Windows DNS Server Remote Code Execution Vulnerability

Introduction

On July 14, 2020, Microsoft released a security update for the issue 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 leveraged 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 applying the security update in order to enable them to update their systems by using a standard deployment cadence.

Workaround

To work around this vulnerability, make the following registry change to restrict the size of the largest inbound TCP-based DNS response packet allowed:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\DNS\Parameters

TcpReceivePacketSize

Value = 0xFF00

Note You must restart the DNS Service for the registry change to take effect.

The Default (also max) Value = 0xFFFF
The Recommended Value = 0xFF00 (255 bytes less than the max)

After the workaround is implemented, a Windows DNS server will be unable to resolve DNS names for its clients when the DNS response from the upstream server is larger than 65280 bytes.

Important information about this workaround

TCP-based DNS response packets that exceed the recommended value will be dropped without error, so it is possible that some queries may not be answered. This could result in an unanticipated failure. A DNS server will only be negatively impacted by this workaround if it receives valid TCP responses that are greater than allowed in the previous mitigation (over 65,280 bytes).

The reduced value is unlikely to affect standard deployments or recursive queries, but a non-standard use-case may be present 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 need to review the log files to identify the presence of anomalously large TCP response packets

For more information, see DNS Logging and Diagnostics on the Microsoft Docs website.