fix: security audit remediation (AUDIT-001, 003, 004, 005, 006, 018)

- AUDIT-001: Validate OpenSSL revoke inputs (hex-only serials, RFC 5280 reasons)
- AUDIT-003: Enforce /20 CIDR size cap at API level (create + update)
- AUDIT-004: Support comma-separated CERTCTL_AUTH_SECRET for zero-downtime key rotation
- AUDIT-005: Add ReadHeaderTimeout (5s) to prevent Slowloris
- AUDIT-006: Document audit trail query parameter exclusion rationale
- AUDIT-018: Add immediate-run-on-start to short-lived expiry scheduler loop

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/connector/issuer/openssl/openssl.go

@@ -32,9 +32,12 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"time"
 
 	"github.com/shankar0123/certctl/internal/connector/issuer"
+	"github.com/shankar0123/certctl/internal/domain"
+	"github.com/shankar0123/certctl/internal/validation"
 )
 
 // Config represents the OpenSSL/Custom CA issuer connector configuration.
@@ -258,6 +261,36 @@ func (c *Connector) RenewCertificate(ctx context.Context, request issuer.Renewal
 	return result, nil
 }
 
+// hexSerialRegex validates that a serial number contains only hexadecimal characters.
+// Certificate serial numbers are integers represented in hex (RFC 5280).
+var hexSerialRegex = regexp.MustCompile(`^[0-9a-fA-F]+$`)
+
+// validateSerial validates a certificate serial number for safe use in shell commands.
+// Serial numbers must be non-empty, hex-only strings with no shell metacharacters.
+func validateSerial(serial string) error {
+	if serial == "" {
+		return fmt.Errorf("serial number cannot be empty")
+	}
+	if !hexSerialRegex.MatchString(serial) {
+		return fmt.Errorf("serial number %q contains non-hex characters (expected ^[0-9a-fA-F]+$)", serial)
+	}
+	if err := validation.ValidateShellCommand(serial); err != nil {
+		return fmt.Errorf("serial number failed shell safety validation: %w", err)
+	}
+	return nil
+}
+
+// validateRevocationReason validates a revocation reason against RFC 5280 reason codes.
+func validateRevocationReason(reason string) error {
+	if !domain.IsValidRevocationReason(reason) {
+		return fmt.Errorf("invalid revocation reason %q (must be a valid RFC 5280 reason code)", reason)
+	}
+	if err := validation.ValidateShellCommand(reason); err != nil {
+		return fmt.Errorf("revocation reason failed shell safety validation: %w", err)
+	}
+	return nil
+}
+
 // RevokeCertificate revokes a certificate by calling the revoke script if configured.
 func (c *Connector) RevokeCertificate(ctx context.Context, request issuer.RevocationRequest) error {
 	if c.config.RevokeScript == "" {
@@ -270,6 +303,14 @@ func (c *Connector) RevokeCertificate(ctx context.Context, request issuer.Revoca
 		reason = *request.Reason
 	}
 
+	// Validate serial number (hex-only) and reason code (RFC 5280) before shell execution
+	if err := validateSerial(request.Serial); err != nil {
+		return fmt.Errorf("revocation input validation failed: %w", err)
+	}
+	if err := validateRevocationReason(reason); err != nil {
+		return fmt.Errorf("revocation input validation failed: %w", err)
+	}
+
 	c.logger.Info("revoking certificate via revoke script",
 		"serial", request.Serial,
 		"reason", reason)

internal/connector/issuer/openssl/openssl_test.go

@@ -289,7 +289,7 @@ func TestOpenSSLConnector(t *testing.T) {
 		}
 
 		revokeReq := issuer.RevocationRequest{
-			Serial: "test-serial-12345",
+			Serial: "ABCDEF1234567890",
 		}
 
 		// Should return nil (no-op) when revoke script not configured
@@ -324,8 +324,10 @@ func TestOpenSSLConnector(t *testing.T) {
 			t.Fatalf("ValidateConfig failed: %v", err)
 		}
 
+		reason := "keyCompromise"
 		revokeReq := issuer.RevocationRequest{
-			Serial: "test-serial-12345",
+			Serial: "ABCDEF1234567890",
+			Reason: &reason,
 		}
 
 		err := connector.RevokeCertificate(ctx, revokeReq)
@@ -334,6 +336,139 @@ func TestOpenSSLConnector(t *testing.T) {
 		}
 	})
 
+	// Test 15: RevokeCertificate rejects injection payloads in serial number
+	t.Run("RevokeCertificate_InjectionSerial", func(t *testing.T) {
+		tmpDir := t.TempDir()
+
+		signScript := filepath.Join(tmpDir, "sign.sh")
+		if err := os.WriteFile(signScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create sign script: %v", err)
+		}
+		revokeScript := filepath.Join(tmpDir, "revoke.sh")
+		if err := os.WriteFile(revokeScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create revoke script: %v", err)
+		}
+
+		config := &openssl.Config{
+			SignScript:   signScript,
+			RevokeScript: revokeScript,
+		}
+		connector := openssl.New(config, logger)
+		rawConfig, _ := json.Marshal(config)
+		if err := connector.ValidateConfig(ctx, rawConfig); err != nil {
+			t.Fatalf("ValidateConfig failed: %v", err)
+		}
+
+		injectionPayloads := []string{
+			"1234;rm -rf /",
+			"1234|cat /etc/passwd",
+			"1234&whoami",
+			"$(id)",
+			"`id`",
+			"1234\nid",
+			"../../../etc/passwd",
+			"test-serial-12345", // hyphens not allowed (not hex)
+		}
+
+		for _, payload := range injectionPayloads {
+			t.Run(payload, func(t *testing.T) {
+				req := issuer.RevocationRequest{Serial: payload}
+				err := connector.RevokeCertificate(ctx, req)
+				if err == nil {
+					t.Errorf("Expected injection payload %q to be rejected, but it was accepted", payload)
+				}
+			})
+		}
+	})
+
+	// Test 16: RevokeCertificate rejects invalid reason codes
+	t.Run("RevokeCertificate_InvalidReason", func(t *testing.T) {
+		tmpDir := t.TempDir()
+
+		signScript := filepath.Join(tmpDir, "sign.sh")
+		if err := os.WriteFile(signScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create sign script: %v", err)
+		}
+		revokeScript := filepath.Join(tmpDir, "revoke.sh")
+		if err := os.WriteFile(revokeScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create revoke script: %v", err)
+		}
+
+		config := &openssl.Config{
+			SignScript:   signScript,
+			RevokeScript: revokeScript,
+		}
+		connector := openssl.New(config, logger)
+		rawConfig, _ := json.Marshal(config)
+		if err := connector.ValidateConfig(ctx, rawConfig); err != nil {
+			t.Fatalf("ValidateConfig failed: %v", err)
+		}
+
+		invalidReasons := []string{
+			"notARealReason",
+			"keyCompromise;rm -rf /",
+			"$(whoami)",
+			"`id`",
+		}
+
+		for _, reason := range invalidReasons {
+			t.Run(reason, func(t *testing.T) {
+				r := reason
+				req := issuer.RevocationRequest{
+					Serial: "ABCDEF1234567890",
+					Reason: &r,
+				}
+				err := connector.RevokeCertificate(ctx, req)
+				if err == nil {
+					t.Errorf("Expected invalid reason %q to be rejected, but it was accepted", reason)
+				}
+			})
+		}
+	})
+
+	// Test 17: RevokeCertificate accepts all valid RFC 5280 reason codes
+	t.Run("RevokeCertificate_ValidReasons", func(t *testing.T) {
+		tmpDir := t.TempDir()
+
+		signScript := filepath.Join(tmpDir, "sign.sh")
+		if err := os.WriteFile(signScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create sign script: %v", err)
+		}
+		revokeScript := filepath.Join(tmpDir, "revoke.sh")
+		if err := os.WriteFile(revokeScript, []byte("#!/bin/sh\nexit 0"), 0755); err != nil {
+			t.Fatalf("Failed to create revoke script: %v", err)
+		}
+
+		config := &openssl.Config{
+			SignScript:   signScript,
+			RevokeScript: revokeScript,
+		}
+		connector := openssl.New(config, logger)
+		rawConfig, _ := json.Marshal(config)
+		if err := connector.ValidateConfig(ctx, rawConfig); err != nil {
+			t.Fatalf("ValidateConfig failed: %v", err)
+		}
+
+		validReasons := []string{
+			"unspecified", "keyCompromise", "caCompromise", "affiliationChanged",
+			"superseded", "cessationOfOperation", "certificateHold", "privilegeWithdrawn",
+		}
+
+		for _, reason := range validReasons {
+			t.Run(reason, func(t *testing.T) {
+				r := reason
+				req := issuer.RevocationRequest{
+					Serial: "ABCDEF1234567890",
+					Reason: &r,
+				}
+				err := connector.RevokeCertificate(ctx, req)
+				if err != nil {
+					t.Errorf("Expected valid reason %q to be accepted, got error: %v", reason, err)
+				}
+			})
+		}
+	})
+
 	// Test 10: GetOrderStatus always returns "completed"
 	t.Run("GetOrderStatus", func(t *testing.T) {
 		tmpDir := t.TempDir()