New Windows Injection Technique Hijacks Win32k Callback Dispatch to Execute Shellcode

A newly documented injection technique abuses the kernel-to-user callback dispatch path used by the Windows graphical subsystem (win32k.sys) to achieve remote code execution while leaving the KernelCallbackTable structurally intact.

Rather than replacing a KernelCallbackTable entry with a shellcode pointer, the operator resolves a legitimate callback target from the table and installs an inline detour inside that user-mode routine causing expected GUI callbacks to jump into attacker-controlled shellcode when invoked.

Because the modification occurs at the callback’s entrypoint rather than the PEB table itself, this method reduces obvious artifacts that many integrity checks target, offering a stealthier alternative to remote thread creation and APC-based injection.

After the kernel switches to user mode, execution arrives at ntdll’s KiUserCallbackDispatcher, which looks up the requested callback index in the target process’s KernelCallbackTable, stored in its Process Environment Block (PEB), and transfers control to the associated user-mode handler typically a function in user32.dll.

By enumerating entries in the KernelCallbackTable, an attacker can resolve a concrete function pointer for a frequently reachable callback such as __fnCOPYDATA and prepare a deterministic trigger.

According to Medium, the technique exploits the standard KeUserModeCallback transition, which win32k.sys uses to ask user-mode code to perform GUI-related work.

Instead of overwriting the KernelCallbackTable entry (a technique defenders can detect by validating PEB structures or table integrity), the operator allocates executable memory in the target, writes shellcode, preserves the original prologue bytes of the legitimate callback routine, and patches the routine’s start with an absolute jump to the injected code.

The choice of __fnCOPYDATA as the detour target is deliberate: it is invoked in response to WM_COPYDATA messages and can be triggered externally via SendMessage, providing a reliable, in-band execution primitive without unusual process state or complex synchronization.

Windows Injection Technique Hijacks Win32k

Technical implementation follows a simple three-step flow. First, the attacker opens the target process, queries its PROCESS_BASIC_INFORMATION to obtain the PEB address, and reads the KernelCallbackTable pointer at the PEB offset.

Second, executable memory is allocated remotely (NtAllocateVirtualMemory) and shellcode is written via NtWriteVirtualMemory.

Third, the attacker reads and preserves the original callback prologue (NtReadVirtualMemory), temporarily changes page protections (NtProtectVirtualMemory), writes an x64 absolute jump stub over the prologue, triggers the callback with SendMessageW/WM_COPYDATA.

Because the modification is limited to the in-memory prologue of a legitimate function, traditional heuristics that focus on altered PEB structures or unexpected callback table addresses may miss this activity.

However, defenders can detect this technique by monitoring for in-process code-page modifications to well-known user32 callback routines, anomalous WriteProcessMemory/NtWriteVirtualMemory sequences followed by immediate SendMessage triggers, or temporary executable allocations near callback invocation windows.

Instrumenting EDR to capture remote memory protection changes (PAGE_EXECUTE_READWRITE) and correlating them with GUI message traffic provides a practical detection avenue.

For additional technical context and mitigation guidance, see Microsoft’s documentation on KeUserModeCallback and KiUserCallbackDispatcher and public write-ups on KernelCallbackTable injection and inline function detouring.

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