Email Security/Exchange Online - Ensure Mail Transport Rules Do Not Whitelist Specific Domains

By Ashwin Venugopal -

 








Summary

The Exchange Online Mail Transport rules should be set, so that, they do not whitelist any specific domains.

Reason

If certain domains are whitelisted in the transport rules, they can bypass the regular malware and phishing scanning, which in turn allows an attacker to launch attacks against any user from a safe haven domain.

What If?

One should be careful while implementing to make sure that there is no business need for case-by-case whitelisting. However, if all the whitelisted domains are removed, then, it will surely affect the incoming mail flow to an organization although modern systems sending legitimate mails should have no issues with it.

How to?

To alter the mail transport rules so they do not whitelist any specific domain, use the Microsoft 365 Admin Center:
  1. Select Exchange.
  2. Go for Mail Flow and Rules.
  3. Now, for each rule that whitelists specific domains, select the rule and click the 'Delete' icon.

To remove mail transport rules, you may also use the Exchange Online PowerShell Module:
  1. Connect to Exchange Online using Connect-ExchangeOnline.
  2. Now, run the following Exchange Online PowerShell command: 

Remove-TransportRule {RuleName}

      3. Verify the rules no longer exists.

Get-TransportRule | WhereObject {($_.setscl -eq -1 -and $_.SenderDomainIs - ne $null)} | ft Name,SenderDomainIs

Monitor:

To verify the mail transport rules do not whitelist any specific domains, use the Microsoft 365 Admin Center:
  1. Select Exchange.
  2. Go for Mail Flow and Rules.
  3. Now, review the rules and verify that none of them whitelists any specific domains.
To verify that mail transport rules do not whitelist any domains, you may also use the Exchange Online PowerShell Module:
  1. Connect to Exchange Online using Connect-ExchangeOnline.
  2. Now, run the following Exchange Online PowerShell command: 

Get-TransportRule | WhereObject {($_.setscl -eq -1 -and $_.SenderDomainIs - ne $null)} | ft Name,SenderDomainIs






































Comments

Post a Comment

Popular posts from this blog

Deployment (Part 3)

By Ashwin Venugopal -
Image
  To read part 1, please click  here To read part 2, please click  here Microsoft Monitoring Agent (MMA) MMA is used across multiple Microsoft solutions and has undergone a few name changes as its features and functionality have evolved. If it is being considered to be used, then following areas should be covered during a Microsoft Sentinel deployment project: Consider the timelines and the strategy for a migration to AMA (Azure Monitor Agent) before the MMA end-of-life.  Consider the central deployment and management methodology for MMA. Determine which endpoints are in scope for deployment as these affect the log ingestion volume significantly.  Azure Monitor Agent (AMA) Organizations with MMA as the main Microsoft Sentinel agent should start planning the migration to AMA as soon as possible to avoid a rushed migration. One of the main advantages of AMA is the ability to control the log collection policy centrally and apply different policies against different...
Read more

Deployment (Part 1)

By Ashwin Venugopal -
Image
  To read part 2, please click  here To read part 3, please click  here Azure Resources Microsoft Sentinel needs following resources to be created: Subscription (if a dedicated subscription(s) will be used) Resource group(s) Log Analytics workspace(s) Automation rules/playbook Alert rules Workbooks Microsoft Sentinel offers hundreds of alert rules, workbooks, and automation playbook templates along with hunting scripts. The templates can be used to activate/deploy schedule alerts, create customized dashboards, create automation playbooks and perform threat-hunting activities. Generally, once deployed, the resources created have to be adjusted to match the existing environment, configure local credentials, etc. Methods of deployment: Manual- Administrator can manually configures the Microsoft Sentinel resources with the help of Azure portal. Any manual process has the inherent risks of human operator error, lack of compliance with potential change control procedures, and u...
Read more

Project Resourcing (Part 2)

By Ashwin Venugopal -
Image
  To read part 1, please click  here Engineering - SIEM The SIEM engineers configures Microsoft Sentinel, including Log Analytics, Logic Apps, workbooks, and playbooks. They are also responsible for the following high-level tasks: Initial configuration of Azure tenant, including provisioning required resources, assigning access roles, and configuring workspace parameters like log retention, resource tagging, and blueprints.  Deployment and configuration of syslog/CEF log collection agents in appropriate locations to collect logs from on-premises devices. Generally, it will be joint effort of both system engineer and SIEM engineer, involving configuration and potential troubleshooting.  Working with system owners to enable log forwarding and configuring any required parsing of log data in Log Analytics.  Working with security operations to create and deploy KQL analytic rules to offer detections for SOC/Computer Security Incident Response Team (CSRIT) use. Tuning...
Read more