Fuzzing with SPIKE

Objective

Learn how to use SPIKE to fuzz a server, uncover vulnerabilities, and understand how fuzzing can test the robustness of applications and protocols.

Purpose

SPIKE is a powerful API that allows us to develop custom stress tests for protocols or applications. By sending unexpected or malformed data, SPIKE helps identify vulnerabilities like buffer overflows or crashes that might not be detected through traditional testing.

Tools Required

• Kali Linux: To run SPIKE and craft fuzzing scripts.
• Windows Machine: Running Vulnserver, a deliberately vulnerable server application.

Lab Topology

The lab setup includes:

1. Kali Linux (Virtual Machine): For running SPIKE.
2. Windows Machine (Physical or Virtual): Hosting Vulnserver to act as the fuzzing target.

Walkthrough

Task 1: Setting Up Vulnserver

1. Download Vulnserver:
   • Access the Vulnserver repository from GitHub.
   • Clone or download the repository onto your Windows machine.
2. Extract and Navigate:
   • Extract the downloaded files to a folder such as C:\Vulnserver.
   • Open Command Prompt and navigate to the directory:

     cd C:\Vulnserver
     

3. Run Vulnserver:
   • Start the server by running:

     vulnserver.exe
     

   • Ensure the server listens on the default port 9999.
   • If prompted, allow the application through your firewall.
4. Verify Accessibility:
   • Test the server connection from Kali Linux:

     nc <target_ip> 9999
     

     • Replace <target_ip> with the Windows machine’s IP address.
     • Type a command like HELP to verify the server responds.

Task 2: Installing SPIKE on Kali Linux

1. Update and Install Dependencies:

   sudo apt update
   sudo apt install build-essential git
   

2. Download and Compile SPIKE:
   • Clone the SPIKE repository:

     git clone https://github.com/SpikeFuzzer/spike.git
     

   • Navigate to the source directory:

     cd spike/src
     

   • Compile the SPIKE tools:

     make
     

3. Verify Installation:
   • Confirm the compiled binaries (e.g., generic_send_tcp) are available in the src directory.

Task 3: Creating and Running a Fuzzing Script

1. Create a SPIKE Script:
   • Create a new .spk file to craft a fuzzing payload:

     nano fuzz_script.spk
     

   • Add the following lines:

     s_string("TRUN ");
     s_string_variable("A");
     

   • Save and close the file.
   • Explanation:
     • s_string("TRUN "): Sends the TRUN command recognized by Vulnserver.
     • s_string_variable("A"): Appends fuzzed data to test the server’s handling.
2. Run the SPIKE Script:
   • Use the generic_send_tcp tool to send fuzzed payloads:

     ./generic_send_tcp <target_ip> 9999 fuzz_script.spk 0 0
     

     • Replace <target_ip> with the Windows machine’s IP address.
3. Monitor the Target:
   • Watch Vulnserver’s output on the Windows machine for crashes or errors.
   • If Vulnserver crashes, note the fuzzing iteration causing the crash for further analysis.

Task 4: Debugging with Immunity Debugger

1. Install Immunity Debugger:
   • Download Immunity Debugger from https://debugger.immunityinc.com/ and install it on the Windows machine.
2. Attach the Process:
   • Open Immunity Debugger.
   • Attach it to the vulnserver.exe process by selecting File > Attach and choosing vulnserver.exe from the list.
3. Reproduce the Crash:
   • Run the SPIKE fuzzing script again from Kali Linux.
   • Observe Immunity Debugger for the crash location and payload data.
4. Analyze the Crash:
   • Use the debugger’s stack and memory inspection tools to identify potential buffer overflows or other vulnerabilities.

Best Practices for Fuzzing

1. Isolate the Environment:
   • Perform fuzzing in a controlled lab setup to prevent unintended impacts on production systems.
2. Analyze Network Traffic:
   • Use Wireshark on Kali Linux to capture and review traffic sent to Vulnserver.
3. Iterate on Fuzzing Scripts:
   • Modify the SPIKE script to test other Vulnserver commands such as GMON, KSTET, or LTER.
4. Document Results:
   • Record the input causing crashes and any observations from the debugger for future testing or reporting.

Key Takeaways

1. SPIKE is a robust tool for fuzzing and vulnerability discovery, offering flexibility for testing various protocols and inputs.
2. Vulnserver is an excellent target for practicing fuzzing techniques in a safe environment.
3. Debugging tools like Immunity Debugger are critical for analyzing crashes and identifying potential exploit opportunities.

Troubleshooting Tips

1. No Response from Vulnserver:
   • Ensure Vulnserver is running and listening on port 9999.
   • Verify network connectivity between the Kali Linux and Windows machines.
2. SPIKE Errors:
   • Double-check the syntax of your SPIKE script.
   • Ensure the correct file paths and IP addresses are specified when running generic_send_tcp.
3. Firewall Blocking Connections:
   • Temporarily disable the Windows firewall or create a rule to allow connections on port 9999.
4. Immunity Debugger Not Capturing Crashes:
   • Confirm the debugger is attached to the correct process.
   • Ensure Vulnserver has not restarted after a crash.