Screen Crab — The Complete Guide
一句話定位：Screen Crab 是一台「藏在 HDMI 線中間的隱形監視器」——把它夾在電腦與螢幕（或遊戲機與電視）之間，它零延遲地把畫面截圖或錄影存進 MicroSD，還能透過 Wi-Fi 串流到 Cloud C²。系統管理員、滲透測試與想「看到別人看到什麼」的人最愛它。
The Screen Crab is the first HDMI man-in-the-middle device made for pentesters. It doesn't intercept network traffic — it intercepts the video signal itself. Because it splits the HDMI signal passively (no re-encoding in the data path), the output display sees zero lag and zero interruption while the Crab quietly captures a second copy.
It's disarmingly simple: plug it inline, power it over USB, and it starts saving screenshots to the MicroSD card out of the box. Edit a config.txt file to change intervals, enable video, or connect it to Wi-Fi + Cloud C² to watch screens live from a browser.
⚠️ Authorised testing only. Capturing someone's screen without consent is illegal (Taiwan: 刑法第三一五條之一妨害秘密罪, among others). Use on your own machines/displays or with explicit authorization.
Specs at a glance
| Item | Specification |
|---|---|
| Interface | 2× full-size HDMI (IN / OUT) + USB-C (power) + MicroSD |
| Standards | HDMI 1.4 / DVI 1.0; 802.11 b/g/n (WiFi, 2.4 GHz) |
| Resolution | Most 16:9 formats up to 1920×1080 (Full HD 1080p), auto up/downscale |
| Capture modes | Interval screenshots, or full-motion video (MPEG-4 at 2 / 4 / 16 Mbps) |
| Storage | MicroSD (SDXC support); loop recording overwrites oldest files |
| Latency | Zero — passive signal splitter, no lag on output |
| WiFi | RP-SMA dipole antenna for Cloud C² streaming |
| Power | USB 5V 1A (5 W) |
| Dimensions | 105 × 51 × 21 mm |
| Official docs | https://docs.hak5.org/screen-crab |
Anatomy
| Part | Purpose |
|---|---|
| HDMI IN | From the source (computer / console) |
| HDMI OUT | To the monitor / TV (pass-through, zero-lag) |
| USB-C | Power |
| MicroSD slot | Screenshots / video storage |
| WiFi antenna (RP-SMA) | Cloud C² streaming |
| RGB LED + button | Status (disable LED for stealth) |
Installation & data flow
- Connect the source to the Crab's HDMI IN.
- Connect your monitor to the Crab's HDMI OUT (normal operation, no lag).
- Power over USB-C. Screenshots begin saving to the MicroSD at default intervals.
Quickstart — first capture in 2 minutes
Step 1 — Insert a MicroSD card
Any MicroSD works; SDXC gives months of storage. The Crab auto-generates a config.txt on first boot.
Step 2 — Wire it inline
- HDMI IN ← your computer.
- HDMI OUT → your monitor.
- USB-C → power.
Expected result: the monitor works exactly as before (no delay), and the MicroSD starts collecting screenshots.
Step 3 — Read the captures
Eject the MicroSD and browse:
/root/
loot/
2026-08-21-1430/screenshot_001.jpg
2026-08-21-1430/screenshot_002.jpg
...
config.txt
Step 4 — Enable video
Edit config.txt on the MicroSD root:
# Capture mode: image or video
capture_mode = video
# MPEG4 quality: 2, 4, or 16 Mbps
bitrate = 4
# Seconds between captures
interval = 30
Safely remove the MicroSD, reinsert, power-cycle. The Crab now records MPEG-4 video.
Cloud C² — watch from anywhere
To stream screens/video remotely:
- Edit
config.txtand add your Wi-Fi network:
wifi_ssid = OfficeWiFi
wifi_pass = hunter2example
- Add your Cloud C² device file to the MicroSD.
- Boot. The Crab connects to Wi-Fi and registers with C² — you can now stream screenshots and download captures from the browser.
You might be asking: "Zero lag — how?" The Crab uses a passive video signal splitter on the HDMI path. The source's signal is mirrored to the monitor untouched; a duplicate is fed to the capture hardware. Nothing in the original path is re-encoded, so there's no added latency. That's also what makes it so invisible.
Advanced
| Capability | How |
|---|---|
| Loop recording | Continuous capture deletes oldest files to make room (never fills up) |
| Interval screenshots | Configurable interval in seconds |
| Multiple bitrates | 2 / 4 / 16 Mbps MPEG-4 quality tiers |
| Stealth LED | Disable the RGB LED for covert operation |
| Resolution handling | Auto up/downscale to 1080p from most sources |
| Remote management | Cloud C²: change settings, download captures, live view |
Troubleshooting
| Symptom | Cause | Fix |
|---|---|---|
| No screenshots saved | MicroSD not seated / write-protected | Re-seat; check lock switch; test with a known-good card |
| Monitor blinks when Crab inserted | Power / HDMI negotiation | Verify 5V/1A USB power; reseat HDMI connections |
| Video choppy | Bitrate too low for the scene | Raise to 16 Mbps |
| Cloud C² never connects | Wi-Fi creds wrong / no device file | Recheck config.txt; copy the C² device file to the card |
| Can't find config.txt | Card not booted once | Insert card, boot once, it auto-generates the config |
Related resources
- Firmware & Downloads — Cloud C² server setup
- Key Croc — keystroke-level interception (complementary: keys and screens)
- Packet Squirrel Mark II — network-side interception
- Troubleshooting Index
- Hak5 overview