Overview
The Model Context Protocol (MCP) is now the standard way to connect AI agents to real-world tools as they go from simple chat interfaces to fully autonomous operators. But this connection has created a new attack surface where the "code" isn't just binary; each tool is a function which takes various parameters as an input, perform some operations with the provided parameters and return the results which LLM can use. Hence, all the traditional web/API vulnerabilities such as injections, improper input validation, information leakage/disclosure, authorization, SSRF are still possible in the tools and those should be reviewed and pen tested before releasing it for production.
Tool Summary
The MCP Fuzzer is a semi-automated security scanner for scanning MCP tools. The tool provides three modes - list, execute and fuzz which are described below -
- The "List" mode helps in enumerating through all the tools and its parameters of an MCP server.
- The "Generate Input File" mode generates the sample inputs for all the parameters of the tools/functions as per the data type. The sample values can be entered in the JSON to be able to use the input file in the "Execute" and "Fuzz" mode.
- The "Execute" mode helps scanner with executing the tool with dummy/sample arguments. The scanner also allows to view all the data being stored at the tool.
- The "Fuzz" mode is a test function to validate parameters and their integrity against malicious data exfiltration patterns by comparing the response and content lengths before and after the payload injection. Along with displaying the "Status" of the attack in the response, the tool also identifies the type of regex or vulnerability pattern present in the input.
Requirements
- This tool requires no software prerequisites and can be run directly from the terminal using the provided executable (.exe) file.
- While using the "—proxy" commands, ensure the proxy is configured to use HTTP/1.1 instead of HTTP/2 so that you can properly capture the request/response.
Core Capabilities
List
Usage: MCPFuzzer list --endpoint <MCP-Endpoint>
The "list" command enumerates tool names, descriptions, parameters, and their associated data types. This metadata can be used by security testers to find malicious system instructions hidden in the tool descriptions or to identify possible tool misconfigurations.
generate-input-file
The "generate-input-file" command creates the tool input file for all the parameters of the tools based on the data type which will be used by ‘execute’ and ‘fuzz’ commands. We propose to change sample data with real data to get better performance of ‘fuzz’ command.
Execute
Usage: MCPFuzzer execute --endpoint <MCP-Endpoint> --tool-name <tool-name> --tool-args-file .\tool-input.json
The “execute” command enables the use of test inputs to verify that the tool functions according to its intended behavior and logic.
Example: tool_input.json
Fuzz
Usage: MCPFuzzer fuzz --endpoint <MCP-Endpoint> --tool-names <tool-name,tool-name> --tool-args-file .\tool-input.json --payload-file payloads.txt --vulnresp-regex-file vulnresponse-regex.txt--tool-names takes a list of tools that we want to do fuzzing for
--tool-args-file takes the input request body for each of the above tools
--payload-file take a list of payloads to be injected
--vulnresp-regex-file takes a list of regular expressions to compare the response against
The fuzz command acts as an additional layer on top of the standard “execute” command. It automatically tests the given input fields against a library of regex patterns and malicious payloads to match JSON key-value pairs. The fuzz command is executed it four different steps (status code):
- Baseline: It is the initial state of the request which captures the original response and the content length for comparison.
- Running: Active fuzzing; It is a state where the active requests are made on target parameter based on the attack payloads from payload file.
- Failed: A payload hit was detected. This suggests a security vulnerability.
- Success: When the payload response matches the baseline content length, indicates that the attack was unsuccessful. This means that the parameters are secure against the tested attacks.
The "fuzz" mode injects payloads into the "value" fields and measures the difference between the original length and the updated length of the response after injecting the payload. This comparison helps in finding what specific parameters are vulnerable to attacks.
For example, below is a screenshot showing that the tool discovered that one of the parameters is susceptible to "SQL Exception/Error," which exposes private database schema information.
Note: For MCP servers with multiple tools, the fuzz command accepts a comma-separated string of all applicable tool names. The tool also allows to test any parameter within the MCP endpoint.
Usage: MCPFuzzer fuzz --endpoint <MCP-Endpoint> --tool-names tool1,tool2,tool3 --tool-args-files .\tool-input.json [--parameter-names para1,para2] --payload-file payloads.txt --vulnresp-regex-file vulnresponse-regex.txt
Operational Features
Authorization and Extra Header Support
Usage: MCPfuzzer execute –endpoint <MCP endpoint> --tool-name <tool-name> --tool-args-file .\tool-input.json –proxy-host <local-host> --proxy-port <porxy-port> --extra-headers-file <headers-file>
The “--extra-headers” parameters allow to specify access or authorization tokens for MCP serves behind gateways that may need read-write access.
Proxy Support
Usage: MCPFuzzer execute –endpoint <MCP-Endpoint> --tool-name <tool-name> --tool-args-file .\tool-input.json --proxy-host <proxy-host> --proxy-port <proxy-port>
Logging Support
Usage: MCPFuzzer fuzz --endpoint <MCP-Endpoint> --tool-name <tool-name> --parameter-names <parameter-list> --tool-args-file .\tool-input.json --payload-file payloads.txt --vulnresp-regex-file .\vulnresponse-regex.txt --proxy-host <proxy-host> --proxy-port <proxy-port> --logfile mcp.log
The “--log-file” parameter logs a complete record of your network activity. Every request-response pair is logged into a destination file.
Structured Output (html format)
Usage: MCPFuzzer execute --endpoint <MCP-Endpoint> --tool-name list_travel_expenses --parameter-names start_date --tool-args-file .\tool-input.json --payload-file payloads.txt --vulnresp-regex-file .\vulnresponse-regex.txt --proxy-host <proxy-host> --proxy-port <proxy-port> --output-type html/json <output-file-name>
The "–output-file" command lets users choose the file type (--output-type), which enables users to create an HTML or JSON report according to their further requirements.
JSON Output
HTML REPORT
The “--output-file” parameter generates a structured JSON report which is wrapped in an HTML file, for all commands with “failed” status.
Why MCP Fuzzer?
Fuzzer is designed to bridge the gap between complex security analysis and seamless developer workflows. Why it’s unique:
• MCP Fuzzer was built to be user-friendly. It finds vulnerabilities and patterns using natural language queries, no hardcore coding skills required.
• It integrates seamlessly with your existing CI/CD pipelines or development cycles, making security part of the build process from day one.
• Provides the supply chain with status flags and confidence ratios so it can be used to automatically trigger alerts when suspicious behavior is detected.
• Stay ahead of emerging threats by modifying test inputs and configuration files to changing attack patterns.
• MCP Fuzzer is a free tool that records all tool invocations and is highly adaptive to your environment.
Conclusion
As the ecosystem of AI agents and MCP integrations continues to grow, security cannot be an afterthought. Fuzzer transforms MCP security from a manual, complex obstacle into a streamlined, automated process that fits seamlessly into the modern developer’s toolkit.
Equipped with deep visibility into request-response pairs, and a secure fuzzing layer, it ensures your application is immune to certain emerging threats. It provides transparency, scalability and control to innovate with confidence whether you’re a single developer or an enterprise with a massive network of endpoints. Build safer, don't build faster.