Exchange Outbound Connection Limit per Domain Count must be controlled.

From MS Exchange 2013 Edge Transport Server Security Technical Implementation Guide

Associated with: CCI-001095

SV-84443r1_rule Exchange Outbound Connection Limit per Domain Count must be controlled.

Vulnerability discussion

Email system availability depends in part on best practice strategies for setting tuning configurations. This configuration controls the maximum number of simultaneous outbound connections from a domain and works in conjunction with the Maximum Outbound Connections Count setting as a delivery tuning mechanism. If the limit is too low, connections may be dropped. If the limit is too high, some domains may use a disproportionate resource share, denying access to other domains. Appropriate tuning reduces the risk of data delay or loss. By default, a limit of 20 simultaneous outbound connections from a domain should be sufficient. The value may be adjusted if justified by local site conditions.

Check content

Review the Email Domain Security Plan (EDSP). Determine the value for Maximum Domain Connections. Open the Exchange Management Shell and enter the following command: Get-TransportService | Select Name, Identity, MaxPerDomainOutboundConnections If the value of MaxPerDomainOutboundConnections is not set to 20, this is a finding. or If the value of MaxPerDomainOutboundConnections is set to a value other than 20 and has signoff and risk acceptance in the EDSP, this is not a finding.

Fix text

Update the EDSP. Open the Exchange Management Shell and enter the following command: Set-TransportService -Identity <'IdentityName'> -MaxPerDomainOutboundConnections 20 Note: The value must be in quotes. or The value as identified by the EDSP that has obtained a signoff with risk acceptance.

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