fix(bundle-4): EST/SCEP Attack Surface Hardening — 3 audit findings closed

Closes 3 findings (1 High + 1 Medium + 1 Low) from
/Users/shankar/Desktop/cowork/comprehensive-audit-2026-04-25/.

Bundle 4 hardens the only attack surface reachable by an anonymous network
attacker in certctl: the unauthenticated EST + SCEP enrollment endpoints.

Findings closed:

  - H-004 (High): Hand-rolled ASN.1 parser had no fuzz target.
    The audit's original framing pointed at internal/pkcs7/, but recon
    confirmed that package is an ASN.1 ENCODER (BuildCertsOnlyPKCS7,
    ASN1Wrap*, ASN1EncodeLength) — not a parser. The actual hand-rolled
    PKCS#7 PARSING reachable via anonymous network is in
    internal/api/handler/scep.go::extractCSRFromPKCS7 +
    parseSignedDataForCSR. Added native go fuzz targets:
      * internal/api/handler/scep_fuzz_test.go::FuzzExtractCSRFromPKCS7
      * internal/api/handler/scep_fuzz_test.go::FuzzParseSignedDataForCSR
      * internal/pkcs7/pkcs7_fuzz_test.go::FuzzPEMToDERChain (defense-in-depth)
      * internal/pkcs7/pkcs7_fuzz_test.go::FuzzASN1EncodeLength (defense-in-depth)
    Local 15s fuzz session: 150k execs on FuzzExtractCSRFromPKCS7,
    937k on FuzzPEMToDERChain, 925k on FuzzASN1EncodeLength — zero panics.

  - M-021 (Medium): EST TLS-Unique channel binding (RFC 7030 §3.2.3).
    Added internal/api/handler/est.go::verifyESTTransport — defense-in-depth
    TLS pre-conditions (r.TLS != nil; HandshakeComplete; TLS ≥ 1.2).
    The full §3.2.3 channel binding only applies when EST mTLS is in use;
    certctl does not currently support EST mTLS, so the §3.2.3 requirement
    is moot today. RFC 9266 (TLS 1.3 tls-exporter) and EST mTLS are
    documented as deferred follow-ups in the verifyESTTransport doc comment.

  - L-005 (Low): EST/SCEP issuer-binding fail-loud at startup.
    Pre-Bundle-4 cmd/server/main.go validated that CERTCTL_EST_ISSUER_ID and
    CERTCTL_SCEP_ISSUER_ID existed in the registry but did NOT validate the
    issuer TYPE could emit a CA cert. An operator binding EST to an ACME
    issuer (whose GetCACertPEM returns explicit error) booted successfully
    and only failed at first /est/cacerts request. Post-Bundle-4: new
    preflightEnrollmentIssuer helper calls GetCACertPEM(ctx) at startup
    with a 10s timeout. Failure logs the connector error + the candidate
    issuer types and os.Exit(1).

Tests added/modified:
  - internal/api/handler/est_transport_test.go (new) — 5 verifyESTTransport
    table cases covering plaintext-rejected, incomplete-handshake-rejected,
    TLS 1.0 rejected, TLS 1.2/1.3 accepted
  - cmd/server/preflight_test.go (new) — TestPreflightEnrollmentIssuer
    covering nil-connector, error-from-issuer, empty-PEM, valid cases
  - internal/api/handler/est_handler_test.go (modified) — 7 POST sites
    now stamp r.TLS to satisfy the new transport pre-condition
  - internal/integration/negative_test.go (modified) — setupTestServer
    wraps the test handler with a fake-TLS-state injector so the EST
    handler receives r.TLS != nil; production paths still rely on the
    real TLS listener

Threat model reference: TB-11 (EST/SCEP client ↔ Server) per
cowork/comprehensive-audit-2026-04-25/threat-model.md.
Standards: RFC 7030 §3.2.3, RFC 8894 §3, RFC 5652, RFC 9266 (deferred).

cmd/server/main.go

@@ -630,6 +630,17 @@ func main() {
 			logger.Error("EST issuer not found in registry", "issuer_id", cfg.EST.IssuerID)
 			os.Exit(1)
 		}
+		// Bundle-4 / L-005: validate the issuer can actually serve a CA certificate
+		// at startup, not at first request time. ACME / DigiCert / Sectigo etc.
+		// return an error from GetCACertPEM because they don't expose a static
+		// CA chain; binding EST to one of those would silently degrade enrollment.
+		preflightCtx, preflightCancel := context.WithTimeout(context.Background(), 10*time.Second)
+		if err := preflightEnrollmentIssuer(preflightCtx, "EST", cfg.EST.IssuerID, issuerConn); err != nil {
+			preflightCancel()
+			logger.Error("startup refused: EST issuer cannot serve CA certificate", "error", err)
+			os.Exit(1)
+		}
+		preflightCancel()
 		estService := service.NewESTService(cfg.EST.IssuerID, issuerConn, auditService, logger)
 		estService.SetProfileRepo(profileRepo)
 		if cfg.EST.ProfileID != "" {
@@ -668,6 +679,15 @@ func main() {
 			logger.Error("SCEP issuer not found in registry", "issuer_id", cfg.SCEP.IssuerID)
 			os.Exit(1)
 		}
+		// Bundle-4 / L-005: validate the issuer can actually serve a CA certificate
+		// at startup. Same rationale as EST above.
+		preflightCtx, preflightCancel := context.WithTimeout(context.Background(), 10*time.Second)
+		if err := preflightEnrollmentIssuer(preflightCtx, "SCEP", cfg.SCEP.IssuerID, issuerConn); err != nil {
+			preflightCancel()
+			logger.Error("startup refused: SCEP issuer cannot serve CA certificate", "error", err)
+			os.Exit(1)
+		}
+		preflightCancel()
 		scepService := service.NewSCEPService(cfg.SCEP.IssuerID, issuerConn, auditService, logger, cfg.SCEP.ChallengePassword)
 		scepService.SetProfileRepo(profileRepo)
 		if cfg.SCEP.ProfileID != "" {
@@ -981,6 +1001,43 @@ func preflightSCEPChallengePassword(enabled bool, challengePassword string) erro
 	return nil
 }
 
+// preflightEnrollmentIssuer validates at startup that an EST/SCEP-bound issuer
+// can actually serve a CA certificate. This closes audit finding L-005:
+// pre-Bundle-4 the EST/SCEP startup path verified the issuer existed in the
+// registry but did not verify the issuer TYPE could emit a CA cert. An
+// operator who bound CERTCTL_EST_ISSUER_ID to an ACME issuer (which does
+// not have a static CA cert — see internal/connector/issuer/acme/acme.go::
+// GetCACertPEM returning an explicit error) would boot successfully and
+// only see failures at the first /est/cacerts request, hiding the misconfig
+// for hours/days behind a degraded enrollment surface.
+//
+// Strategy: call issuerConn.GetCACertPEM(ctx) at startup with a short
+// timeout. If the issuer can serve a CA cert (local, vault, openssl,
+// stepca, awsacmpca, etc.), the call succeeds and we proceed. If not
+// (acme, digicert, sectigo, entrust, googlecas, ejbca, globalsign — most
+// vendor-CA issuers that hand back chains per-issuance), the call fails
+// loudly with the connector's own error string, and the caller os.Exit(1)s.
+//
+// Returns nil on success, non-nil error suitable for structured logging
+// + os.Exit(1) by the caller. Caller is responsible for the timeout context.
+func preflightEnrollmentIssuer(ctx context.Context, protocol, issuerID string, issuerConn service.IssuerConnector) error {
+	if issuerConn == nil {
+		return fmt.Errorf("%s issuer %q: connector is nil", protocol, issuerID)
+	}
+	caCertPEM, err := issuerConn.GetCACertPEM(ctx)
+	if err != nil {
+		return fmt.Errorf("%s issuer %q: cannot serve CA certificate (%w); "+
+			"choose an issuer type that exposes a static CA chain "+
+			"(local / vault / openssl / stepca / awsacmpca) or disable %s",
+			protocol, issuerID, err, protocol)
+	}
+	if caCertPEM == "" {
+		return fmt.Errorf("%s issuer %q: GetCACertPEM returned empty PEM with no error; "+
+			"choose an issuer type that exposes a static CA chain", protocol, issuerID)
+	}
+	return nil
+}
+
 // buildFinalHandler builds the outer HTTP dispatch handler that routes incoming
 // requests to either the authenticated apiHandler chain or the unauthenticated
 // noAuthHandler chain based on URL path prefix. Extracted from main() so the

cmd/server/preflight_test.go

@@ -0,0 +1,100 @@
+package main
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/shankar0123/certctl/internal/service"
+)
+
+// fakeIssuerConn implements service.IssuerConnector enough for preflight tests.
+type fakeIssuerConn struct {
+	caCertPEM string
+	caCertErr error
+}
+
+func (f *fakeIssuerConn) IssueCertificate(ctx context.Context, commonName string, sans []string, csrPEM string, ekus []string, maxTTLSeconds int) (*service.IssuanceResult, error) {
+	return nil, nil
+}
+func (f *fakeIssuerConn) RenewCertificate(ctx context.Context, commonName string, sans []string, csrPEM string, ekus []string, maxTTLSeconds int) (*service.IssuanceResult, error) {
+	return nil, nil
+}
+func (f *fakeIssuerConn) RevokeCertificate(ctx context.Context, serial string, reason string) error {
+	return nil
+}
+func (f *fakeIssuerConn) GenerateCRL(ctx context.Context, revokedCerts []service.CRLEntry) ([]byte, error) {
+	return nil, nil
+}
+func (f *fakeIssuerConn) SignOCSPResponse(ctx context.Context, req service.OCSPSignRequest) ([]byte, error) {
+	return nil, nil
+}
+func (f *fakeIssuerConn) GetCACertPEM(ctx context.Context) (string, error) {
+	return f.caCertPEM, f.caCertErr
+}
+func (f *fakeIssuerConn) GetRenewalInfo(ctx context.Context, certPEM string) (*service.RenewalInfoResult, error) {
+	return nil, nil
+}
+
+// TestPreflightEnrollmentIssuer covers Bundle-4 / L-005 startup validation
+// for EST/SCEP issuer binding.
+func TestPreflightEnrollmentIssuer(t *testing.T) {
+	cases := []struct {
+		name        string
+		issuer      service.IssuerConnector
+		wantErr     bool
+		errContains string
+	}{
+		{
+			name:        "nil_connector_fails",
+			issuer:      nil,
+			wantErr:     true,
+			errContains: "connector is nil",
+		},
+		{
+			name: "issuer_returns_error_fails",
+			issuer: &fakeIssuerConn{
+				caCertErr: errStub("ACME issuers do not provide a static CA certificate"),
+			},
+			wantErr:     true,
+			errContains: "cannot serve CA certificate",
+		},
+		{
+			name: "issuer_returns_empty_pem_fails",
+			issuer: &fakeIssuerConn{
+				caCertPEM: "",
+				caCertErr: nil,
+			},
+			wantErr:     true,
+			errContains: "empty PEM",
+		},
+		{
+			name: "issuer_returns_valid_pem_succeeds",
+			issuer: &fakeIssuerConn{
+				caCertPEM: "-----BEGIN CERTIFICATE-----\nMIIB...\n-----END CERTIFICATE-----",
+				caCertErr: nil,
+			},
+			wantErr: false,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := preflightEnrollmentIssuer(context.Background(), "EST", "iss-test", tc.issuer)
+			if tc.wantErr && err == nil {
+				t.Fatalf("expected error, got nil")
+			}
+			if !tc.wantErr && err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if tc.wantErr && tc.errContains != "" && !strings.Contains(err.Error(), tc.errContains) {
+				t.Fatalf("error %q missing substring %q", err.Error(), tc.errContains)
+			}
+		})
+	}
+}
+
+// errStub is a tiny error wrapper so test cases can use string literals
+// without importing fmt in every test struct entry.
+type errStub string
+
+func (e errStub) Error() string { return string(e) }