Pushing the Limits of Cyber Threat Intelligence: Extending STIX to Support Complex Patterns

Nowadays, attacks against single computer systems or whole infrastructures pose a significant risk. Although deployed security systems are often able to prevent and detect standard attacks in a reliable way, it is not uncommon that more sophisticated attackers are capable to bypass these systems and stay undetected. To support the prevention and detection of attacks, the sharing of cyber threat intelligence information becomes increasingly important. Unfortunately, the currently available threat intelligence formats, such as YARA or STIX (Structured Threat Information eXpression), cannot be used to describe complex patterns that are needed to share relevant attack details about more sophisticated attacks.In this paper, we propose an extension for the standardized STIX format that allows the description of complex patterns. With this extension it is possible to tag attributes of an object and use these attributes to describe precise relations between different objects. To evaluate the proposed STIX extension we analyzed the API calls of the credential dumping tool Mimikatz and created a pattern based on these calls. This pattern precisely describes the performed API calls of Mimikatz to access the LSASS (Local Security Authority Subsystem Service) process, which is responsible for authentication procedures in Windows. Due to the specified relations, it is possible to detect the execution of Mimikatz in a reliable way.