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KB4569509: Guidance for DNS Server Vulnerability CVE-2020-1350

Introduction On July 14, 2020, Microsoft released a security update for the issue that is described in CVE-2020-1350 | Windows DNS Server Remote Code Execution Vulnerability. This advisory describes a Critical Remote Code Execution (RCE) vulnerability that affects Windows servers that are configured to run the DNS Server role. We strongly recommend that server administrators apply the security update at their earliest convenience. A registry-based workaround can be used to help protect an affected Windows server, and it can be implemented without requiring an administrator to restart the server. Because of the volatility of this vulnerability, administrators may have to implement the workaround before they apply the security update in order to enable them to update their systems by using a standard deployment cadence.

Workaround Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur. To work around this vulnerability, make the following registry change to restrict the size of the largest inbound TCP-based DNS response packet that's allowed: Subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters  Value: TcpReceivePacketSize Type: DWORD  Value data: 0xFF00 Notes

  • The default (also maximum) Value data = 0xFFFF.

  • The recommended Value data = 0xFF00 (255 bytes less than the maximum).

  • You must restart the DNS Service for the registry change to take effect. To do this, run the following command at an elevated command prompt:

net stop dns && net start dns After the workaround is implemented, a Windows DNS server will be unable to resolve DNS names for its clients if the DNS response from the upstream server is larger than 65,280 bytes. Important information about this workaround TCP-based DNS response packets that exceed the recommended value will be dropped without error. Therefore, it is possible that some queries might not be answered. This could cause an unanticipated failure. A DNS server will be negatively impacted by this workaround only if it receives valid TCP responses that are greater than allowed in the previous mitigation (more than 65,280 bytes). The reduced value is unlikely to affect standard deployments or recursive queries. However, a non-standard use-case may exist in a given environment. To determine whether the server implementation will be adversely affected by this workaround, you should enable diagnostic logging, and capture a sample set that is representative of your typical business flow. Then, you will have to review the log files to identify the presence of anomalously large TCP response packets For more information, see DNS Logging and Diagnostics.

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