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T1055.004 Asynchronous Procedure Call

Adversaries may inject malicious code into processes via the asynchronous procedure call (APC) queue in order to evade process-based defenses as well as possibly elevate privileges. APC injection is a method of executing arbitrary code in the address space of a separate live process.

APC injection is commonly performed by attaching malicious code to the APC Queue 1 of a process’s thread. Queued APC functions are executed when the thread enters an alterable state.1 A handle to an existing victim process is first created with native Windows API calls such as OpenThread. At this point QueueUserAPC can be used to invoke a function (such as LoadLibrayA pointing to a malicious DLL).

A variation of APC injection, dubbed “Early Bird injection”, involves creating a suspended process in which malicious code can be written and executed before the process’ entry point (and potentially subsequent anti-malware hooks) via an APC. 2 AtomBombing 3 is another variation that utilizes APCs to invoke malicious code previously written to the global atom table.4

Running code in the context of another process may allow access to the process’s memory, system/network resources, and possibly elevated privileges. Execution via APC injection may also evade detection from security products since the execution is masked under a legitimate process.

Item Value
ID T1055.004
Sub-techniques T1055.001, T1055.002, T1055.003, T1055.004, T1055.005, T1055.008, T1055.009, T1055.011, T1055.012, T1055.013, T1055.014, T1055.015
Tactics TA0005, TA0004
Platforms Windows
Version 1.2
Created 14 January 2020
Last Modified 24 October 2025

Procedure Examples

ID Name Description
S0438 Attor Attor performs the injection by attaching its code into the APC queue using NtQueueApcThread API.15
S1081 BADHATCH BADHATCH can inject itself into a new svchost.exe -k netsvcs process using the asynchronous procedure call (APC) queue.1314
S1039 Bumblebee Bumblebee can use asynchronous procedure call (APC) injection to execute commands received from C2.8
S0484 Carberp Carberp has queued an APC routine to explorer.exe by calling ZwQueueApcThread.10
G0061 FIN8 FIN8 has injected malicious code into a new svchost.exe process.17
S0483 IcedID IcedID has used ZwQueueApcThread to inject itself into remote processes.11
S0260 InvisiMole InvisiMole can inject its code into a trusted process via the APC queue.7
S0517 Pillowmint Pillowmint has used the NtQueueApcThread syscall to inject code into svchost.exe.6
S1018 Saint Bot Saint Bot has written its payload into a newly-created EhStorAuthn.exe process using ZwWriteVirtualMemory and executed it using NtQueueApcThread and ZwAlertResumeThread.9
S1085 Sardonic Sardonic can use the QueueUserAPC API to execute shellcode on a compromised machine.16
S0199 TURNEDUP TURNEDUP is capable of injecting code into the APC queue of a created Rundll32 process as part of an “Early Bird injection.”2
S1207 XLoader XLoader injects code into the APC queue using NtQueueApcThread API.12

Mitigations

ID Mitigation Description
M1040 Behavior Prevention on Endpoint Some endpoint security solutions can be configured to block some types of process injection based on common sequences of behavior that occur during the injection process.

References

  1. Microsoft. (n.d.). Asynchronous Procedure Calls. Retrieved December 8, 2017. ↩↩

  2. Gavriel, H. & Erbesfeld, B. (2018, April 11). New ‘Early Bird’ Code Injection Technique Discovered. Retrieved May 24, 2018. ↩↩

  3. Liberman, T. (2016, October 27). ATOMBOMBING: BRAND NEW CODE INJECTION FOR WINDOWS. Retrieved December 8, 2017. ↩

  4. Microsoft. (n.d.). About Atom Tables. Retrieved December 8, 2017. ↩

  5. Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017. ↩

  6. Trustwave SpiderLabs. (2020, June 22). Pillowmint: FIN7’s Monkey Thief . Retrieved July 27, 2020. ↩

  7. Hromcova, Z. and Cherpanov, A. (2020, June). INVISIMOLE: THE HIDDEN PART OF THE STORY. Retrieved July 16, 2020. ↩

  8. Merriman, K. and Trouerbach, P. (2022, April 28). This isn’t Optimus Prime’s Bumblebee but it’s Still Transforming. Retrieved August 22, 2022. ↩

  9. Hasherezade. (2021, April 6). A deep dive into Saint Bot, a new downloader. Retrieved June 9, 2022. ↩

  10. Giuliani, M., Allievi, A. (2011, February 28). Carberp - a modular information stealing trojan. Retrieved September 12, 2024. ↩

  11. Kessem, L., et al. (2017, November 13). New Banking Trojan IcedID Discovered by IBM X-Force Research. Retrieved July 14, 2020. ↩

  12. Zscaler Threatlabz. (2025, January 27). Technical Analysis of Xloader Versions 6 and 7 | Part 1. Retrieved March 11, 2025. ↩

  13. Savelesky, K., et al. (2019, July 23). ABADBABE 8BADFOOD: Discovering BADHATCH and a Detailed Look at FIN8’s Tooling. Retrieved September 8, 2021. ↩

  14. Vrabie, V., et al. (2021, March 10). FIN8 Returns with Improved BADHATCH Toolkit. Retrieved September 8, 2021. ↩

  15. Hromcova, Z. (2019, October). AT COMMANDS, TOR-BASED COMMUNICATIONS: MEET ATTOR, A FANTASY CREATURE AND ALSO A SPY PLATFORM. Retrieved May 6, 2020. ↩

  16. Symantec Threat Hunter Team. (2023, July 18). FIN8 Uses Revamped Sardonic Backdoor to Deliver Noberus Ransomware. Retrieved August 9, 2023. ↩

  17. Martin Zugec. (2021, July 27). Deep Dive Into a FIN8 Attack - A Forensic Investigation. Retrieved September 1, 2021. ↩