C++ License Check Bypass

TL;DR

Many C++ applications use simple if statements to check for valid licenses. This guide shows how to bypass these checks using a debugger and disassembler, or by patching the executable directly.

Understanding the Problem

A common licensing scheme involves checking for a specific file, registry entry, or other indicator of a valid license. The code usually looks something like this:

if (license_valid) {
    // Run application
} else {
    // Show error message and exit
}

Our goal is to make the license_valid condition always true.

Step-by-Step Bypass Guide

1. Identify the License Check Function: Use a disassembler (like IDA Pro, Ghidra, or x64dbg) to find the code responsible for license validation. Look for strings related to licensing (e.g., “License Key”, “Valid License”, file names like “license.dat”).
2. Locate the Conditional Jump: Once you’ve found the license check function, identify the if statement that determines whether to run the application or display an error. This will be a conditional jump instruction (e.g., JE – Jump if Equal, JNZ – Jump if Not Zero).
3. Method 1: Debugging and Patching in Memory
   • Attach a Debugger: Attach a debugger (like x64dbg) to the running process.
   • Set a Breakpoint: Set a breakpoint on the conditional jump instruction identified in Step 2.
   • Run the Application: Start the application and let it hit the breakpoint.
   • Modify the Instruction: Change the opcode of the conditional jump to an unconditional jump (e.g., replace JE with JMP). Alternatively, change the condition code so that the jump is always taken or never taken. For example, if it’s a comparison instruction setting the Zero Flag, you can modify the operands to force the flag to be set/unset appropriately.
   • Continue Execution: Continue execution of the application. It should now bypass the license check.
4. Method 2: Patching the Executable File
   • Open in a Hex Editor: Open the executable file in a hex editor (like HxD or 010 Editor).
   • Locate the Conditional Jump: Use the address found in Step 2 to locate the conditional jump instruction in the hex editor.
   • Modify the Instruction: Change the opcode of the conditional jump to an unconditional jump (e.g., replace JE with JMP). Be very careful when modifying binary files; incorrect changes can render the application unusable.
   • Save Changes: Save the modified executable file.
   • Run the Application: Run the patched application. It should now bypass the license check.
5. Method 3: Modifying Variables (Less Common)
   • If the license check relies on a global variable, find that variable in memory using the debugger.
   • Change the value of the variable to indicate a valid license. This might involve changing a boolean from false to true or setting an integer to a non-zero value.

Important Considerations

• Anti-Debugging Techniques: Some applications employ anti-debugging techniques to make it harder to attach a debugger. You may need to bypass these techniques first (e.g., using a debugger that can handle them, or patching the executable to remove the checks).
• Code Obfuscation: More sophisticated licensing schemes use code obfuscation to hide the license check logic. This makes it more difficult to find and modify the relevant code.
• Checksums and Integrity Checks: Some applications perform checksum or integrity checks on their files. Patching the executable may invalidate these checks, causing the application to crash or refuse to run.
• Legality: Bypassing license checks is illegal in many jurisdictions. This guide is for educational purposes only; do not use it to violate software licenses.