Potential File Transfer via Curl for Windows

This threat brief highlights the potential misuse of Curl for Windows by adversaries to facilitate command and control activities and data exfiltration. Curl, a legitimate command-line tool used for transferring data with URLs, can be leveraged to download malicious payloads or upload sensitive information to remote servers. The detection rule focuses on identifying instances where curl.exe is executed with HTTP parameters, especially when initiated from common scripting environments. The goal of this detection is to identify suspicious file transfers indicating potential malicious activity originating from Windows systems within the organization. This activity is often seen after initial compromise and can be indicative of lateral movement or data theft.

Attack Chain

1. An attacker gains initial access to a Windows system, potentially through phishing or exploiting a vulnerability.
2. The attacker executes a command shell (cmd.exe or powershell.exe) on the compromised host.
3. Curl for Windows (curl.exe) is invoked from the command shell, using parameters to download a file from a remote HTTP server.
4. The downloaded file is saved to disk, potentially in a temporary directory or a user’s profile.
5. The attacker executes the downloaded file, which could be malware, a script, or another malicious tool.
6. Curl is used to exfiltrate data from the victim machine back to an attacker-controlled server.
7. The attacker may use curl to download additional tools or scripts to further compromise the system or network.
8. The attacker maintains persistence and continues to exfiltrate sensitive data from the victim machine.

Impact

Successful exploitation can lead to the introduction of malware onto the system, potentially compromising sensitive data, enabling lateral movement within the network, and establishing a persistent presence for the attacker. The impact could range from data theft and system disruption to complete network compromise, with potential financial losses, reputational damage, and legal repercussions. The severity is dependent on the sensitivity of the data compromised and the extent of the attacker’s access within the network.

Recommendation

• Deploy the “Potential File Transfer via Curl for Windows” Sigma rule to your SIEM to detect suspicious Curl activity on Windows endpoints.
• Enable Sysmon process creation logging to capture detailed process execution data, enhancing the effectiveness of the detection rule.
• Investigate any alerts generated by the Sigma rule, focusing on the parent process of curl.exe and the destination of the HTTP request.
• Monitor network traffic for connections to known malicious domains or IP addresses, especially those associated with Curl-initiated transfers.
• Implement application control policies to restrict the execution of unauthorized or untrusted executables, including curl.exe, in sensitive environments.