Asynchronous Procedure Call

Asynchronous Procedure Call [T1055.004]

Information

• Name: Asynchronous Procedure Call

• ID: T1055.004

• Tactics: TA0005, TA0004

• Technique: T1055

Introduction

Asynchronous Procedure Call (APC) injection, identified as sub-technique T1055.004 in the MITRE ATT&CK framework, involves adversaries injecting malicious code into a thread's APC queue to execute arbitrary code within the context of a legitimate process. APC injection allows attackers to evade detection and achieve persistence by leveraging legitimate system processes, making it challenging for defenders to identify malicious activities. This sub-technique is a subset of Process Injection (T1055), specifically focusing on exploiting APC mechanisms within Windows operating systems.

Deep Dive Into Technique

An Asynchronous Procedure Call (APC) is a Windows mechanism used to execute code asynchronously in the context of a particular thread. APC injection involves inserting malicious code into the APC queue of a thread within a legitimate process, causing this code to execute when the thread enters an alertable state. Key technical points include:

• Alertable State: Threads periodically enter alertable states when they call certain Windows API functions, such as:

  • SleepEx

  • WaitForSingleObjectEx

  • WaitForMultipleObjectsEx

  • MsgWaitForMultipleObjectsEx

  • SignalObjectAndWait

• Execution Methodology:

  1. Identify a suitable target process/thread.

  2. Allocate memory within the target process using APIs like VirtualAllocEx.

  3. Write malicious payload into the allocated memory using WriteProcessMemory.

  4. Queue the APC using Windows APIs such as QueueUserAPC, specifying the thread handle and the address of the malicious code.

  5. The APC executes when the targeted thread enters an alertable state, running the injected payload.

• Advantages for Attackers:

  • Leveraging legitimate processes reduces suspicion.

  • APC injection can evade traditional antivirus and endpoint detection systems.

  • Difficult to detect due to its subtlety and reliance on system-level functionality.

• Commonly Targeted Processes:

  • explorer.exe

  • svchost.exe

  • Web browsers and other frequently running processes.

When this Technique is Usually Used

Attackers typically use APC injection at various stages of the attack lifecycle, including:

• Execution Stage:

  • Initial payload execution after gaining initial access.

  • Execution of secondary-stage malware or payloads.

• Persistence Stage:

  • Maintaining long-term access by injecting code into legitimate system processes that run persistently.

• Defense Evasion Stage:

  • Avoiding detection by injecting into legitimate system processes.

  • Obfuscating malicious behavior by running code within trusted processes.

• Privilege Escalation:

  • Potentially injecting code into higher-privileged processes to escalate privileges.

• Lateral Movement:

  • Injecting code into processes across the network to move laterally without detection.

How this Technique is Usually Detected

Detection of APC injection is challenging but achievable through a combination of behavioral monitoring, memory analysis, and endpoint detection tools. Effective detection methods include:

• Endpoint Detection and Response (EDR) Tools:

  • Monitor API calls such as QueueUserAPC, VirtualAllocEx, and WriteProcessMemory.

  • Detect suspicious memory allocation and injection patterns.

• Behavioral Monitoring:

  • Identify unusual activity within legitimate processes, such as unexpected network connections or file access.

  • Monitor threads entering alertable states frequently or abnormally.

• Memory Forensics:

  • Analyze process memory to detect anomalies or injected code.

  • Identify suspicious memory pages with execute permissions allocated dynamically.

• Event Log Analysis:

  • Monitor Windows Security logs for process injection-related events.

  • Track process creation and injection attempts via Sysmon logs.

• Indicators of Compromise (IoCs):

  • Unusual memory allocation patterns.

  • Suspicious API calls logged by EDR solutions.

  • Unexpected thread behavior within legitimate processes.

  • Anomalous network traffic originating from system processes.

Why it is Important to Detect This Technique

Early detection of APC injection is critical due to its potential severe impacts on systems and networks, including:

• Stealthy Persistence:

  • Attackers can maintain long-term access without detection, allowing prolonged reconnaissance and exploitation.

• Privilege Escalation:

  • Injecting code into higher-privileged processes can lead to elevated privileges, enabling further exploitation.

• Data Exfiltration:

  • Malicious code within legitimate processes can facilitate covert data theft and exfiltration.

• Defense Evasion:

  • Difficult to detect using traditional antivirus solutions, allowing attackers to operate undetected.

• System Instability:

  • Injection into critical processes may cause system instability or crashes, impacting system reliability.

• Lateral Movement:

  • APC injection can enable attackers to traverse networks discreetly, compromising multiple systems.

Detecting APC injection early limits attackers' ability to establish persistence, escalate privileges, and exfiltrate sensitive data, significantly reducing the overall impact of a breach.

Examples

Real-world examples of APC injection include various malware families and threat actor campaigns, demonstrating the technique's effectiveness and popularity:

• DoublePulsar (NSA Exploit):

  • Utilized APC injection to execute payloads within the context of legitimate processes.

  • Enabled stealthy persistence and lateral movement across compromised networks.

• Dridex Banking Trojan:

  • Employed APC injection to inject malicious payloads into legitimate Windows processes, evading detection and facilitating credential theft.

• Cobalt Strike Framework:

  • Provides built-in capability for APC injection as part of its post-exploitation modules.

  • Frequently leveraged by threat actors for stealthy persistence and privilege escalation.

• FIN7 Threat Group:

  • Known to utilize APC injection techniques to compromise financial institutions and retail organizations.

  • Enabled persistent, stealthy access and data exfiltration from compromised systems.

• PowerSploit Framework:

  • Open-source penetration testing toolkit that includes APC injection modules (Invoke-APCInjection).

  • Used by both penetration testers and threat actors to validate security posture or exploit vulnerabilities.

These examples highlight the widespread adoption of APC injection by both sophisticated threat actors and common malware families, underscoring the importance of effective detection, prevention, and response strategies.