The firewall implementation must generate error messages that provide information necessary for corrective actions without revealing information that could be exploited by adversaries.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
SRG-NET-000273-FW-000152	SRG-NET-000273-FW-000152	SRG-NET-000273-FW-000152_rule		Medium

Description
Internet Control Message Protocol (ICMP) messages are used to provide feedback about problems in the network. These messages are sent back to the sender to support diagnostics. However, some also provide information that an attacker can use in reconnaissance of a network. A firewall rule or ACL entry must be configured to “silently drop” the packet and not send an ICMP control message back to the source. In some cases, it may be necessary to direct the traffic to a null interface. Three ICMP messages are commonly used by attackers for network mapping: Destination Unreachable, Redirect, and Address Mask Reply. These responses must be blocked on external interfaces; however, blocking the Destination Unreachable response will prevent Path Maximum Transmission Unit Discovery (PMTUD), which relies on the response “ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set”. PMTUD is a useful function and should only be “broken” after careful consideration. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces.

Description

Internet Control Message Protocol (ICMP) messages are used to provide feedback about problems in the network. These messages are sent back to the sender to support diagnostics. However, some also provide information that an attacker can use in reconnaissance of a network. A firewall rule or ACL entry must be configured to “silently drop” the packet and not send an ICMP control message back to the source. In some cases, it may be necessary to direct the traffic to a null interface. Three ICMP messages are commonly used by attackers for network mapping: Destination Unreachable, Redirect, and Address Mask Reply. These responses must be blocked on external interfaces; however, blocking the Destination Unreachable response will prevent Path Maximum Transmission Unit Discovery (PMTUD), which relies on the response “ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set”. PMTUD is a useful function and should only be “broken” after careful consideration. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces.

STIG	Date
Firewall Security Requirements Guide	2014-07-07

Details

Check Text ( C-SRG-NET-000273-FW-000152_chk )
Review the configuration of the firewall implementation; verify that the device does not send the following ICMP replies out any external interfaces: Destination Unreachable, Redirect, and Address Mask. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces. If these responses have not been blocked at the external interfaces, this is a finding.

Check Text ( C-SRG-NET-000273-FW-000152_chk )

Review the configuration of the firewall implementation; verify that the device does not send the following ICMP replies out any external interfaces: Destination Unreachable, Redirect, and Address Mask. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces. If these responses have not been blocked at the external interfaces, this is a finding.

Fix Text (F-SRG-NET-000273-FW-000152_fix)
Configure the firewall implementation to not send the following ICMP replies out any external interfaces: Destination Unreachable, Redirect, and Address Mask. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces.

Fix Text (F-SRG-NET-000273-FW-000152_fix)

Configure the firewall implementation to not send the following ICMP replies out any external interfaces: Destination Unreachable, Redirect, and Address Mask. An acceptable alternative to blocking all Destination Unreachable responses is to filter Destination Unreachable messages generated by the firewall implementation to allow ICMP Destination Unreachable—Fragmentation Needed but DF Bit Set (Type 3, Code 4) and apply this filter to the external interfaces.