fix: prevent zip slip attacks in DXT unpack function (#74)

* fix: prevent zip slip attacks in DXT unpack function

Add path validation to prevent directory traversal attacks when unpacking DXT files.
The fix validates that extracted file paths remain within the intended output directory
before writing files to disk.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: make zip slip protection cross-platform compatible

Use path.sep instead of hardcoded '/' separator to ensure path validation
works correctly on both Windows and Unix systems. This fixes failing tests
on Windows while maintaining security protection against directory traversal.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

CLI.md

@@ -157,10 +157,12 @@ dxt unsign my-extension.dxt
 For signing extensions, you need:
 
 1. **Certificate**: X.509 certificate in PEM format
+
    - Should have Code Signing extended key usage
    - Can be self-signed (for development) or CA-issued (for production)
 
 2. **Private Key**: Corresponding private key in PEM format
+
    - Must match the certificate's public key
 
 3. **Intermediate Certificates** (optional): For CA-issued certificates

MANIFEST.md

@@ -290,12 +290,14 @@ The `server` object defines how to run the MCP server:
 ### Server Types
 
 1. **Python**: `server.type = "python"`
+
    - Requires `entry_point` to Python file
    - All dependencies must be bundled in the DXT
    - Can use `server/lib` for packages or `server/venv` for full virtual environment
    - Python runtime version specified in `compatibility.runtimes.python`
 
 2. **Node.js**: `server.type = "node"`
+
    - Requires `entry_point` to JavaScript file
    - All dependencies must be bundled in `node_modules`
    - Node.js runtime version specified in `compatibility.runtimes.node`

src/cli/unpack.ts

@@ -1,6 +1,6 @@
 import { unzipSync } from "fflate";
 import { chmodSync, existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
-import { join, resolve } from "path";
+import { join, resolve, sep } from "path";
 
 import { extractSignatureBlock } from "../node/sign.js";
 import { getLogger } from "../shared/log.js";
@@ -85,6 +85,17 @@ export async function unpackExtension({
       if (Object.prototype.hasOwnProperty.call(decompressed, relativePath)) {
         const data = decompressed[relativePath];
         const fullPath = join(finalOutputDir, relativePath);
+
+        // Prevent zip slip attacks by validating the resolved path
+        const normalizedPath = resolve(fullPath);
+        const normalizedOutputDir = resolve(finalOutputDir);
+        if (
+          !normalizedPath.startsWith(normalizedOutputDir + sep) &&
+          normalizedPath !== normalizedOutputDir
+        ) {
+          throw new Error(`Path traversal attempt detected: ${relativePath}`);
+        }
+
         const dir = join(fullPath, "..");
         if (!existsSync(dir)) {
           mkdirSync(dir, { recursive: true });