Attack Chain

1. A user downloads a malicious file, often delivered via phishing or drive-by download, containing a crafted .theme file.
2. The malicious file is saved to a common user directory such as Downloads.
3. The attacker executes code (e.g., via a script or executable) that creates or copies a specially crafted .theme file in a location such as C:\Users\<username>\Downloads.
4. The user or attacker interacts with the .theme file, triggering the parsing of its contents.
5. The crafted .theme file contains malicious directives that exploit vulnerabilities to execute arbitrary code or initiate NTLM authentication to a rogue server controlled by the attacker.
6. If code execution is achieved, the attacker gains control of the user’s system.
7. If NTLM coercion is successful, the attacker captures the user’s credentials.
8. The attacker uses compromised credentials or system control for lateral movement or data exfiltration.

Impact

Successful exploitation via malicious theme files can lead to remote code execution, allowing attackers to gain control over the victim’s system. NTLM coercion can result in credential theft, enabling lateral movement and further compromise of the network. The scope of impact depends on the attacker’s objectives, but may include data exfiltration, ransomware deployment, or long-term persistence within the environment.

Recommendation

• Enable Sysmon EventID 11 logging to collect file creation events, which is required for the detections to function.
• Deploy the Sigma rules provided below to your SIEM to detect suspicious theme file creation and tune for your environment.
• Investigate any alerts generated by these rules, paying close attention to the process creating the theme file and the user context.
• Implement strict file download policies to reduce the risk of users downloading and executing malicious files.
• Educate users about the risks associated with opening untrusted files, especially those with unusual extensions like .theme.