specterops BOF Development Skill
When to Use
Use this skill when developing BOF files (Beacon Object Files):
- Creating new BOFs from scratch
- Porting existing tools or repos into BOF format
- Looking up BOF API functions (BeaconPrintf, BeaconDataParse, etc.)
- Understanding Dynamic Function Resolution (DFR) conventions
- Compiling and building BOFs
- Linting BOF code with
boflint.py
When NOT to Use
Do not use this skill unless you are certain that you are creating a BOF file, or working on a BOF project.
Terminology
- BOF - Beacon Object File - An object file format used by C2 frameworks for loading additional in-memory functionality
- DFR - Dynamic Function Resolution - A convention to declare and call Win32 APIs as
LIBRARY$Function
BOF Overview
BOFs are compiled C/C++ object files that execute in-memory within a C2 implant's process space. They provide a way to add functionality without writing artifacts to disk.
Example BOF that queries the primary Domain Controller:
#include <windows.h>
#include <stdio.h>
#include <dsgetdc.h>
#include "beacon.h"
DECLSPEC_IMPORT DWORD WINAPI NETAPI32$DsGetDcNameA(LPVOID, LPVOID, LPVOID, LPVOID, ULONG, LPVOID);
DECLSPEC_IMPORT DWORD WINAPI NETAPI32$NetApiBufferFree(LPVOID);
void go(char * args, int alen) {
PDOMAIN_CONTROLLER_INFO pdcInfo;
DWORD dwRet = NETAPI32$DsGetDcNameA(NULL, NULL, NULL, NULL, 0, &pdcInfo);
if (ERROR_SUCCESS == dwRet) {
BeaconPrintf(CALLBACK_OUTPUT, "%s", pdcInfo->DomainName);
}
NETAPI32$NetApiBufferFree(pdcInfo);
}
BOF Considerations
- Minimal footprint — reduce memory usage, only include what's needed
- Stability — a BOF crash kills the parent process; handle errors carefully
- No disk artifacts — avoid writing to disk; advise the user when unavoidable
- Windows x64 — assume Windows x64 target unless told otherwise
Linting
Use the included linter to check BOF code for common issues:
python3 ~/.agents/skills/bof/scripts/boflint.py <bof_source.c>
References
- DFR Information - Dynamic Function Resolution process used in BOF development
- API - BOF APIs for communication with C2 frameworks
- Building - How to compile a BOF