Exploring PCjs: A Browser-Based Emulator for Retro Computing and Cybersecurity Training

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Introduction

PCjs is an open-source, browser-based emulator that recreates vintage computing environments, including classic IBM PCs, early Windows versions, and DOS-based systems. Beyond nostalgia, this tool offers cybersecurity professionals and IT enthusiasts a sandbox to study legacy vulnerabilities, malware, and low-level system interactions.

Learning Objectives

  • Understand how PCjs emulates vintage hardware and software.
  • Explore cybersecurity applications, such as analyzing legacy malware or insecure protocols.
  • Learn how to integrate PCjs into IT training for historical context and low-level system experimentation.

1. Running Legacy DOS for Cybersecurity Analysis

Command:

debug.exe

Step-by-Step Guide:

  1. Launch PCjs and load an MS-DOS environment (e.g., PC-DOS 1.1).
  2. Type `debug.exe` to access the DOS debugger, a tool often exploited in early malware.
  3. Use debug commands like `-a` (assemble) to write simple assembly programs or `-d` (dump) to inspect memory.

– Example: `-d 0100` displays memory contents at address 0100h.
4. This helps analyze how legacy viruses (e.g., Brain Virus) manipulated system interrupts.

2. Emulating Early Windows Vulnerabilities

Command (Windows 1.0):

win.com

Step-by-Step Guide:

1. Load Windows 1.x or 3.1 in PCjs.

  1. Observe how early Windows handled memory management (no protected mode).
  2. Simulate a “DLL hijacking” attack by replacing `gdi.exe` with a malicious file.

– Use PCjs’s disk tools (DiskImage.js) to modify the virtual disk.
4. Note how lack of ASLR (Address Space Layout Randomization) made these systems prone to exploitation.

3. Analyzing Boot Sector Malware

Command (DOS):

fdisk /mbr

Step-by-Step Guide:

  1. Use PCjs to boot a DOS system with a corrupted Master Boot Record (MBR).
  2. Run `fdisk /mbr` to repair the MBR, simulating a recovery from a bootkits like Stoned.
  3. Compare with modern UEFI secure boot to highlight security evolution.

4. Studying Legacy Network Protocols

Tool: PCjs’s VT100 terminal emulator.

Step-by-Step Guide:

  1. Emulate a VT100 terminal connected to a telnet server (e.g., telnet example.com).
  2. Capture plaintext credentials sent over telnet using PCjs’s debug mode.
  3. Discuss why modern protocols (SSH, TLS) replaced insecure telnet.

5. Reverse Engineering Classic Malware

Tool: PCjs’s `FileImage.js` for disk analysis.

Step-by-Step Guide:

  1. Load a disk image containing a DOS-era virus (e.g., Jerusalem).
  2. Use `debug.exe` to disassemble the virus code (-u command).
  3. Trace how it hooks interrupts (e.g., INT 21h) to spread.

6. Cloud Integration for Training

Tool: GitHub + PCjs.

Step-by-Step Guide:

  1. Fork the PCjs GitHub repo.
  2. Modify configurations to emulate a networked DOS environment.
  3. Deploy the emulator to a cloud platform (e.g., AWS S3) for team training.

7. Hardware-Level Exploits

Command (BIOS):

int 13h

Step-by-Step Guide:

  1. Use PCjs to simulate an `int 13h` call (disk interrupt).
  2. Write a simple bootloader that overwrites disk sectors.
  3. Compare with modern BIOS/UEFI security features (e.g., Secure Boot).

What Undercode Say

  • Key Takeaway 1: PCjs bridges historical computing and modern cybersecurity training, offering hands-on experience with legacy threats.
  • Key Takeaway 2: Emulators like PCjs are invaluable for understanding low-level exploits, from bootkits to memory corruption.

Analysis:

PCjs’s accuracy in replicating vintage systems makes it a unique tool for studying cybersecurity’s evolution. For example, analyzing how MS-DOS’s lack of memory protection led to rampant malware reveals why modern OSes prioritize isolation (e.g., sandboxing). Additionally, PCjs’s open-source nature allows customization for red-team drills, such as simulating attacks on deprecated protocols (NetBIOS, IPX/SPX). As AI and quantum computing advance, preserving retro systems ensures we learn from past vulnerabilities to harden future infrastructures.

Prediction

Legacy system emulation will grow in importance for cybersecurity, particularly as attackers target aging industrial control systems (ICS) and embedded devices. Tools like PCjs will become standard in training programs to teach mitigation strategies for “old but still operational” technologies.

Further Reading:

IT/Security Reporter URL:

Reported By: Razvan Alexandru – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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