Some organisations do not use Exchange Online Protection and Microsoft Defender for 365 to protect their Exchange Online tenant and use 3rd party message hygiene services like Mimecast and Proof Point. This blog will demonstrate a a scenario where securing Exchange Online message routing is configured incorrectly and could be classified a vulnerability,

SCENARIO 2

1. Contoso.com MX record is pointed at Mimecast; Mimecast provides spam protection
2. Mimecast then passes the messages to Exchange Online
3. If there are any remaining Exchange on-premise recipients, Exchange online will route the messages to the Exchange Hybrid servers via the secure Exchange Hybrid connectors
3. The Exchange Online inbound connector that accepts traffic from the Mimecast service is secured via TLS

VULNERABILITY
Anyone in the world can send an email to to an Exchange Online recipient with a *.mail.onmicrosoft.com or *.onmicrosoft.com alias, when sending to these domains. The messages completely bypasses , the organisation’s Mimecast’s message hygiene services and can route messages to Exchange Online recipients and Exchange on-premise recipients. If the organisation has not configured Exchange Online protection and Microsoft Defender for 365, then the organisation is vulnerable to malware and phishing emails.

Another problem: Office365 groups, When a Teams Channel is created, it creates an Office365 group that will have a *.onmicrosoft.com alias. Bad actors sending emails to these aliases can once again completely bypass the organisation’s message hygiene services.

SOLUTION
The Hybrid configuration wizard creates an Exchange Online inbound connector that is locked down with TLS via a public trusted certificate on the Exchange Hybrid servers.

The default inbound Exchange Online connector that was created by the Exchange Hybrid wizard, can be modified to only accept inbound messages from the IP ranges of the Mimecast service and TLS.

This script queries the existing inbound connector and creates an inbound connector that blocks messages recipients using the *.mail.onmicrosoft.com to only accept traffic from a service using the TLS certificate and connector that has been modified or it can query a new inbound connector

In this scenario , when a message is sent to an Exchange online recipient , the message flows as follows.
1. MX contoso.com
2. Mimecast
3. Contoso.com aliases & all consto.com *.onmicrosoft.com will only accept messages from Mimecast

Note: run this script using the latest Exchange Online PowerShell module

New-InboundConnector -Name ‘Restrict inbound mail flow to hybrid domains’ -ConnectorType Partner -SenderDomains * -TlsSenderCertificateName (Get-InboundConnector $InboundConnectorName).TlsSenderCertificateName -RestrictDomainsToCertificate $true -RequireTls $true