Stop leaking PIMONITOR_API_KEY into vcgencmd's environment

SECURITY.md

@@ -64,6 +64,11 @@ reporters and reviewers have context:
 - Shell-outs (`apt list --upgradable`, optional `vcgencmd measure_temp`) are
   invoked with fixed argument lists (no user input is interpolated into
   shell commands), to avoid command injection.
+- Shell-outs are also invoked with an explicit, minimal environment rather
+  than inheriting the service's own — neither `apt` nor `vcgencmd` needs
+  anything beyond `PATH` (and, for `apt`, a couple of apt-specific
+  variables), so `PIMONITOR_API_KEY` is never copied into a child process's
+  `/proc/<pid>/environ`.
 
 If you believe any of these assumptions are violated by the current
 implementation, please report it as described above.

internal/collector/vcgencmd.go

@@ -86,7 +86,15 @@ func (r *vcgencmdRunner) run(ctx context.Context, subcommand string) (string, er
 	ctx, cancel := context.WithTimeout(ctx, vcgencmdTimeout)
 	defer cancel()
 
-	out, err := exec.CommandContext(ctx, path, subcommand).Output()
+	cmd := exec.CommandContext(ctx, path, subcommand)
+	// Build the child environment explicitly rather than inheriting the
+	// service's: PiMonitor's own environment may carry PIMONITOR_API_KEY, and
+	// there is no reason for that secret to be visible in vcgencmd's
+	// /proc/<pid>/environ. path is already the absolute location resolved by
+	// exec.LookPath at detection time, so no PATH is needed either.
+	cmd.Env = []string{}
+
+	out, err := cmd.Output()
 	if err != nil {
 		return "", fmt.Errorf("run vcgencmd %s: %w", subcommand, err)
 	}

internal/collector/vcgencmd_test.go

@@ -3,7 +3,11 @@ package collector
 import (
 	"context"
 	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 	"time"
 )
@@ -88,6 +92,34 @@ func TestVcgencmdRunner_Run_CommandFails(t *testing.T) {
 	}
 }
 
+func TestVcgencmdRunner_Run_DoesNotLeakParentEnvironment(t *testing.T) {
+	t.Setenv("PIMONITOR_API_KEY", "test-secret-should-not-leak")
+
+	dir := t.TempDir()
+	envFile := filepath.Join(dir, "env.out")
+	path := writeFakeVcgencmd(t, dir, "fake-vcgencmd", fmt.Sprintf("env > %q", envFile))
+	r := &vcgencmdRunner{detected: true, path: path}
+
+	if _, err := r.run(context.Background(), "measure_temp"); err != nil {
+		t.Fatalf("run: %v", err)
+	}
+
+	out, err := os.ReadFile(envFile)
+	if err != nil {
+		t.Fatalf("read captured child environment: %v", err)
+	}
+	if strings.Contains(string(out), "PIMONITOR_API_KEY") {
+		t.Fatalf("child environment leaked PIMONITOR_API_KEY: %q", out)
+	}
+	// PATH is not required: path is already the absolute location resolved
+	// by exec.LookPath at detection time, so its absence here confirms the
+	// environment was built explicitly rather than inherited (PWD is set by
+	// the shell itself, not inherited, so its presence is expected).
+	if strings.Contains(string(out), "PATH=") {
+		t.Fatalf("child environment unexpectedly contains PATH: %q", out)
+	}
+}
+
 func TestNewVcgencmdRunner_NilClockDefaultsToTimeNow(t *testing.T) {
 	r := newVcgencmdRunner(nil)