Ensure mail transport rules do not whitelist specific domains

MédiaNível 1Revisão manualm365

Descrição

Mail flow rules (transport rules) in Exchange Online can be configured to set the spam confidence level (SCL) of a message to -1, which bypasses spam and phishing filtering. When a rule applies this action to messages based on the sender's domain, all mail from that domain is treated as trusted and skips anti-malware and anti-phishing evaluation regardless of message content. Whitelisting domains in transport rules bypasses regular malware and phishing scanning, which can enable an attacker to launch attacks against your users from a safe haven domain. Note: If an organization identifies a business need for an exception, the domain should only be whitelisted if inbound emails from that domain originate from a specific IP address. These exceptions should be documented and regularly reviewed.

Como é verificado

Método de auditoria: Revisão manual

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To audit using the UI: 1. Navigate to Exchange admin center https://admin.exchange.microsoft.com 2. Click to expand Mail Flow and then select Rules. 3. Review each rule and ensure that a single rule does not contain both of these properties together: o Under Apply this rule if: Sender's address domain portion belongs to any of these domains: '<domain>' o Under Do the following: Set the spam confidence level (SCL) to '-1' Note: Setting the spam confidence level to -1 indicates the message is from a trusted sender, so the message bypasses spam filtering. The recommendation fails if any external domain has a SCL of -1. To audit using PowerShell: 1. Connect to Exchange online using Connect-ExchangeOnline. 2. Run the following PowerShell command: Get-TransportRule | Where-Object { $_.setscl -eq -1 -and $_.SenderDomainIs - ne $null } | ft Name,SenderDomainIs,SetSCL 3. Transport rules that fail the audit will be shown. If no output is shown, the recommendation passes. To pass, all rules with SetSCL set to -1 must not include any domains in the SenderDomainIs property.

Como é corrigido

Método de correção: Orientação ou patch de infraestrutura

To remediate using the UI: 1. Navigate to Exchange admin center https://admin.exchange.microsoft.com 2. Click to expand Mail Flow and then select Rules. 3. For each rule that sets the spam confidence level to -1 for a specific domain, select the rule and click Delete. To remediate using PowerShell: 1. Connect to Exchange online using Connect-ExchangeOnline. 2. To remove a specific non-compliant rule: Remove-TransportRule -Identity "RuleName" Note: If the rule serves a legitimate purpose beyond domain whitelisting, consider modifying it to remove the SenderDomainIs condition or the SetSCL -1 action rather than deleting it entirely.

Propriedades

Sistemas operacionais: Qualquer
Tags: m365saas
Referência: CIS Microsoft 365 Foundations Benchmark v7.0.0 - 6.2.2 Ensure mail transport rules do not whitelist specific domains