Unusual Child Processes of RunDLL32 Execution Without Arguments

This detection identifies instances where rundll32.exe is executed without arguments or with malformed arguments, immediately followed by the execution of a child process. This behavior is atypical, as rundll32.exe is normally invoked with specific parameters indicating a DLL, export, or Control_RunDLL target. Attackers may exploit this by using rundll32.exe as a proxy to execute other malicious payloads or for command and control. The detection logic focuses on identifying instances where the argument count is one and the command line does not conform to expected patterns. This behavior has been observed being used by malware to evade traditional detection methods by proxying execution through a trusted Windows utility. This rule is applicable to endpoint telemetry, Windows event logs, and Crowdstrike FDR data.

Attack Chain

1. An initial access vector, such as a phishing email or exploit, delivers an initial payload to the system.
2. The initial payload executes, potentially dropping or creating a file on disk, or directly invoking rundll32.exe.
3. rundll32.exe is executed without arguments, or with malformed arguments, bypassing typical usage patterns. This is the key indicator the rule detects.
4. rundll32.exe spawns a child process, which could be a script interpreter (e.g., powershell.exe, cmd.exe), another executable, or a network utility.
5. The child process executes malicious code, downloads additional payloads, or establishes a command and control connection.
6. The attacker leverages the child process for lateral movement or privilege escalation within the network.
7. The final objective could include data exfiltration, ransomware deployment, or persistent access to the compromised system.
8. The adversary uses rundll32.exe to hide the execution of the malicious process and blend into normal system activity.

Impact

Successful exploitation can lead to arbitrary code execution, allowing attackers to gain control of the affected system. This can result in data breaches, system compromise, and potential lateral movement within the network. The use of a trusted system binary like rundll32.exe makes detection more challenging. It affects Windows systems and can be used in targeted attacks as well as widespread campaigns. Organizations failing to detect this behavior are at risk of significant data loss and operational disruption.

Recommendation

• Deploy the Sigma rule Unusual RunDLL32 Child Process to your SIEM and tune for your environment to detect the execution of rundll32.exe without arguments, followed by a child process.
• Enable Sysmon process creation logging (Event ID 1) to collect the necessary data for the Sigma rule.
• Investigate any alerts generated by this rule to determine the legitimacy of the rundll32.exe execution and the spawned child process.
• Implement application control policies to restrict the execution of unsigned or untrusted executables in your environment, mitigating the impact of this technique.
• Monitor process execution events for unusual parent-child relationships involving rundll32.exe.