TCP connection response retransmissions are not controlled.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-4437	5.097	SV-29612r1_rule	ECSC-1	Low

Description
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.

STIG	Date
Windows 2003 Domain Controller Security Technical Implementation Guide	2015-03-09

Details

Check Text ( C-2235r1_chk )
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.

Check Text ( C-2235r1_chk )

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 (F-6084r1_fix)
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”.

Fix Text (F-6084r1_fix)

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”.