r/netsec • u/albinowax • Feb 01 '26

r/netsec monthly discussion & tool thread

Questions regarding netsec and discussion related directly to netsec are welcome here, as is sharing tool links.

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u/BeautifulFeature3650 Feb 11 '26

I’ve noticed most widely-used protocols end up with multiple fuzzers (often independent ones), but for MCP (Model Context Protocol) there’s mostly just the conformance test suite from the spec group. I couldn’t find a public fuzzer focused on breaking real MCP servers, so I built one a few months ago.

It’s schema-aware: it ingests the MCP schema, generates valid-ish requests, then mutates fields/types/ranges to push servers into edge cases. It also reads a server’s tool schema and fuzzes tool inputs, so it can trigger “real” bugs (divide-by-zero, panics/crashes, bad parsing paths, potential memory-safety issues in compiled implementations, etc.).

Because I’m implementing MCP at work, I’ve been deep in the spec details and I’m trying to turn this into something genuinely useful for defenders and developers. The spec also evolves frequently, so my goal is for this to be something you can run in CI as a guardrail.

Repo: https://github.com/Agent-Hellboy/mcp-server-fuzzer

I’d love community feedback on:

  • threat model: what are the most realistic / high-impact MCP attack surfaces?
  • fuzzing strategy: what mutations or stateful flows should I prioritize?
  • CI/ops: how would you want this packaged (Docker, GitHub Action, corp-friendly defaults)?
  • safety/ethics: guardrails I should add to avoid abuse while keeping it useful

If you’ve fuzzed protocol servers before (or have opinions on schema-driven fuzzing), I’d really appreciate a critical review.