security: fix SSH command injection vulnerabilities in pulse-sensor-proxy

CRITICAL security fixes for pulse-sensor-proxy:

1. Strengthened hostname validation regex:
   - Now requires hostnames to start with alphanumeric character
   - Prevents SSH option injection via hostnames starting with '-'
   - Pattern: ^[a-zA-Z0-9][a-zA-Z0-9._-]{0,63}$ (1-64 chars total)
   - Added IPv4 and IPv6 validation regexes for future use

2. Added validation to vulnerable V1 RPC handlers:
   - handleGetTemperature: Now validates node parameter before SSH
   - handleRegisterNodes: Now validates discovered cluster nodes
   - Previously these handlers passed unsanitized input directly to SSH

3. Defense in depth:
   - V2 handlers already had validation (now using improved regex)
   - Multiple layers of protection against malicious node identifiers
   - Validation prevents container from passing SSH options as hostnames

Without these fixes, a compromised container could potentially inject SSH
options by providing malicious node names, though the 'root@' prefix
provided some mitigation.

Addresses high-severity finding from security audit.

cmd/pulse-sensor-proxy/main.go

@@ -39,6 +39,10 @@ func defaultWorkDir() string {
 	return "/var/lib/pulse-sensor-proxy"
 }
 
+var (
+	configPath string
+)
+
 var rootCmd = &cobra.Command{
 	Use:     "pulse-sensor-proxy",
 	Short:   "Pulse Sensor Proxy - Secure sensor data bridge for containerized Pulse",
@@ -65,6 +69,7 @@ var versionCmd = &cobra.Command{
 
 func init() {
 	rootCmd.AddCommand(versionCmd)
+	rootCmd.PersistentFlags().StringVar(&configPath, "config", "", "Path to configuration file (default: /etc/pulse-sensor-proxy/config.yaml)")
 }
 
 func main() {
@@ -135,12 +140,16 @@ func runProxy() {
 	}
 
 	// Load configuration
-	configPath := os.Getenv("PULSE_SENSOR_PROXY_CONFIG")
+	// Priority: --config flag > PULSE_SENSOR_PROXY_CONFIG env > default path
-	if configPath == "" {
+	cfgPath := configPath // from flag
-		configPath = defaultConfigPath
+	if cfgPath == "" {
+		cfgPath = os.Getenv("PULSE_SENSOR_PROXY_CONFIG")
+	}
+	if cfgPath == "" {
+		cfgPath = defaultConfigPath
 	}
 
-	cfg, err := loadConfig(configPath)
+	cfg, err := loadConfig(cfgPath)
 	if err != nil {
 		log.Fatal().Err(err).Msg("Failed to load configuration")
 	}
@@ -151,7 +160,7 @@ func runProxy() {
 	log.Info().
 		Str("socket", socketPath).
 		Str("ssh_key_dir", sshKeyPath).
-		Str("config_path", configPath).
+		Str("config_path", cfgPath).
 		Str("version", Version).
 		Msg("Starting pulse-sensor-proxy")
 
@@ -568,6 +577,13 @@ func (p *Proxy) handleRegisterNodes(req RPCRequest) RPCResponse {
 	// Test SSH connectivity to each node
 	nodeStatus := make([]map[string]interface{}, 0, len(nodes))
 	for _, node := range nodes {
+		// Validate node name to prevent SSH command injection
+		node = strings.TrimSpace(node)
+		if err := validateNodeName(node); err != nil {
+			log.Warn().Str("node", node).Msg("Invalid node name format from cluster discovery")
+			continue
+		}
+
 		status := map[string]interface{}{
 			"name": node,
 		}
@@ -609,6 +625,15 @@ func (p *Proxy) handleGetTemperature(req RPCRequest) RPCResponse {
 		}
 	}
 
+	// Validate node name to prevent SSH command injection
+	node = strings.TrimSpace(node)
+	if err := validateNodeName(node); err != nil {
+		return RPCResponse{
+			Success: false,
+			Error:   "invalid node name format",
+		}
+	}
+
 	// Fetch temperature data
 	tempData, err := p.getTemperatureViaSSH(node)
 	if err != nil {

cmd/pulse-sensor-proxy/validation.go

@@ -9,7 +9,14 @@ import (
 
 var (
 	// nodeNameRegex validates node names (alphanumeric, dots, underscores, hyphens, 1-64 chars)
-	nodeNameRegex = regexp.MustCompile(`^[a-zA-Z0-9._-]{1,64}$`)
+	// Must not start with hyphen to prevent SSH option injection
+	nodeNameRegex = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._-]{0,63}$`)
+
+	// ipv4Regex validates IPv4 addresses
+	ipv4Regex = regexp.MustCompile(`^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$`)
+
+	// ipv6Regex validates IPv6 addresses (simplified)
+	ipv6Regex = regexp.MustCompile(`^[0-9a-fA-F:]+$`)
 )
 
 // sanitizeCorrelationID validates and sanitizes a correlation ID