security: improve path validation in get_file handler

The previous check only looked for ".." substring, which missed:
- Absolute paths (/etc/passwd)
- URL-encoded traversal patterns
- Paths that clean to traversal (./../../etc)

Now uses filepath.Clean() and filepath.IsAbs() for robust validation:
- Rejects absolute paths
- Cleans paths before checking for traversal
- Uses cleaned path for database lookup

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

internal/mcp/server_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"git.t-juice.club/torjus/labmcp/internal/database"
+	"git.t-juice.club/torjus/labmcp/internal/nixos"
 )
 
 func TestServerInitialize(t *testing.T) {
@@ -144,6 +145,91 @@ func TestServerNotification(t *testing.T) {
 	}
 }
 
+func TestGetFilePathValidation(t *testing.T) {
+	store := setupTestStore(t)
+	server := setupTestServer(t, store)
+
+	// Create a test revision and file
+	ctx := context.Background()
+	rev := &database.Revision{
+		GitHash:     "abc123",
+		OptionCount: 0,
+	}
+	if err := store.CreateRevision(ctx, rev); err != nil {
+		t.Fatalf("Failed to create revision: %v", err)
+	}
+
+	file := &database.File{
+		RevisionID: rev.ID,
+		FilePath:   "nixos/modules/test.nix",
+		Extension:  ".nix",
+		Content:    "{ }",
+	}
+	if err := store.CreateFile(ctx, file); err != nil {
+		t.Fatalf("Failed to create file: %v", err)
+	}
+
+	tests := []struct {
+		name      string
+		path      string
+		wantError bool
+		errorMsg  string
+	}{
+		// Valid paths
+		{"valid relative path", "nixos/modules/test.nix", false, ""},
+		{"valid simple path", "test.nix", false, ""},
+
+		// Path traversal attempts
+		{"dotdot traversal", "../etc/passwd", true, "directory traversal"},
+		{"dotdot in middle", "nixos/../../../etc/passwd", true, "directory traversal"},
+		{"multiple dotdot", "../../etc/passwd", true, "directory traversal"},
+
+		// Absolute paths
+		{"absolute unix path", "/etc/passwd", true, "absolute paths"},
+
+		// Cleaned paths that become traversal
+		{"dot slash dotdot", "./../../etc/passwd", true, "directory traversal"},
+
+		// Paths that clean to valid (no error expected, but file won't exist)
+		{"dotdot at end cleans to valid", "nixos/modules/..", false, ""}, // Cleans to "nixos", which is safe
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input := `{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"get_file","arguments":{"path":"` + tt.path + `","revision":"abc123"}}}`
+			resp := runRequest(t, server, input)
+
+			result, ok := resp.Result.(map[string]interface{})
+			if !ok {
+				t.Fatalf("Expected map result, got %T", resp.Result)
+			}
+
+			isError, _ := result["isError"].(bool)
+
+			if tt.wantError {
+				if !isError {
+					t.Errorf("Expected error for path %q, got success", tt.path)
+				} else {
+					content := result["content"].([]interface{})
+					text := content[0].(map[string]interface{})["text"].(string)
+					if tt.errorMsg != "" && !strings.Contains(text, tt.errorMsg) {
+						t.Errorf("Error message %q doesn't contain %q", text, tt.errorMsg)
+					}
+				}
+			} else {
+				// For valid paths that don't exist, we expect a "not found" error, not a security error
+				if isError {
+					content := result["content"].([]interface{})
+					text := content[0].(map[string]interface{})["text"].(string)
+					if strings.Contains(text, "traversal") || strings.Contains(text, "absolute") {
+						t.Errorf("Got security error for valid path %q: %s", tt.path, text)
+					}
+				}
+			}
+		})
+	}
+}
+
 // Helper functions
 
 func setupTestStore(t *testing.T) database.Store {
@@ -165,6 +251,16 @@ func setupTestStore(t *testing.T) database.Store {
 	return store
 }
 
+func setupTestServer(t *testing.T, store database.Store) *Server {
+	t.Helper()
+
+	server := NewServer(store, nil)
+	indexer := nixos.NewIndexer(store)
+	server.RegisterHandlers(indexer)
+
+	return server
+}
+
 func runRequest(t *testing.T, server *Server, input string) *Response {
 	t.Helper()