Prologue — Strings, Stacks, and Legacy Tricks

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Legacy systems are a goldmine. I was rummaging through an old factory control rig running Windows XP and found a small utility called DevMon Status Tool. No ASLR, no stack cookies, no DEP. Real 2003 energy.

It printed connected devices through a local GUI—harmless on the surface. But once I reversed the binary, I hit a beautiful red flag: printf(user_input) with no format control.

That’s when things got interesting.

Platform

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• Windows XP SP3 x86
• DevMon Status Tool v1.0.7

Overview

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The vulnerability lies in a direct printf() call using unsanitized user input. That means:

• You control the format string.
• You can leak stack values.
• You can write arbitrary values using %n.

That’s full stack control without breaking a sweat.

Sample Code (Decompiled)

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void showDeviceStatus(char* user_input) {
    printf(user_input); // directly using user-controlled input
}

This was pulled from Ghidra. No wrapper, no format string defined—just raw input to printf.

Exploit Strategy

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The approach was textbook:

1. Inject %x format specifiers to leak stack data.
2. Use the output to locate key pointers and buffer offsets.
3. Craft an input with %n to overwrite return address (EIP) or function pointer.
4. Redirect execution to injected shellcode.

The %n format specifier writes the number of printed characters into a memory location. With the right padding and target address on the stack, it’s game over.

Payload Example

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AAAA%08x.%08x.%08x.%n

Once offsets were mapped, I replaced the prefix with the address I wanted to overwrite (padded correctly), followed by enough characters to write a value like the shellcode address.

Advanced Payload Breakdown

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4Vx%123x%4$n

• 4Vx: Address to write to (return pointer)
• %123x: Padding to set the number of characters printed
• %4$n: Write that count to the 4th stack argument

With careful positioning and known stack layout, this gave full EIP control.

Shellcode Use

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I dropped a short WinExec shellcode into the stack buffer before the format string and redirected EIP into it.

    mov ecx, offset cmd_string
    call WinExec
    ret

cmd_string db "cmd.exe", 0

Outcome

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Triggered the exploit through a malformed device name entry:

• Leaked multiple stack values
• Calculated buffer offset to return address
• Overwrote EIP using %n with precision padding
• Jumped to shellcode
• Spawned a local shell as SYSTEM

No external scripts or tools needed. Just a classic format string abuse through a legacy binary still running in production.

Impact

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• Privilege Escalation: Yes (user to SYSTEM)
• Remote Vector: No (local user interaction)
• Mitigations Bypassed: All (none present)
• Exploit Method: Format string to EIP overwrite and shellcode execution

Lessons

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This bug type might feel ancient, but it still lives in legacy systems that haven’t been audited in decades. When format strings go unchecked, they let attackers whisper to the stack—and the stack listens.

Endgame

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Old system, ancient bug, clean exploit. Dropped the payload, hijacked control, and summoned a SYSTEM shell. And all it took was a few sneaky characters passed into printf().