Title

TCP connection response retransmissions are not controlled. (Cat III impact)

Discussion

In a SYN flood attack, the attacker sends a continuous stream of SYN packets to a server, and the server leaves the half-open connections open until it is overwhelmed and no longer is able to respond to legitimate requests.

Check Content

Analyze the system using the Security Configuration and Analysis snap-in. Expand the Security Configuration and Analysis tree view. Navigate to Local Policies -> Security Options. If the value for “MSS: (TcpMaxConnectResponseRetransmissions) SYN-ACK retransmissions when a connection is not acknowledged” is not set to “3 & 6 seconds, half-open connections dropped after 21 seconds”, “3 seconds, half-open connections dropped after 9 seconds” or “No retransmission, half-open connections dropped after 3 seconds”, then this is a finding. The policy referenced configures the following registry value: Registry Hive: HKEY_LOCAL_MACHINE Registry Path: \System\CurrentControlSet\Services\Tcpip\Parameters\ Value Name: TcpMaxConnectResponseRetransmissions Value Type: REG_DWORD Value: 2 (3 & 6 seconds), 1 (3 seconds), 0 (No retransmission) Warning: Microsoft cautions that setting this to “No retransmission, half-open connections dropped after 3 seconds” may cause legitimate connection attempts from distant clients to fail due to time-out.

Fix Text

Configure the policy value for Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options -> “MSS: (TcpMaxConnectResponseRetransmissions) SYN-ACK retransmissions when a connection is not acknowledged” to “3 & 6 seconds, half-open connections dropped after 21 seconds”, “3 seconds, half-open connections dropped after 9 seconds” or “No retransmission, half-open connections dropped after 3 seconds”.

Additional Identifiers

Rule ID: SV-4437r1_rule

Vulnerability ID: V-4437

Group Title: TCP Connection Response Retransmissions

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