security: harden EXTRA_READ_PATHS with validation and blocklist

Add security controls to the EXTRA_READ_PATHS feature (PR #126) to prevent
path traversal attacks and accidental exposure of sensitive directories.

Changes:
- Add EXTRA_READ_PATHS_BLOCKLIST constant blocking credential directories
  (.ssh, .aws, .azure, .kube, .gnupg, .docker, .npmrc, .pypirc, .netrc)
- Create get_extra_read_paths() function with comprehensive validation:
  - Path canonicalization via Path.resolve() to prevent .. traversal
  - Validates paths are absolute (rejects relative paths)
  - Validates paths exist and are directories
  - Blocks paths that are/contain sensitive directories
  - Blocks paths that would expose sensitive dirs (e.g., home dir)
- Update create_client() to use validated getter function
- Improve logging to show validated paths instead of raw input
- Document security controls in CLAUDE.md under Security Model section

Security considerations:
- Addresses path traversal risk similar to CVE-2025-54794
- Prevents accidental exposure of SSH keys, cloud credentials, etc.
- All validation happens before permissions are granted to the agent

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -211,6 +211,46 @@ Defense-in-depth approach configured in `client.py`:
 2. Filesystem restricted to project directory only
 3. Bash commands validated using hierarchical allowlist system
 
+#### Extra Read Paths (Cross-Project File Access)
+
+The agent can optionally read files from directories outside the project folder via the `EXTRA_READ_PATHS` environment variable. This enables referencing documentation, shared libraries, or other projects.
+
+**Configuration:**
+
+```bash
+# Single path
+EXTRA_READ_PATHS=/Users/me/docs
+
+# Multiple paths (comma-separated)
+EXTRA_READ_PATHS=/Users/me/docs,/opt/shared-libs,/Volumes/Data/reference
+```
+
+**Security Controls:**
+
+All paths are validated before being granted read access:
+- Must be absolute paths (not relative)
+- Must exist and be directories
+- Paths are canonicalized via `Path.resolve()` to prevent `..` traversal attacks
+- Sensitive directories are blocked (see blocklist below)
+- Only Read, Glob, and Grep operations are allowed (no Write/Edit)
+
+**Blocked Sensitive Directories:**
+
+The following directories (relative to home) are always blocked:
+- `.ssh`, `.aws`, `.azure`, `.kube` - Cloud/SSH credentials
+- `.gnupg`, `.gpg`, `.password-store` - Encryption keys
+- `.docker`, `.config/gcloud` - Container/cloud configs
+- `.npmrc`, `.pypirc`, `.netrc` - Package manager credentials
+
+**Example Output:**
+
+```
+Created security settings at /path/to/project/.claude_settings.json
+   - Sandbox enabled (OS-level bash isolation)
+   - Filesystem restricted to: /path/to/project
+   - Extra read paths (validated): /Users/me/docs, /opt/shared-libs
+```
+
 #### Per-Project Allowed Commands
 
 The agent's bash command access is controlled through a hierarchical configuration system:

client.py

@@ -47,6 +47,23 @@ API_ENV_VARS = [
 # Example: EXTRA_READ_PATHS=/Volumes/Data/dev,/Users/shared/libs
 EXTRA_READ_PATHS_VAR = "EXTRA_READ_PATHS"
 
+# Sensitive directories that should never be allowed via EXTRA_READ_PATHS
+# These contain credentials, keys, or system-critical files
+EXTRA_READ_PATHS_BLOCKLIST = {
+    ".ssh",
+    ".aws",
+    ".azure",
+    ".kube",
+    ".gnupg",
+    ".gpg",
+    ".password-store",
+    ".docker",
+    ".config/gcloud",
+    ".npmrc",
+    ".pypirc",
+    ".netrc",
+}
+
 
 def get_playwright_headless() -> bool:
     """
@@ -85,6 +102,79 @@ def get_playwright_browser() -> str:
     return value
 
 
+def get_extra_read_paths() -> list[Path]:
+    """
+    Get extra read-only paths from EXTRA_READ_PATHS environment variable.
+
+    Parses comma-separated absolute paths and validates each one:
+    - Must be an absolute path
+    - Must exist and be a directory
+    - Cannot be or contain sensitive directories (e.g., .ssh, .aws)
+
+    Returns:
+        List of validated, canonicalized Path objects.
+    """
+    raw_value = os.getenv(EXTRA_READ_PATHS_VAR, "").strip()
+    if not raw_value:
+        return []
+
+    validated_paths: list[Path] = []
+    home_dir = Path.home()
+
+    for path_str in raw_value.split(","):
+        path_str = path_str.strip()
+        if not path_str:
+            continue
+
+        # Parse and canonicalize the path
+        try:
+            path = Path(path_str).resolve()
+        except (OSError, ValueError) as e:
+            print(f"   - Warning: Invalid EXTRA_READ_PATHS path '{path_str}': {e}")
+            continue
+
+        # Must be absolute (resolve() makes it absolute, but check original input)
+        if not Path(path_str).is_absolute():
+            print(f"   - Warning: EXTRA_READ_PATHS requires absolute paths, skipping: {path_str}")
+            continue
+
+        # Must exist
+        if not path.exists():
+            print(f"   - Warning: EXTRA_READ_PATHS path does not exist, skipping: {path_str}")
+            continue
+
+        # Must be a directory
+        if not path.is_dir():
+            print(f"   - Warning: EXTRA_READ_PATHS path is not a directory, skipping: {path_str}")
+            continue
+
+        # Check against sensitive directory blocklist
+        is_blocked = False
+        for sensitive in EXTRA_READ_PATHS_BLOCKLIST:
+            sensitive_path = (home_dir / sensitive).resolve()
+            try:
+                # Block if path IS the sensitive dir or is INSIDE it
+                if path == sensitive_path or path.is_relative_to(sensitive_path):
+                    print(f"   - Warning: EXTRA_READ_PATHS blocked sensitive path: {path_str}")
+                    is_blocked = True
+                    break
+                # Also block if sensitive dir is INSIDE the requested path
+                if sensitive_path.is_relative_to(path):
+                    print(f"   - Warning: EXTRA_READ_PATHS path contains sensitive directory ({sensitive}): {path_str}")
+                    is_blocked = True
+                    break
+            except (OSError, ValueError):
+                # is_relative_to can raise on some edge cases
+                continue
+
+        if is_blocked:
+            continue
+
+        validated_paths.append(path)
+
+    return validated_paths
+
+
 # Feature MCP tools for feature/test management
 FEATURE_MCP_TOOLS = [
     # Core feature operations
@@ -209,12 +299,10 @@ def create_client(
     ]
 
     # Add extra read paths from environment variable (read-only access)
-    extra_read_paths = os.getenv(EXTRA_READ_PATHS_VAR, "")
-    if extra_read_paths:
-        for path in extra_read_paths.split(","):
-            path = path.strip()
-            if path:
-                # Add read-only permissions for each extra path
+    # Paths are validated, canonicalized, and checked against sensitive blocklist
+    extra_read_paths = get_extra_read_paths()
+    for path in extra_read_paths:
+        # Add read-only permissions for each validated path
         permissions_list.append(f"Read({path}/**)")
         permissions_list.append(f"Glob({path}/**)")
         permissions_list.append(f"Grep({path}/**)")
@@ -246,7 +334,7 @@ def create_client(
     print("   - Sandbox enabled (OS-level bash isolation)")
     print(f"   - Filesystem restricted to: {project_dir.resolve()}")
     if extra_read_paths:
-        print(f"   - Extra read paths: {extra_read_paths}")
+        print(f"   - Extra read paths (validated): {', '.join(str(p) for p in extra_read_paths)}")
     print("   - Bash commands restricted to allowlist (see security.py)")
     if yolo_mode:
         print("   - MCP servers: features (database) - YOLO MODE (no Playwright)")