name to an email message using cryptographic authentication. Email systems that get email from this domain can use a digital signature to help verify whether incoming email is legitimate. Use of DKIM in addition to SPF and DMARC to help prevent malicious actors using spoofing techniques from sending messages that look like they are coming from your domain.

Rationale

By enabling DKIM with Office 365, messages that are sent from Exchange Online will be cryptographically signed. This will allow the receiving email system to validate that the messages were generated by a server that the organization authorized and not being spoofed.

Impact

There should be no impact of setting up DKIM however, organizations should ensure appropriate setup to ensure continuous mail-flow.

Note: Initial (MOERA, e.g. contoso.onmicrosoft.com) and coexistence (*.mail.onmicrosoft.com) domains are excluded from this check, as Microsoft automatically signs outbound mail for those domains.

Remediation action

To remediate using a DNS Provider:

For each accepted domain in Exchange Online, two DNS entries are required.

Host name: selector1._domainkey
Points to address or value: selector1-<domainGUID>._domainkey.<initialDomain>
TTL: 3600
Host name: selector2._domainkey
Points to address or value: selector2-<domainGUID>._domainkey.<initialDomain>
TTL: 3600

For Office 365, the selectors will always be selector1 or selector2.

domainGUID is the same as the domainGUID in the customized MX record for your custom domain that appears before mail.protection.outlook.com. For example, in the following MX record for the domain contoso.com, the domainGUID is contoso-com:

contoso.com. 3600 IN MX 5 contoso-com.mail.protection.outlook.com

The initial domain is the domain that you used when you signed up for Office 365. Initial domains always end with onmicrosoft.com.

After the DNS records are created, enable DKIM signing in Defender.
Navigate to Microsoft 365 Defender
Expand Email & collaboration > Policies & rules > Threat policies.
Under Rules section click Email authentication settings.
Select DKIM
Click on each domain and click Enable next to Sign messages for this domain with DKIM signature.

Final remediation step using the Exchange Online PowerShell Module:

Connect to Exchange Online service using Connect-ExchangeOnline.
Run the following Exchange Online PowerShell command:

Set-DkimSigningConfig -Identity < domainName > -Enabled $True

Mito check, based on CIS Microsoft 365 Foundations Benchmark. Author: NZLostboy; contributors: JeanPhilippeGeorge, brianreidc7, amlhive-tech. Open-source licences.

Open-source licences. CIS Benchmark material © Center for Internet Security, Inc.; CISA SCuBA baselines are public domain (CC0).