How to defend ourselves?

We can use MITRE Att&ck framework to asses defensive capability across your security architecture.

The MITRE ATT&K® framework helps provide context to the Sunburst campaign. The following represent known tactics and techniques:

Query Registry [T1012]
Obfuscated Files or Information [1027]
Obfuscated Files or Information: Steganography [T1027.003]
Process Discovery [T1057]
Indicator Removal on Host: File Deletion [T1070.004]
Application Layer Protocol: Web Protocols [T1071.001]
Application Layer Protocol: DNS [T1071.004]
File and Directory Discovery [T1083]
Ingress Tool Transfer [T1105]
Data Encoding: Standard Encoding [T1132.001]
Supply Chain Compromise: Compromise Software Dependencies and Development Tools [ [T1195.001]
Supply Chain Compromise: Compromise Software Supply Chain [T1195.002]
Software Discovery [T1518]
Software Discovery: Security Software Discovery [T1518.001]
Create or Modify System Process: Windows Service [T1543.003]
Subvert Trust Controls: Code Signing [T1553.002]
Dynamic Resolution: Domain Generation Algorithms [T1568.002]
System Services: Service Execution [T1569.002]
Compromise Infrastructure [T1584]

Mitigation steps

Implementing multi factor authentication.
Monitoring all services for any changes in tokens or keys and for malicious activities.
Re-evaluating API key integrations, SAML integrations and website configuration files.
Review all system and security policies.
Resetting user credentials.
Consider security auditing.