What I Learned Reverse Engineering Legacy Windows Apps

Why Reverse Engineer Old Software?

Legacy applications are everywhere. Banks, hospitals, and government agencies run critical systems on code written decades ago. Understanding how these systems work—and where they're vulnerable—is increasingly valuable.

My Approach

I spent several months reverse engineering 4 legacy Windows applications using:

• Ghidra for static analysis
• x64dbg for dynamic analysis
• IDA Free for cross-referencing

What I Found

1. Buffer Overflows Everywhere

Older C/C++ code rarely validates input lengths. I found multiple instances of strcpy and sprintf without bounds checking.

// Vulnerable pattern I kept seeing
char buffer[64];
strcpy(buffer, user_input);  // No length check!

2. Hardcoded Credentials

Two applications had credentials embedded in the binary. One was XOR'd with a single byte (security theater).

3. Unsafe Type Casts

Integer overflows from unsafe casts between signed/unsigned types. These can lead to heap corruption.

The Remediation Process

For each vulnerability, I:

1. Documented the issue with reproduction steps
2. Wrote a safer C equivalent
3. Created a patch diff for the vendor

Key Takeaways

• Learn x86 assembly. You don't need to be an expert, but understanding stack frames and calling conventions is essential
• Ghidra is incredible. And it's free. The decompiler output is surprisingly readable
• Old != simple. Legacy code often has complex, undocumented behavior

This research is ongoing. If you're interested in collaborating on security research, reach out!