add(plugin): mcp-server-dev — skills for building MCP servers (#731)

plugins/mcp-server-dev/skills/build-mcpb/references/local-security.md

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+# Local MCP Security
+
+**MCPB provides no sandbox.** There's no `permissions` block in the manifest, no filesystem scoping, no network allowlist enforced by the platform. The server process runs with the user's full privileges — it can read any file the user can, spawn any process, hit any network endpoint.
+
+Claude drives it. That combination means: **tool inputs are untrusted**, even though they come from an AI the user trusts. A prompt-injected web page can make Claude call your `delete_file` tool with a path you didn't intend.
+
+Your tool handlers are the only defense. Everything below is about building that defense yourself.
+
+---
+
+## Path traversal
+
+The #1 bug in local MCP servers. If you take a path parameter and join it to a root, **resolve and check containment**.
+
+```typescript
+import { resolve, relative, isAbsolute } from "node:path";
+
+function safeJoin(root: string, userPath: string): string {
+  const full = resolve(root, userPath);
+  const rel = relative(root, full);
+  if (rel.startsWith("..") || isAbsolute(rel)) {
+    throw new Error(`Path escapes root: ${userPath}`);
+  }
+  return full;
+}
+```
+
+`resolve` normalizes `..`, symlink segments, etc. `relative` tells you if the result left the root. Don't just `String.includes("..")` — that misses encoded and symlink-based escapes.
+
+**Python equivalent:**
+
+```python
+from pathlib import Path
+
+def safe_join(root: Path, user_path: str) -> Path:
+    full = (root / user_path).resolve()
+    if not full.is_relative_to(root.resolve()):
+        raise ValueError(f"Path escapes root: {user_path}")
+    return full
+```
+
+---
+
+## Roots — ask the host, don't hardcode
+
+Before hardcoding `ROOT` from a config env var, check if the host supports `roots/list`. This is the spec-native way to get user-approved workspace boundaries.
+
+```typescript
+import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
+
+const server = new McpServer({ name: "...", version: "..." });
+
+let allowedRoots: string[] = [];
+server.server.oninitialized = async () => {
+  const caps = server.getClientCapabilities();
+  if (caps?.roots) {
+    const { roots } = await server.server.listRoots();
+    allowedRoots = roots.map(r => new URL(r.uri).pathname);
+  } else {
+    allowedRoots = [process.env.ROOT_DIR ?? process.cwd()];
+  }
+};
+```
+
+```python
+# fastmcp — inside a tool handler
+async def my_tool(ctx: Context) -> str:
+    try:
+        roots = await ctx.list_roots()
+        allowed = [urlparse(r.uri).path for r in roots]
+    except Exception:
+        allowed = [os.environ.get("ROOT_DIR", os.getcwd())]
+```
+
+If roots are available, use them. If not, fall back to config. Either way, validate every path against the allowed set.
+
+---
+
+## Command injection
+
+If you spawn processes, **never pass user input through a shell**.
+
+```typescript
+// ❌ catastrophic
+exec(`git log ${branch}`);
+
+// ✅ array-args, no shell
+execFile("git", ["log", branch]);
+```
+
+If you're wrapping a CLI, build the full argv as an array. Validate each flag against an allowlist if the tool accepts flags at all.
+
+---
+
+## Read-only by default
+
+Split read and write into separate tools. Most workflows only need read. A tool that's read-only can't be weaponized into data loss no matter what Claude is tricked into calling it with.
+
+```
+list_files   ← safe to call freely
+read_file    ← safe to call freely
+write_file   ← separate tool, separate scrutiny
+delete_file  ← consider not shipping this at all
+```
+
+Pair this with tool annotations — `readOnlyHint: true` on every read tool, `destructiveHint: true` on delete/overwrite tools. Hosts surface these in permission UI (auto-approve reads, confirm-dialog destructive). See `../build-mcp-server/references/tool-design.md`.
+
+If you ship write/delete, consider requiring explicit confirmation via elicitation (see `../build-mcp-server/references/elicitation.md`) or a confirmation widget (see `build-mcp-app`) so the user approves each destructive call.
+
+---
+
+## Resource limits
+
+Claude will happily ask to read a 4GB log file. Cap everything:
+
+```typescript
+const MAX_BYTES = 1_000_000;
+const buf = await readFile(path);
+if (buf.length > MAX_BYTES) {
+  return {
+    content: [{
+      type: "text",
+      text: `File is ${buf.length} bytes — too large. Showing first ${MAX_BYTES}:\n\n`
+            + buf.subarray(0, MAX_BYTES).toString("utf8"),
+    }],
+  };
+}
+```
+
+Same for directory listings (cap entry count), search results (cap matches), and anything else unbounded.
+
+---
+
+## Secrets
+
+- **Config secrets** (`sensitive: true` in manifest `user_config`): host stores in OS keychain, delivers via env var. Don't log them. Don't include them in tool results.
+- **Never store secrets in plaintext files.** If the host's keychain integration isn't enough, use `keytar` (Node) / `keyring` (Python) yourself.
+- **Tool results flow into the chat transcript.** Anything you return, the user (and any log export) can see. Redact before returning.
+
+---
+
+## Checklist before shipping
+
+- [ ] Every path parameter goes through containment check
+- [ ] No `exec()` / `shell=True` — `execFile` / array-argv only
+- [ ] Write/delete split from read tools; `readOnlyHint`/`destructiveHint` annotations set
+- [ ] Size caps on file reads, listing lengths, search results
+- [ ] Secrets never logged or returned in tool results
+- [ ] Tested with adversarial inputs: `../../etc/passwd`, `; rm -rf ~`, 10GB file