harden: detected non-static command inside command in run.go (#1109)

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Matt Van Horn <mvanhorn@users.noreply.github.com>

changelog.d/1109.security.md

@@ -0,0 +1,3 @@
+Require an absolute path for automatically discovered MCP Python interpreters,
+including when Go's built-in relative-path protection is explicitly disabled.
+Explicit interpreter overrides retain their existing behavior.

mcp/internal/engine/run.go

@@ -121,7 +121,9 @@ func resolvePython(override string) (string, error) {
 		return override, nil
 	}
 	path, err := exec.LookPath(DefaultPythonBinary)
-	if err == nil {
+	// Go normally rejects relative results with ErrDot. Keep this invariant
+	// even when that protection is disabled with GODEBUG=execerrdot=0.
+	if err == nil && filepath.IsAbs(path) {
 		return path, nil
 	}
 	return "", fmt.Errorf(

mcp/internal/engine/run_test.go

@@ -238,6 +238,45 @@ func TestRunMissingPython(t *testing.T) {
 	}
 }
 
+func TestResolvePythonRejectsRelativePATH(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("POSIX executable fixture")
+	}
+	t.Chdir(t.TempDir())
+	if err := os.WriteFile(DefaultPythonBinary, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", ".")
+	// Exercise our own guard even when Go's ErrDot protection is disabled.
+	t.Setenv("GODEBUG", "execerrdot=0")
+	if path, err := resolvePython(""); err == nil || path != "" {
+		t.Fatalf("resolvePython accepted relative executable: path=%q err=%v", path, err)
+	}
+}
+
+func TestResolvePythonAcceptsAbsolutePATH(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("POSIX executable fixture")
+	}
+	dir := t.TempDir()
+	want := filepath.Join(dir, DefaultPythonBinary)
+	if err := os.WriteFile(want, []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", dir)
+	if path, err := resolvePython(""); err != nil || path != want {
+		t.Fatalf("resolvePython = %q, %v; want %q", path, err, want)
+	}
+}
+
+func TestResolvePythonPreservesExplicitOverride(t *testing.T) {
+	t.Setenv("PATH", "")
+	want := filepath.Join("explicit", "python")
+	if path, err := resolvePython(want); err != nil || path != want {
+		t.Fatalf("resolvePython = %q, %v; want trusted override %q", path, err, want)
+	}
+}
+
 func TestRunMissingScript(t *testing.T) {
 	stub := makeStubPython(t)
 	// CacheDir exists but contains no last30days.py.